CERN-SPSC-2004-024 SPSC-M-722

30 August 2004






 
 


Workshop on

PHYSICS WITH A  

MULTI-MW PROTON SOURCE

CERN, Geneva, May 25-27, 2004

 

 

Figure 1

 

 


Physics with a multi-Megawatt proton source

 

Summary

The workshop ‘Physics with a multi-Megawatt (MMW) Proton Source’ was held at CERN on 25-27 May 2004. It was organized by the ECFA/BENE working groups for future neutrino facilities in Europe, and by the EURISOL nuclear physics community to explore the physics opportunities offered by a new high intensity proton accelerator. It confirmed the important synergies that exist between these communities. This document summarises the main design choices for the accelerators and experiments involved, the physics possibilities and the most critical R&D that will be necessary.  A preliminary baseline road map is delineated.

A strong and diverse physics programme was highlighted.

 

The leading particle physics case is provided by the study of neutrino oscillations with the ultimate aim of discovery and study of leptonic CP violation. Two main options emerge.

1) A Neutrino Factory based on a high brilliance muon beam would provide high-energy electron neutrinos (up to 20-50 GeV). Aimed at dense magnetic detectors situated ~700 to 4000 km away, this is by far the best tool to perform very precise and unambiguous measurements of oscillation parameters, neutrino mass hierarchy, CP violation and tests of universality in the neutrino sector.

2) A neutrino beta-beam facility  would provide a very clean beam of electron (anti) neutrinos from beta decay of high energy radioactive ion beams of characteristics similar to those of EURISOL. The very large detectors that are required are the same as those needed to extend the search for proton decay and astrophysical neutrinos. In combination with a low energy conventional neutrino beam (superbeam), this would provide interesting sensitivity in search for leptonic CP and T violation. 

In addition, the near detectors of these facilities offer the possibility to study neutrino interactions with great precision in various energy regimes. The envisaged proton driver offers great flexibility of time structures and would allow cutting edge experiments in low energy muon physics.  It will be of great utility for the high intensity operation of the LHC, and will allow higher intensities to be achieved throughout the present CERN complex and fixed target experiments.

A high intensity proton accelerator has been advocated as opening a wide field of new possibilities in nuclear physics as well, with the availability of intense beams of rare radio-isotopes. The detailed study of properties of nuclei at the edges of stability should project decisive light on the fundamental but still largely mysterious question of nuclear matter and its dynamical symmetries. The simultaneous availability of radioactive ion beams and of low energy muon and antiproton beams should allow a new class of experiments to be performed., several of which are critical for the understanding of star formation and supernovae.

The choices will need to be made in the near future require the critical R&D and studies to be carried out. The list comprises i) the study of high power target, particle collection elements and target stations, ii) the study of beam activations in the proton driver and subsequent accelerators, iii) the demonstration of ionisation cooling, iv) the feasibility study of large underground caverns and v) R&D on the very large detectors that are envisaged.

Table of Contents

1     Foreword: High Energy and High Intensity Frontiers. 12

2     The High Intensity Frontier. 14

2.1       Nuclear physics. 15

2.2       Particle physics. 18

2.3       Conclusions. 25

3     Planned High Intensity facilities in the world. 26

3.1       The Japanese Proton Accelerator Reseach Center J-PARC.. 26

3.2       U.S. Plans for High Power Proton Drivers. 29

3.3       Fair, the GSI New Facility For Antiproton and ion research. 33

4     High Intensity Proton Source. 38

4.1       The future of proton accelerators at CERN.. 38

4.2       The Superconducting Proton Linac (SPL) and its potential 43

4.3       The Rapid Cycling Synchrotron options. 47

5     Facilities around the intense proton source. 56

5.1       Additional installations for a neutrino physics facility. 56

5.2       Hadro-production experiments. 63

5.3       EURISOL : a new nuclear physics facility. 67

5.4       A neutrino beta beam facility at CERN.. 70

6     Particle Physics case – neutrino oscillations. 74

6.1       Overview of neutrino oscillation physics. 74

6.2       A possible large underground laboratory: status and plans of the Fréjus international laboratory  84

6.3       A megaton water Cherenkov detector 85

6.4       Ideas for a next generation liquid Argon TPC detector for neutrino physics and nucleon decay searches  92

7     Other particle physics opportunities. 98

7.1       Short baseline neutrino physics. 98

7.2       High intensity muon physics. 102

7.3       Physics at Higher Intensity PS or SPS at CERN.. 105

8     Nuclear physics. 108

8.1       The future of nuclear physics studies. 108

8.2       Nuclear dynamics and the nuclear equation of state. 109

8.3       Astrophysics with R.I.B. 112

8.4       Fundamental symmetries and interactions. 116

8.5       New approaches to the study of the nucleus with muons and antiprotons. 120

9     Conclusions and outlook. 123

9.1       Accelerators. 123

9.2       Nuclear Physics. 125

9.3       Particle physics. 126

9.4       General outlook. 127

10       Recommendations. 132

10.1     Recommended accelerator R&D.. 132

10.2     Recommended detector R&D.. 133

10.3     Milestones. 134

References. 135

Workshop Organization, Participants and Programme. 142

 

Table of illustrations

Figure 1 Possible layout of the Superconducting Proton Linac on the CERN site. 15

Figure 2 Possible location of the EURISOL complex at CERN. 16

Figure 3 Several aspects of physics with Radioactive Nuclear Beams. 16

Figure 4: illustration of some fundamental questions in Nuclear Physics; on the left, the formation of bosons in the nucleus (‘nucleonic Cooper pairs’) is an important ingredient to understand why some nuclei are stable and others not; on the right, the neutron rich nuclei have a ‘skin’ of neutrons which display shape oscillations in various modes. 17

Figure 5 Novae and Supernovae constitute nuclear reactions at a grand scale. The explanation of the energy production in explosive thermonuclear burning probably requires two distinct neutron-capture processes as the origin of heavy nuclei beyond iron. 18

Figure 6 Graphical representation of the present knowledge of the neutrino mixing matrix. The rotations by the successive Euler angles q12, q13, q23  that transform the mass eigenstates n1 n2 n3  into the flavour eigenstates ne  nm  nt  are shown on the left, while the mass splittings are shown on the right. The best present values are q12 =320, q23 = 45 0, q13 < 130, and  Dm212 = + 8 10-5 eV2,  Dm223 = ± 2.5 eV2. In addition there is a phase, d, which generates CP and T violating effects in neutrino oscillations. 19

Figure 7 Present determination of the neutrino mixing parameters. Left:  q23 and Dm223 from atmospheric neutrinos and the K2K experiment; right:  q12 and Dm212  from the solar neutrinos and the KAMLAND experiment [neutrinofits]. 19

Figure 8 Three possible neutrino facilities based on the same proton driver. Top: a low energy conventional pion decay beam; middle the beta beam where 6He and 18Ne RIBs are accelerated using the PS and SPS as accelerators; bottom, the neutrino factory where muons are accelerated and stored at 20-50 GeV. 21

Figure 9 relative sensitivity to q13 (top) and to the CP violating phase d (bottom), of various options of Figure 8 for future neutrino facilities based on a high intensity proton driver. 22

Figure 10 Event rates at the exit of the straight sections in a Neutrino Factory. Note the scale. 23

Figure 11 Supersymmetric See-Saw mechanism (left) leads to predictions (right) for the branching ratio that are close to or even beyond the present limits of ~10-11 . 24

Figure 12 Principle of the measurement of the muon EDM.. 24

Figure 13 Left, the dependence of the rare decays  , and  upon the parameters of the unitarity triangle; right, sensitivity of to the Susy mass scale m0 . 25

Figure 14 Artistic aerial view of the Japanese Proton Accelerator Reseach Complex J-PARC.. 26

Figure 15 Layout of theT2K experiment 27

Figure 16 The J-Nufact scheme for a Neutrino Factory, based on a chain of FFAG accelerators  28

Figure 17 Schematic view of the AGS upgrade concept. Existing accelerators are in red, new facilities are in blue  31

Figure 18 Schematic layout of the superconducting linac based Proton Driver at Fermilab. Linac variants under consideration include higher beam power (2 MW), lower rf frequencies (1207 MHz and its derivatives), and a warm front end. 32

Figure 19 Schematic layout for the Facility for Antiproton and Ion Research, FAIR, at GSI. Details are given in the CDR [CDRFair]. 33

Figure 20 The Superconducting Fragment Separator (Super-FRS) at the                 FAIR facility. 34

Figure 21 In-flight radioactive beam facilities and yields of fully stripped ions as a function of energy. 35

Figure 22 Production of secondary beams by 1 GeV/u uranium ions impinging on a hydrogen target. Over 1000 products were identified by the fragment separator FRS at GSI. 35

Figure 23: Expected yields at the Super Fragment Separator at FAIR. 36

Figure 24 : SPL schematic layout (CDR 1 design for the superconducting part). 44

Figure 25 : Proton driver complex on the CERN site. 45

Figure 26: Indicative multi-year planning for the full SPL project. 46

Figure 27 Rapid Cycling Synchrotron models at 5 GeV (left) and 15 GeV (right) 52

Figure 28  Schematic layout of a Neutrino Factory. 56

Figure 29 Possible layout of a Neutrino Factory on the CERN site. 57

Figure 30 Experiments with liquid mercury jets. On the left is seen a jet exposed to a beam of 4 1012 protons of 24 GeV at BNL; the jet explosion begins long enough after the impact. On the right is shown the behaviour of the jet inside a high magnetic field; the jet is able to penetrate the intense magnetic field, and Eddie currents smoothen it. 59

Figure 31  Possible layout of a Hg jet target and a horn. On the right, a prototype horn built at CERN  [Gilardoni] 60

Figure 32 Layout of the MICE experiment. 61

Figure 33 The HARP experiment 63

Figure 34 Transversal momentum resolution as a function of pT, as  measured by the TPC. 65

Figure 35 (left) dE/dX as a function of p, showing different particle populations (p,m and protons); (right) energy peaks for the radioactive sources used to calibrate the detector: (a) overall picture with 55Fe and 83Kr. (b) Fe peaks (at 5.9 and 3.0 keV) and Kr peaks (the main one being at 41.6 keV). This case is obtained using equalised pads. 65

Figure 36 (p,q) distribution for pions in the K2K case as described in the text. The parameter space is well within the reach of the forward HARP detectors. 66

Figure 37 (left) momentum distribution, integrated over q, for secondary pions from 12.9 GeV/c protons impinging onto a 5% l Al target. (right) angle distribution, integrated over p, for the same sample. Vertical axis is in arbitrary units [Cervera04]. 66

Figure 38 : Conceptual layout of the EURISOL facility. 68

Figure 39 Beta-beam base line design, partially using existing CERN accelerator infrastructure (parts in black). 70

Figure 40 Left: fluxes in the BB from 1018 18Ne and 3 1018 6He ion decays per year at g=100;60 and L=130 km. Right: fluxes at the NF from 2 1020  50GeV m+ , m- decays and L=3000 km. 75

Figure 41 Coupling allowed for neutrinos in the Standard Model, which is not allowed for the other fermions. 75

Figure 42 :  Four classes of events studied in a liquid argon TPC  with  muon charge identification.  From the top, left to right:  events with high-energy electrons, right-sign muons, wrong-sign muons,  no charged leptons [Bueno00]. 78

Figure 43  The sensitivity reaches as functions of sin22 q13 for sin22 q13 itself, the neutrino mass hierarchy, and maximal CP Violation (dCP = p/2) for each of the indicated baseline combinations. The bars show the ranges in sin22 q13  where sensitivity to the corresponding quantity can be achieved at the 3s CL. The dark (red) bars show the variation in the result as Dm221 is varied within its present uncertainty. Figure from [Huber03]. 78

Figure 44 :  Beta Beam fluxes at the Frejus location (130 km baseline). Also the SPL Super Beam nm and nm fluxes are shown in the plot [Bouchez03]. 80

Figure 45 :  Left:  90%CL sensitivity of the disappearance channel to q13 in a 5 yrs run drawn as dotted lines.  The labels 0.5%, 1% and 2% indicate the systematic errors with which are computed.  Also shown are the appearance sensitivities of Beta and SPL beams, computed for d=0, sign(Dm2)=+1. Right:  90%CL sensitivity expressed as function of d for dm223 =2.5×10-3eV2. All the appearance sensitivities are computed for sign(Dm2)=+1. 82

Figure 46 : 99%CL d sensitivity of the Beta Beam, of the SPL-SuperBeam, and of their combination, see text.  Dotted line is the combined SPL+Beta sensitivity computed for sign(Dm2)=-1. Sensitivities are compared with a 50 GeV Neutrino Factory producing 2×1020m decays/straight section/year, and two 40 kton detectors at 3000 and 7000 km   82

Figure 47 Solving degeneracies (from [Rigolin04]). The parameter space shown is the variation {Dq13, dCP} around the true solution in the  {q13, dCP} plane. The lines show the locus where the same number of events would be observed. Full lines, neutrino exposure; dashed lines antineutrino exposure. On the left, the red and blue lines show two different base lines (730 and 3500 km) while on the right the red and blue lines show the golden and silver channel. 83

Figure 48 UNO Detector Conceptual Design. 86

Figure 49 Conceptual Design of Hyper Kamiokande. 87

Figure 50 Candidate sites for the US National Underground Science & Engineering Laboratory (NUSEL) 89

Figure 51 Our past, present and possible future knowledge of nucleon decay. 90

Figure 52 Recent HPD Prototypes from Hamamatsu. 91

Figure 53 Reference Photosensor R&D in the USA.. 92

Figure 54:   Schematic layout of the inner detector of a future Large Liquid Argon detector 95

Figure 55 : Artist view of a possible 100 kton liquid argon TPC. 96

Figure 56 NC 1p cross sections. Top left: (nm p ® nm p p0); top right: (nm n ® nm n p0);; bottom left: (nm p ® nm n p+); bottom right: (nm n ® nm p p-). Shown are the free nucleon cross section predictions from NUANCE [Casper02] and NEUGEN [Gallager02] with mA = 1.032 GeV, mV = 0.84 GeV, and sin2q w = 0.233. 100

Figure 57  Sensitivity to sin2qme  as a function of the exposure for different estimates of the systematic errors [Itow01]. 101

Figure 58 Dm2 atmospheric and sin2q23 determination at T2K by assuming present 101

Figure 59 Left: SUSY SU(5) predictions for the µ®eg  branching ratio. Right:   Predictions from a SUSY model including a see-saw mechanism for neutrino masses generation (see text) 102

Figure 60 A sketch of the MEG detector 103

Figure 61 Computed ratio of BRµe/BRµ®eg.                         Figure 62 Results of the SINDRUM II experiment 103

Figure 63 The MECO experiment 104

Figure 64  Left side: normalized configurational energy fluctuations and canonical reference (dark grey) measured in peripheral Au+Au collisions at 35 A.MeV (light grey), and central collisions of an Au beam on different targets (symbols). Abnormally high fluctuations are observed in the energy range 2<E*/A<6.5. Right side: corresponding heat capacity deduced in the framework of thermodynamic equilibrium. All data are from the Multics-Miniball collaboration [Schrenberg01],[D’Agostino04] 110

Figure 65 left side: theoretical prediction for the coexistence zone of asymmetric nuclear matter as a function of proton and neutron density from ref. [Mueller95]]. Right side: relative production yield of light isobars (gas phase) as a function of temperature from ref. [Chomaz99]. A neutron enrichment (upper line) is predicted for this neutron rich hot nucleus (160 neutrons and 96 protons) respect to the simple combinatorial expectation (lower line). 111

Figure 66 Time integrated reaction flow for a complete X-ray burst. The thermonuclear runaway is triggered by the 3 a-reaction and the break-out reactions of the hot CNO cycle into the ap-process, which provides the seed for the hydrogen burning via the rp-process. The inset shows the Sn-Sb-Te cycle in detail. For more details, see reference [Schatz01]. 113

Figure 67  Schematic curve of Solar-System abundances as a function of atomic weight, based on the 1956 data of Suess and Urey ] [Suess56]. 114

Figure 68 3 (Left:) The r-process waiting points for mass models with strong neutron shell closures (such as FRDM or ETFSI-1) are shown in the vicinity of the neutron shell closure at N=82 for a neutron density of nn=9.5×1020 cm-3 and a temperature of 1.2 ×109K. Nuclei with more than 10% of the population of an isotopic chain are represented with an open square. Open squares with a cross indicate waiting-point nuclei with the maximum abundance in an isotopic chain. The nuclei in the valley of stability are displayed as full squares. The bottom part shows the expected waiting points using the masses close to N=82 from models which contain shell quenching (such as HFB/SkP or ETFSI-Q). It is shown that the region within the circle at Z≈40, A≈78 contains more progenitors [Pfeiffer96]. (Right:) Comparison of calculated abundances with the Nr,Θ distribution for ‘unquenched’ (top) and ‘quenched’ masses (bottom). 115

Figure 69 Limits of experimentally determined masses in the neutron-rich closed-shell regions N=50, 82 and 126. 116

Figure 70        Feeding of excited states in 78Ni by muon capture on muonic 78Cu according to RPA calculations. 121

Figure 71 Experimental reach expressed in beam energy and halflives for ``hot'' (directly following production) and ``cold'' (following improvements of beam properties) beams from ISOL-type and in-flight facilities. 122

Figure 72 SPL Block diagram.. 124

Table 1 Performance parameters for Fermilab and BNL concepts for Proton Driver upgrades. 30

Table 2 Main users’ requests. 38

Table 3 Proton flux in 2007 without improvement to the accelerators  (“pot” stands for protons on target). 39

Table 4 Proton flux in 2010 after implementation of the improvements recommended by the HIP WG. 40

Table 5 Possible improvement to the accelerator complex. 41

Table 6 Main SPL parameters (CDR 1) 43

Table 7 Linac 4 specifications. 44

Table 8 Parameters of Existing and Proposed Proton Sources. 47

Table 9 Comparison of unloaded Neutrino Factory costs estimates for the US Study II design and for the latest updated US design. Costs are shown including or not including the Proton Driver and Target station in the estimates. The New design cost estimate has not yet benefited from the level of engineering effort included in the Study II work. Table from Ref. [APS04]. 62

Table 10 Summary of the HARP data taking campaign (2001-2002). Many materials have been tested ranging from H to Pb, at proton momenta covering an entire decade (1.5 to 15 GeV/c). Some special targets (like the K2K and MiniBooNE replicas and cryogenic targets) have been thoroughly studied. 64

Table 11: Ion intensities for 6He and 18Ne operation along the accelerator chain for the beta-beam base line scenario (only beta-decay losses are taken into account). 72

Table 12 :   Event rates for a 4400 kt-y exposure.  The signals are computed for q13 =3°, d=90° sign(Dm2)=+1. “d-oscillated” events indicates the difference between the oscillated events computed with d=90° and with d=0. “Oscillated at the Chooz limit” events are computed for sin22q13=0.12, d=0. 80

Table 13  Measurements of NC/CC single pion cross-section ratios. The Gargamelle data has been corrected to a free nucleon ratio [Krenz78]. Also quoted are the free nucleons cross-section predictions from NUANCE assuming mA = 1.032 GeV, mV = 0.84, and sin2qw  = 0.2319 in each case. * In their later paper [Derrick81], Derrick et al. remark that while this result is 1.6_ smaller than their previous result [Barish74], the neutron background in this case was better understood. ** The BNL NC p0 data was later reanalysed after properly taking into account multi-pion backgrounds and found to have a larger fractional cross-section [Nienaber]. 99

Table 14 Comparison between the current NA48/2 beam and the future one. The figures in brackets in the last column refer to increase in rate with respect to the sum of the positive and negative NA48/2 beams. 106

Table 15 Physics possibilities at a Multi-Megawatt Proton Driver. Most experiments would benefit from a pulsed time structure of the beam. 118

Table 16 List of participants: 142

Table 17  Workshop Agenda. 146

 

1         Foreword: High Energy and High Intensity Frontiers

(R. Aymar, V. Palladino)

In his welcome address, CERN DG Prof. R. Aymar outlined his general view of CERN mission and options for the future. 

He welcomed and thanked all participants for their contribution to the preparation of the meeting organized for that purpose by the CERN SPSC next September in Villars. The CERN management will welcome all help in the definition the optimum changes to be made to the CERN accelerators, so to cover the most ambitious and promising spectrum of physics experiments.

He outlined the two driving scientific motivations of a new vigorous physics program at the high-intensity frontier, and thus for a new MMW proton facility and this Workshop:

Among the possibilities, a superconducting proton linac (SPL) delivering MWatts of beam power at a few GeV has already attracted the interest of the communities interested in radio-active ion beams and in neutrinos. It is clear that such an accelerator could also be beneficial to the whole CERN accelerator complex, renovating the low energy injectors (Linac 2 and PSB), and increasing drastically their performance.

Rapid cycling synchrotrons delivering MMW proton beams at higher energies deserve also to be considered as alternative or complement to the SPL option.  The CERN management would like the help of all communities concerned to refine the specifications of the future CERN proton complex to optimize its interest and potential. Additional extensions should be considered. Comparison with alternative solutions is encouraged. Needless to say, resources are limited, and convergence among  requests has to be fostered. A clear outlook of the future must be the task of the workshop.

2         The High Intensity Frontier

(A. Blondel, J. Ellis)

The last few decades in particle physics have seen the extraordinary success of the Standard Model, which explains most observed phenomena in terms of gauge theories. In particular, experiments at the CERN PS and SPS, followed by the  and LEP Colliders, discovered and studied precisely the Neutral Currents and the W± and Z bosons. This established the validity of the Standard Model, the quantum theory of particles and their strong and electroweak interactions, to distance scales of ~ 10-18m.

Despite these recent successes, a number of extremely fundamental physics questions remain unanswered. Extensions to the Standard Model are almost certainly required. Among the most important issues, one can mention:

-   what is the origin of the widely different masses of elementary particles, and more precisely what is the process that breaks electroweak symmetry;

-   what are the origins of the different fundamental forces, and can a unified description of the forces be made;

-   is the proton stable;

-   what is the origin of matter-antimatter asymmetry in the universe;

-   what is the composition of dark matter, and the nature of dark energy in the universe;

 -  what is the dimensionality of space-time and the role of gravity?

Laboratory exploration in particle physics has traditionally progressed along different and truly complementary lines. The first one is the high-energy frontier, which has been embodied at CERN by the SS, LEP and will soon advance further with the LHC. There is no doubt that LHC will open a domain of yet unexplored energies and answer a number of these questions, in particular in the domain of the electroweak symmetry breaking, addressing the burning issues of the existence and nature of the Higgs boson and supersymmetric particles.

Many of the other questions require different means of investigations with very detailed studies of the properties of already known particles. This requires well controlled experimental conditions as well as high statistics. In the last few years, the requirements for neutrino physics have led to the study of high intensity neutrino sources, such as the recently published ‘CERN/ECFA studies of a European Neutrino Factory Complex’ [ECFAreport]. Beyond neutrino oscillation physics, this report emphasized the wide interest of a high-intensity complex based on a multi-Megawatt proton accelerator, for studies of rare decays and precise properties of muons and kaons, deep-inelastic scattering with neutrinos, as well as the longer-term possibility of muon colliders. At the same time, the nuclear-physics community, a long-time user of CERN with ISOLDE, has been stressing the virtues of a high- intensity upgrade of the existing facility, with the EURISOL study [Eurisole]. The convergence of interest between the particle-physics and nuclear-physics communities  is one of the striking aspects of this high-intensity frontier.  Without going into details which will appear in the following chapters, a foretaste of the physics issues is presented in the following.

The leading possibility for a high-intensity proton accelerator at CERN is the Superconducting Proton Linac (SPL). As shown in Figure 1, it could fit nicely adjacent to the CERN site and feed into the existing CERN infrastructure.

 

Figure 1 Possible layout of the Superconducting Proton Linac on the CERN site.

As discussed in chapter 4 , (see, in particular , Table 2) a survey [HIP_web] has shown that the high-intensity requirements from CERN users cover the CNGS and fixed-target programme, the LHC upgrade, and on a more ambitious scale, the nuclear-physics community,  and the high intensity particle physics programme in neutrino, muon and kaon physics in particular. 

 

Clearly, ensuring that the LHC can reach the highest possible luminosity will be of tremendous importance, especially if this can be achieved in a way which is convenient to the experiments (avoiding excessive pile-up of events) while minimizing the integrated radiation dose in the vicinity of the experiments. Although it is not entirely clear at the moment how this can be best achieved, the flexibility offered by a high brilliance proton source with flexible time structure will in all likelihood be very valuable.

2.1      Nuclear physics

As shown in Figure 2, the SPL protons could be served to an important complex of Radioactive Ion Beams (RIB), EURISOL, which is described in detail in section 5.3. The Isotope Separation On-Line (ISOL) technique to produce radioactive beams has clear complementary aspects to the In Flight Fragmentation (IFF) method which will be used at the project FAIR recently approved in GSI (see section 3.3). First-generation ISOL-based facilities have produced their first results and have been convincingly shown to work. EURISOL aims at increasing the variety of radioactive beams and their intensities by orders of magnitude over the ones available at present. As shown in Figure 3, this will offer rich opportunities for various scientific disciplines including fundamental nuclear physics, nuclear astrophysics and the study of fundamental symmetries, as well as a number of other applications (radioactive spies, curing chemical blindness, positron annihilation studies, applications to biomedicine, etc).

 

Figure 2 Possible location of the EURISOL complex at CERN.

Figure 3 Several aspects of physics with Radioactive Nuclear Beams.

2.1.1       Fundamental nuclear physics.

Although nuclei can be seen as composed of few well known elements bound by a well-known force, the resulting complexity is such that many fundamental questions remain unanswered. In particular it is not clear what are the boundaries of the domain in which nuclei are stable, either at the neutron and proton drip lines, or in the limit of  super-heavy elements. Establishing the limit of nuclear existence involves rare reactions and thus will require the very high intensities that  EURISOL would allow.

 

Among other fundamental questions, one can cite the role played by nucleon pairing (nucleonic cooper pairs), the appearance and behavior of neutron haloes and skins in neutron-rich nuclei, and more generally the modification of shell closures and magic numbers in the vicinity of the drip-lines (see sections 8.1 and 8.2). 

 

  

Figure 4: illustration of some fundamental questions in Nuclear Physics; on the left, the formation of bosons in the nucleus (‘nucleonic Cooper pairs’) is an important ingredient to understand why some nuclei are stable and others not; on the right, the neutron rich nuclei have a ‘skin’ of neutrons which display shape oscillations in various modes.

2.1.2       Nuclear Physics and Astrophysics

Many phenomena in the universe, such as novae and supernovae, X-ray busts, black hole formation or primordial nucleo-synthesis, represent nuclear experiments on a grand scale, often under conditions that – at least for the time being – cannot be replicated on earth. Nuclear physics is an essential provider of the experimental and theoretical data which are needed to model these phenomena. Figure 5 illustrates this point and describes the path through largely unknown nuclear regions that the nuclear reactions follow to produce the heavier nuclei.  The most important parameters needed to be able to improve these calculations are nuclear masses (including n- and p-separation energies) near the proton drip-lines, b-decay half-lives along the process paths and n- and p-capture rates on ‘short-lived’ nuclei. As discussed in chapter  8.3,  many of these data are very poor or completely missing at present and the whole field would benefit considerably from systematic data and understanding of these processes. 

 
 

 

 

 

Figure 5 Novae and Supernovae constitute nuclear reactions at a grand scale. The explanation of the energy production in explosive thermonuclear burning probably requires two distinct neutron-capture processes as the origin of heavy nuclei beyond iron.

2.1.3       Other applications

As will be discussed in section 8.4, the availability of intense beams of various nuclei, but also pions and muons should allow very precise search for violations of fundamental symmetries. Among these one could cite the search for neutron and muon electric dipole moment (EDM), search for T violation in beta decay (using reconstruction of final-state momenta and spin polarisation). Also the search and study of rare or forbidden nuclear decays should be of great interest to improve the understanding of nuclear matrix elements, possibly shining some light on those necessary for the interpretation of neutrino-less double beta decay.

The combined availability of intense of radioactive ion beams with intense muon or antiproton sources opens the very interesting possibility of probing nuclear matter with muonic and anti-protonic atoms, in which the nuclei is already rare or unstable. This technique is both a probe of the structure of these nuclei, as well as a means to produce nuclei one or several steps closer to the drip lines (section 8.5).

Finally one cannot close this discussion on nuclear physics without mentioning the industrial applications such as radioactive spies, curing chemical blindness, positron annihilation studies, applications to biomedicine, and material science that benefits from intense beams of radioactive ions, neutrons and muons.

2.2      Particle physics

The leading particle-physics motivation for a high-intensity proton driver is the study of neutrino oscillations, but the facility would also offer great opportunity for the neutrino, muon and kaon physics. Last but not least, a Neutrino Factory would be the first step towards muon colliders.

2.2.1       Neutrino oscillations

The observation of neutrino oscillations has now established beyond doubt that neutrinos have mass and mix. This existence of neutrino masses is in fact the first solid experimental fact requiring physics beyond the Standard Model.  It was soon realized that with three families and for a favourable set of parameters, it would be possible to observe experimentally violation of CP or T symmetries in the neutrino oscillation phenomenon [Ruj99]. This observation reinforced the already considerable interest for precision measurements of neutrino oscillation parameters. We know since 2003 and the results from SNO and KAMLAND [SNO02], [Kamland02] that the neutrino parameters belong to the so-called LMA solution which suggests that Leptonic CP violation should be large enough to be observed in high-energy neutrino oscillation appearance experiments.  

Figure 6 Graphical representation of the present knowledge of the neutrino mixing matrix. The rotations by the successive Euler angles q12, q13, q23  that transform the mass eigenstates n1 n2 n3  into the flavour eigenstates ne  nm  nt  are shown on the left, while the mass splittings are shown on the right. The best present values are q12 =320, q23 = 45 0, q13 < 130, and  Dm212 = + 8 10-5 eV2,  Dm223 = ± 2.5 eV2. In addition there is a phase, d, which generates CP and T violating effects in neutrino oscillations.  

Figure 7 Present determination of the neutrino mixing parameters. Left:  q23 and Dm223 from atmospheric neutrinos and the K2K experiment; right:  q12 and Dm212  from the solar neutrinos and the KAMLAND experiment [neutrinofits].

 

The fact that neutrinos have masses that are so much smaller than those of the other known fermions (12 orders of magnitude smaller than the electroweak mass scale or the top quark mass) is a mystery that could open the way to the solution of a number of fundamental questions. One can find a more or less natural explanation in the so called ‘See-Saw’ mechanism [seesaw]. In this scenario, the neutrinos are very light because the observed light neutrinos are low-lying states of split doublets with heavy neutrinos of a mass scale M which is interestingly similar to the grand unification scale:

mn M =  v2        with  v » mtop » 174  GeV  Þ       M £ O (1015 ) GeV

The combination of this scenario with the fact that neutrinos could violate CP symmetry opens the fascinating possibility to explain is simple terms the matter-antimatter asymmetry of the universe [leptogenesis], if for instance the decays of heavy Majorana neutrinos violate CP violation, in a way similar to what happens in K0L decays:

N ® +X ¹ N ® -X,

with the resulting lepton-antilepton asymmetry propagating into a baryon-antibaryon asymmetry.

Clearly the study of neutrino properties and in particular the search for leptonic CP violation is a subject of the highest scientific priority.

The tools of choice to search for leptonic CP violation are the oscillations , and at a later stage . The search for an asymmetry between neutrino and anti-neutrino oscillations requires dedicated experiments with high statistics and well-defined flux.  Solar neutrinos and  reactor neutrinos are of too low energy to allow appearance experiments, and atmospheric neutrinos being an intrinsic mix of neutrinos and anti-neutrinos are inadequate. Invariably, the requirement of high flux accelerator-generated neutrino beams leads to the need for a high-intensity proton source. 

Several possibilities exist for neutrino oscillation experiments. First the CERN to Gran Sasso beam, which is optimized for the search for the appearance, would certainly benefit from an increased availability of protons. Then, using more directly the SPL, have been envisaged the following possibilities: the low energy superbeam [Gomez01], beta-beams [Zucchelli], and the Neutrino Factory [Geer98], as sketched on  Figure 8.

 

Figure 8 Three possible neutrino facilities based on the same proton driver. Top: a low energy conventional pion decay beam; middle the beta beam where 6He and 18Ne RIBs are accelerated using the PS and SPS as accelerators; bottom, the neutrino factory where muons are accelerated and stored at 20-50 GeV. 

 

 

The superbeam and beta beam have the advantage of having similar energies which allows usage of the same far detector that could be located for instance in a new international laboratory in the Fréjus tunnel (see sections 6.2, 6.3 and 0). A comparison of performances of these options is made in Figure 9.  All three proposals are superior to existing or planned facilities in the world, but the Neutrino Factory is clearly a much more powerful device. It must use a different detector, however, and may be more difficult and expensive than the others.  As pointed out in chapter  6.1.3, using a high energy beta-beam could partly compensate the performance deficit of the low-energy one, but the technical and financial implications have not yet been established.

Figure 9 relative sensitivity to q13 (top) and to the CP violating phase d (bottom), of various options of Figure 8 for future neutrino facilities based on a high intensity proton driver.

2.2.2       High-precision neutrino scattering

As discussed in  [Mangano01], [Bigi01], the neutrino beams at the end of the straight section of a neutrino factory offer an improvement in flux by several orders of magnitude over conventional beams, allowing several times 108 events to be collected per kilogram and per year. (Figure 10). This could allow a new generation of neutrino experiments, where very detailed studies of nucleon structure, nuclear effects, spin structure functions and final state exclusive processes can be made. Also precision tests of the Standard Model could be carried out in neutrino scattering on nucleon or electron target, as well as a precise determination of neutrino cross-sections and flux monitoring with permil accuracy.  

Figure 10 Event rates at the exit of the straight sections in a Neutrino Factory. Note the scale.

As discussed in chapter 7.1, in the case of a superbeam or beta beam, the study of low-energy neutrino interactions in a near detector is mandatory for the sake of the oscillations analysis. In addition, the understanding of the low-energy transition from elastic reactions to the resonance region and the deep inelastic region is in great need of accurate data. 

2.2.3       Muon physics

A high-intensity proton source could certainly produce many low-energy muons and thus, provided the beam and experiments can be designed to do so, provide opportunities to explore rare decays, such as  , , or the muon conversion , which are lepton-number-violating processes. While the See-Saw mechanism provides a very appealing explanation of neutrino masses and mixings, its inclusion in supersymmetric models almost invariably leads to predictions for these processes that are in excess or close to the present limits.

 

Figure 11 Supersymmetric See-Saw mechanism (left) leads to predictions (right) for the branching ratio that are close to or even beyond the present limits of ~10-11 .

It is therefore quite possible that one of these processes will be discovered in the upcoming generation of experiments (MEG at PSI, MECO at BNL) in which case a detailed study would become mandatory. If not, further search with higher sensitivity would be in demand.  It should be emphasized that the three processes are actually sensitive to different parameters of these models, and thus complementary from both the experimental and theoretical points of view. 

Another fundamental search would clearly be the search for a muon electric dipole moment (EDM), which would require modulation of a transverse electric field for muons situated already at the magic velocity where the magnetic precession and the anomalous (g-2) precession mutually cancel.

Figure 12 Principle of the measurement of the muon EDM

 

2.2.4       Kaon physics

Kaons have played a fundamental role in the discovery of both parity violation and CP violation. Despite the great advance due to B factories, there is a growing interest in challenging channels such as , and , which is a CP-violating process. Discovery or precision study of these processes constitute further test of our understanding of CP violation in the quark sector.

In addition the search for rare processes such as is a sensitive probe of new physics.

 

Figure 13 Left, the dependence of the rare decays  , and  upon the parameters of the unitarity triangle; right, sensitivity of to the Susy mass scale m0 .

2.2.5       Muon Colliders

Finally, although it was not discussed a the workshop, it is worth keeping in mind that the Neutrino Factory is the first step towards muon colliders. As shown in [muoncollider], the relevant characteristics of muons are that, compared to electrons,  i) they hardly undergo synchrotron radiation or beamstrahlung, ii) their coupling to the Higgs bosons is multiplied by the ratio (mm/me)2  , thus allowing s-channel production with a useful rate.

These remarkable properties make muon colliders superb tools for the study of Higgs resonances, especially if, as predicted in supersymmetry, there exist a pair H,A of opposite CP quantum numbers which are nearly degenerate in mass. The study of this system is extremely difficult with any other machine and a unique investigation of the possible CP violation in the Higgs system would become possible.

2.3      Conclusions

A proton accelerator of multi-Megawatt  power would offer a very strong physics case with opportunities for a large variety of cutting-edge experiments. It is synergetic with the CERN middle-term programme providing an upgrade of the LHC luminosity as well as the intensity of the fixed-target programme. It allows one to envisage a long-range neutrino-physics programme, with superbeam, beta-beam and/or Neutrino Factory, and a complementary high- intensity muon physics programme, possibly leading to muon colliders. In addition, it is the backbone of the next generation facility for nuclear physics with important consequences for astrophysics and precision tests of the Standard Model. It certainly constitutes an interesting project – and CERN would be a good place for it!

 

 

3         Planned High Intensity facilities in the world

3.1      The Japanese Proton Accelerator Reseach Center J-PARC

(S. Nagamya, V. Palladino)

The physics program and the experience of this facility was presented by its DG, Prof. S. Nagamiya. As the highest power proton facility presently approved and being built, JPARC [Jparc] represents the natural reference and benchmark for any future high power project. So does the multi-disciplinarity and variety of its program that proved indispensable, and so far unique, to make it possible.

After the groundbreaking ceremony of June 2002, the centre is in advanced state of construction at the JAERI site in Tokai, on the Pacific coast, about 2 hours NE of Tokyo. Once completed, the complex (Figure 14) will comprise a 350 m long Linac, a 1 MW 3 GeV Synchrotron at 25 Hz, and a 0.75 MW 50 GeV Synchrotron. It will be a multipurpose installation serving four different facilities: Nuclear Transmutation, Materials and Life Science, Nuclear & Particle Physics and Neutrino to Superkamioka [Fukuda98].

Figure 14 Artistic aerial view of the Japanese Proton Accelerator Reseach Complex J-PARC

The Nuclear Transmutation facility will operate at 0.6 GeV. The proton beam at 3 Gev will drive a source of separated radioactive nuclei source and the high intensity pulsed Japanese spallation neutron source (JSNS) whose 23 neutron beam lines will be exploited by the Materials and Life Science facility (magnetism, fractals, polymers, structural biolgy etc). The beam 50 GeV will produce 1) high intensity muon beams from pion decay, for fundamental and applied muon science 2) hadronic beams (pions, kaons, antiprotons etc) to study mesons in nuclear matter, rare kaon decays, hyper-nuclei, antimatter 3) a conventional  neutrino (super)beam for the study of neutrino oscillations.

 

The approved neutrino oscillation program [T2K] is based on a conventional nm superbeam derived from the 50 GeV synchrotron. It has become known as the T2K (Tokai to Kamioka) project (Figure 15), successor of the K2K (KEK to Kamioka) experiment [K2K] that has confirmed, with man made laboratory neutrinos, the phenomena of nm disappearance first observed in the Kamioka detectors with atmospheric neutrinos. The T2K neutrino flux will be about 100 times larger that K2K’s. nm  disappearance, presumably dominated by the transition into nt , will be studied with superior precision. The compelling search of the subdominant transition into ne , that promises the answers to the remaining and most intriguing questions concerning neutrino mixing, will be possible with unprecedented sensitivity.

 

Figure 15 Layout of theT2K experiment

If transition nm, to ne are discovered in the first phase of T2K and if it will prove possible to run the 50 GeV accelerator at significantly higher power, the project may have a second phase with a 50 times larger Hyper-Kamiokande detector, 1 Mton or so, aiming at discovery of leptonic CP violation.

 

Construction of all elements of the complex has started. In spite of different causes of delay, among them the lucky discovery on site of invaluable archeological findings from the Japanese middle ages, accelerator beams are expected to be operating by the end of 2007 and the experimental facilities will be coming into operations early in 2009, at the latest, when the neutrino superbeam will also start operation. The total capital cost will be 151 GYens, grossly equivalent to 1.5 G$, for the first seven years phase. This is still mostly still to be spent, in the years 2004-2009; about 80% of purchase commitments are done, however. The cost will rise to 189 GYens, when including its phase II after 2009.

 

The origin of the JPARC is rooted in the 1998 negotiations and in 1999 MOU between the Japanese applied nuclear physics (JAERI) and particle physics (KEK) agencies. Strongly encouraged in the following years, under the auspices of the newly established fundamental & applied science & technology joint MEXT ministry, the project obtained the construction budget for its first phase in 2001. Notably, however, ideas for a joint large hadron facility project have to be traced, at least back to 1985!

 

A first call in 2002 resulted in 30 LOI [jparclois], signed by 478 physicist, about 2/3 non- Japanese. The call for proposal is expected in 2004 An International Advisory Committee (IAC) has long been in place.

 

On October 30, 2003,the LINAC had a successful acceleration of 6 mA at 20 MeV. On November 7, 30 mA was achieved. Among the recent realizations are the vacuum pipe for the 3 GeV beam and the first RF cavities, dipole and quadrupole magnets for the 50 GeV beam. Impressive progress in construction was manifest from pictures taken at the annual meeting of the IAC in March 2004.

 

Due to financial difficulties, the energy of the front end LINAC will be initially limited to 200 MeV, but work towards its upgrade to the original 400 MeV will start immediately after. This will result in a delay in the achievement of the target intensity (1 MW at 3 GeV and 0.75 MW at 50 GeV) of the first phase. Achieving higher intensities, up to 4 MW, represents one of the major  challenges and unknowns for the successive phases of the facility.

 

 

Figure 16 The J-Nufact scheme for a Neutrino Factory, based on a chain of FFAG accelerators

The number of facilities is also expected to evolve. It is worth mentioning, because relevant to the neutrino sector, that preliminary plans have been outlined, following an independent and original Japanese scheme [Jnufact], to use the high intensity muon area as the front end of a neutrino factory (Figure 16).

 

3.2      U.S. Plans for High Power Proton Drivers

(S. Holmes)

3.2.1       Motivations

A “proton driver” is defined as an accelerator capable of delivering beam power in excess of 1 MW onto a target, derived from a proton beam of at least 1 GeV. A broad range of motivations for high intensity proton drivers exist as described by Ellis [Ellis04]. Within the United States both Fermi National Accelerator Laboratory (Fermilab) and Brookhaven National Laboratory (BNL) are developing concepts for proton drivers with 1-2 MW of beam power [Weng03],[Fermi02]. For Fermilab and BNL, with traditions in high energy and nuclear physics, the motivations center around neutrino physics, rare kaon and muon decays, pulsed neutrons, and/or ultimate utilization as an injector into a hadron collider beyond the Large Hadron Collider at CERN.

3.2.2       U.S. Design Concepts

The Fermilab and BNL concepts for 1-2 MW proton drivers have several elements in common:

1.       An increase in the repetition rate of an existing accelerator, the Main Injector in the case of Fermilab, and the AGS in the case of Brookhaven.

2.       An increase in the injected beam intensity through construction of a new linac (or possibly a synchrotron).

3.       A decrease in the filling time of the existing machine. (This is most straightforwardly accomplished by utilizing a linac rather than a synchrotron.)

4.       Reliance on previously developed superconducting rf technologies.

5.      Upgrade paths are identified that could yield additional factors of 2 to 4.

 

The BNL concept features a 1.2 GeV superconducting linac as the injector into the upgraded AGS. This configuration would allow the delivery of 1 MW of beam power from the AGS at 28 GeV. Fermilab has two implementations under evaluation, each with the capability of providing high power beams simultaneously at 120 GeV, from the Main Injector, and at 8 GeV. The two possibilities are an 8 GeV synchrotron or an 8 GeV superconducting linac, with the linac preferred.

Performance goals and underlying parameters for the various concepts are shown in Table 1. The columns labelled “Present” represent typical performance with the currently configured accelerator complex.


 

Table 1 Performance parameters for Fermilab and BNL concepts for Proton Driver upgrades.

 

 

Fermilab Options

 

Brookhaven

 

 

 

 

Present

Synchrotron

Linac

 

Present

AGS Upgrade

 

 

 

 

 

 

 

 

 

 

 

 

Linac

 

 

 

 

 

 

 

 

 

Kinetic Energy

 

400

600

8000

 

200

1200

 

MeV

Peak Current

 

40

50

25

 

40

30

 

mA

Pulse Length

 

90

90

1000

 

60

720

 

msec

Protons/pulse

 

2.3E+13

2.8E+13

1.6E+14

 

1.5E+13

1.0E+14

 

 

Repetition Rate

 

15

15

10

 

15

2.5

 

Hz

Average Beam Power

 

0.02

0.04

2.00

 

0.007

0.05

 

MW

 

 

 

 

 

 

 

 

 

 

Booster

 

 

 

 

 

 

 

 

 

Kinetic Energy (Out)

 

8

8

 

 

1.5

 

 

GeV

Protons per Pulse

 

5.0E+12

2.5E+13

 

 

1.5E+13

 

 

 

Repetition Rate

 

7.5

15

 

 

6.7

 

 

Hz

Protons/hour

 

1.4E+17

1.4E+18

 

 

3.6E+17

 

 

 

Average Beam Power

 

0.05

0.5

 

 

0.02

 

 

MW

 

 

 

 

 

 

 

 

 

 

Main Injector

 

 

 

 

AGS

 

 

 

 

Kinetic Energy (Out)

 

120

120

120

 

24

28

 

GeV

Protons per Pulse

 

3.0E+13

1.5E+14

1.6E+14

 

6.0E+13

9.0E+13

 

 

Repetition Rate

 

0.54

0.65

0.67

 

0.33

2.50

 

Hz

Protons/hour

 

5.8E+16

3.5E+17

3.8E+17

 

7.2E+16

8.1E+17

 

 

Average Beam Power

 

0.3

1.9

2.0

 

0.1

1.0

 

MW

 

 

3.2.3       The Brookhaven AGS Upgrade

The Brookhaven AGS upgrade is based on the direct injection of approximately 1´1014 protons via a 1.0 GeV superconducting extension to the existing 200 MeV drift tube linac. The extension bypasses the existing 1.5 GeV Booster currently used for injection into the AGS. The injector upgrade is accompanied by modifications to the AGS rf and power supply systems to enable 2.5 Hz operations. The net result is a factor of ten increase, to 1.0 MW, in the beam power available at 28 GeV. Subsequent upgrades of either the linac intensity, to 2´1014, and/or the repetition rate, to 5 Hz, would enable additional factors of 2-4 in delivered beam power. A schematic view of the AGS upgrade is given in Figure 17.

Figure 17 Schematic view of the AGS upgrade concept. Existing accelerators are in red, new facilities are in blue

3.2.4       The Fermilab Proton Driver

The goal of the Fermilab Proton is to increase the intensity delivered to the Main Injector to approximately 1.5´1014 protons, allowing full exploitation of the large aperture of the Main Injector and enabling, following relatively modest upgrades to the MI, 2 MW of beam power delivered at 120 GeV. Inspired by the outstanding successes of the TESLA program [Tesla01] the idea has emerged of an 8 GeV superconducting proton linac, injecting H- directly into the Main Injector. The linac concept is represented schematically in Figure 18. The concept utilizes technologies currently under development for both the RIA and TESLA proposals. A number of variations on this basic scheme are still being explored including:  2.0 MW beam power at 8 GeV, frequencies based on 1207.5 MHz and derivatives thereof, and a warm front end. The key element in the design is the utilization of a “TESLA style” rf distribution system in which 36 cavities are driven from each klystron. Realization of this design configuration is dependent on the development of a ferrite based phase/amplitude controller. In addition, the utilization of such controllers would allow the acceleration of electrons interleaved with proton cycles in the downstream 7 GeV of the linac.

 

Figure 18 Schematic layout of the superconducting linac based Proton Driver at Fermilab. Linac variants under consideration include higher beam power (2 MW), lower rf frequencies (1207 MHz and its derivatives), and a warm front end.

3.2.5       Summary

Design concepts for Proton Drivers operating in the range 1-2 MW are being developed by both BNL and Fermilab. Both laboratories are motivated by a variety of physics opportunities, with neutrinos in the forefront. Active R&D programs are underway on critical components, aimed at demonstrating technical and cost performance. The recently released Fermilab Long Range Plan [Lrplan04] identifies a 2 MW Proton Driver as the preferred option in the event a linear collider is constructed elsewhere or is delayed. To that end, Fermilab is currently preparing documentation sufficient to support a “statement of mission need”, know as Critical Decision 0 within the U.S. Department of Energy project management system. Brookhaven is preparing a design study that could serve as the basis for a subsequent proposal.


 

3.3      Fair, the GSI New Facility For Antiproton and ion research

(H.-J. Kluge)

The physics program at the future international facility FAIR at Darmstadt (Facility for Antiproton and Ion Research) addresses a broad research spectrum: In hadron and nuclear physics, it ranges from studies of the sub-nuclear degrees of freedom, of the origin of the nuclear force and the quark-gluon structure of extended nuclear matter, to the exploration of the structure of nuclei, the nuclear many-body system, far from stability. Furthermore, other fields of physics will be exploited such as quantum electrodynamics in extreme electromagnetic fields, atomic physics and fundamental tests by use of antiprotons, the physics of dense plasmas or materials science and biophysics.

The tools for the FAIR research program are intense primary and secondary ion beams, including beams of antiprotons. These beams are generated in the FAIR facility (Figure 19) which makes use of the existing Unilac-SIS18 accelerator as injector. For antiproton production, a linear proton injector and accelerator will be added. The double-ring synchrotrons SIS100/300 will provide a major step in primary and, thus, in secondary ion beam intensities and, for certain demands, also in beam energy. Since the present synchrotron SIS18 is already at its space charge limit of about 1010 ions per second with beams of highly stripped uranium U73+, two steps are foreseen to increase the intensity by two orders of magnitude: a faster cycling of the injector (from 0.3 Hz to 3 Hz) and the use of a lower charge states as, for example, q = 28+ for the case of uranium.

Figure 19 Schematic layout for the Facility for Antiproton and Ion Research, FAIR, at GSI. Details are given in the CDR [CDRFair].

To achieve 1.5 to 2.0 GeV per nucleon for the secondary radioactive beams, an accelerator ring with correspondingly higher magnetic rigidity (≈ 100 Tm) is required. The SIS100 synchrotron will also provide 30 GeV protons, the optimum energy for antiproton production. The second ring, SIS300, serves as a stretcher for slow extraction with high duty cycle and for high charge-state heavy ions and provides energies up to 35 GeV per nucleon at somewhat lower intensities for nucleus-nucleus collisions.

The science case has been worked out in detail in the Conceptual Design Report (CDR) for the facility [CDRFair]. Within the context of this workshop, discussing a high-power proton driver, there is an overlap of FAIR with such a MEGAWATT Facility in the case of research programs with beams of short-lived nuclei. Therefore, the following discussion will be focused on the production and on experiments with radioactive beams (RIB).

Figure 20 presents the layout of the facility for RIB production, separation, and experiments with such beams. The central instrument is the large-acceptance high-resolution spectrometer for exotic nuclei, a two-stage super-conducting fragment separator, called Super-FRS, plus three areas for experiments with stopped (or slowed-down), with fast, and with stored and cooled beams. A key requirement for obtaining uncontaminated secondary beams is the high energy of the primary heavy-ion beam.

Figure 21 shows existing or planned facilities for in-flight production and separation of RIB and indicates at what energy per nucleon the secondary products are fully stripped from electrons that leads to a clean m/q = m/Z separation. The benefit of primary heavy-ion beams well above 1 GeV/nucleon is evident. Important features of in-flight production of RIB are that the radionuclides are available for experiments irrespective of their chemical properties, and that the half-lives of the accessible radionuclides are only limited by their time of flight from the production target to the detector. For relativistic RIB, this time span is of the order of microseconds.

Figure 20 The Superconducting Fragment Separator (Super-FRS) at the
                FAIR facility.

Figure 21 In-flight radioactive beam facilities and yields of fully stripped ions as a function of energy.

 

An example for production and identification of RIB is shown in Figure 22. In this case, a 1 GeV/u uranium beam impinged on a hydrogen target of the present fragment separator FRS at GSI. Over 1000 reaction products could be separated and identified. This fast and universal production mechanism is complementary to the isotope separator on-line (ISOL) approach which has the advantage of higher production yields, as compared to the in-flight technique, for longer-lived radio-nuclides of volatile elements which are released quickly from the target matrix.

 

 

Figure 22 Production of secondary beams by 1 GeV/u uranium ions impinging on a hydrogen target. Over 1000 products were identified by the fragment separator FRS at GSI.

 A second key feature of FAIR is cooling and storage. This technology is going to play an important role at the future facility, for radioactive ion beams as well as for high-energy anti-protons. Beam storage and cooling for high-energy heavy-ion beams has evolved at the present GSI facility as a technology with novel applications and research opportunities [STORI02].

Presently, nuclear physics experiments by use of the Experimental Storage Ring, ESR, at GSI concern mainly mass spectrometry and lifetime measurements. In the future, reaction experiments in inverse kinematics will play a major role. Here, stored and cooled highly-charged radioactive ions react with the atoms of the gas jet target installed in the storage ring. In this context, it should be noted that acceleration by synchrotrons and, in this way, production of pulsed beams of radionuclides are perfectly tailored for highly efficient injection into storage rings. Storing and cooling of radionuclides are effective means to increase the luminosity for reaction experiments by many orders of magnitude since one gains by the factor of the revolution frequency (typically 1 MHz) and the extremely small diameter of the cooled circulating ion beams.

Therefore, also nuclei very far from stability, which are produced with very low yields at FAIR, will become accessible to experimental investigation. The yields expected at the FAIR facility are shown graphically in the chart of nuclei of Figure 23. Here, the known isotopes are enclosed by a black line. The red lines indicate possible routes of the r-process. High yields of up to 106 will be available for more detailed studies of presently known nuclei over nearly the whole chart of nuclei. It is evident that highly efficient detection techniques are required to expand our knowledge to shorter-lived nuclei farther away from stability.

Figure 23: Expected yields at the Super Fragment Separator at FAIR.

 

 

 

 

 

 

4         High Intensity Proton Source

4.1      The future of proton accelerators at CERN

(R. Garoby, W. Scandale)

The Large Hadron Collider will be filled through a set of high performance proton accelerators providing the high brightness beam needed to reach the foreseen luminosity. Although this difficult project has top priority and uses most of the CERN resources, it is nevertheless time investigating improvements of the proton accelerator complex for physical cases beyond the LHC expectations. The needs of multiple physics communities have to be taken into account, as well as the necessity of consolidating the installations while keeping high reliability. This paper starts from the analysis and proposals made by the “High Intensity Proton” (HIP) working group [Hip_web], [Benedikt04] to improve the performances of the PS and the SPS complex and better match the users requests in a staged scenario at short and medium term, and complement it, addressing the main possibilities beyond that horizon.

4.1.1       Outcome of the High Intensity Protons Working Group

The HIP working group, mandated by the direction of the AB department, has recently established a list of requests from the physics teams already working at CERN and recommended a path for the upgrade of the proton accelerators [Hip_web], [Benedikt04]. The needs of LHC, COMPASS, neutrino and radio-active ion beam physics have been taken into account. For the other present users, i.e. AD, PS East area and nToF, the assumption has been that their requirements do not significantly influence the choice, and that every upgrade scenario would be compatible. In terms of schedule and resources, the requests fall into three main categories: (i) the short term, “low” (ideally zero) cost demands, which match the present commitments of CERN and belong to the approved physics programme, (ii) the medium term, “medium” cost requests, which correspond to modest and progressive increases of performance for the present experiments, (iii) the long term, “high” cost  wishes, which are linked to major equipment upgrades and to new experiments suggested for integration inside the future physics programme of CERN. These are summarised in Table 2.

Table 2 Main users’ requests

User

CERN commitment*

Users’ wishes

Short term

Medium term [~asap!]

Long term [> 2014]

LHC

Nominal luminosity

1034 cm-2s-1

Ultimate luminosity

2.6´1034 cm-2s-1

Luminosity upgrade

(tenfold)

SFT (COMPASS)

4.3´105 spills/year?

7.2´105 spills/year

 

CNGS

4.5´1019 p/year

Upgrade ~ ´ 2

 

ISOLDE

1.92 μA**

Upgrade ~ ´ 5

 

Future n beams

 

 

> 2 GeV, 4 MW

EURISOL

 

 

> 1-2 GeV, 5 MW

* Reference value for analysis.     ** 1350 pulses/hour – 3.21013 protons per pulse (ppp).

The Linac2, PSB, PS and SPS have been built more than 35 years ago. Although a significant fraction of their equipment has been renovated, the most expensive ones are the oldest and show weaknesses. This has been aggravated by the reduction or even by the suppression of preventive maintenance due to the lack of resources during the past years. Therefore consolidation is essential in the near future and, in the medium term, the replacement of these accelerators deserves serious consideration. Moreover, the reduction of beam losses is a major issue in order to minimize the material irradiation for improved reliability and, even more important, the dose taken by the personnel during maintenance.

The analysis of the proton flux available to the users starts in 2007, corresponding to the first year of LHC operation, under the following assumptions:

o        Accelerators operating time per year

PS: 5400 h (without setting-up)

SPS/LHC: 4700 h (without setting-up)

SPS in LHC filling mode: 15% (5%) of the time

SPS in LHC pilot mode: 35% (10%) of the time

SPS in CNGS&SFT mode: 50% (85%) of the time

o        Availability

PS & PSB: 90%

SPS : 80%

o        Beam intensities

SPS for CNGS: 4.4´1013 and 7´1013 ppp

PS for CNGS: 3´1013 and 4´1013 ppp.

 

The LHC pilot beam is a “safety beam” to be used to establish circulating beam. The following supercycles have been assumed:

o         LHC filling super-cycle:

1 LHC filling (flat porch for 4 PS injections), nominal length ³ 21.6 s

o         LHC pilot super-cycle:

1 LHC pilot + 2 CNGS, nominal length: 22.8 s

o         CNGS&SFT super-cycle:

3 CNGS + 1 SFT + 1 MD (Machine Development), nominal length: 34.8 s.

 

Without any improvement, the basic requests cannot be met, especially for the ISOLDE and SPS users (Table 3 illustrates a case where priority is given to CNGS, resulting in a low flux for COMPASS). Moreover, the large level of beam loss associated with the CNGS operation in the PS will cause irradiation of PS equipment with detrimental consequences on reliability and maintenance.

 

Table 3 Proton flux in 2007 without improvement to the accelerators  (“pot” stands for protons on target).

 

Available flux

Basic user’s requests

 

CNGS flux

4.4

4.5

1019 pot/year

COMPASS spills

1.9

7.2

105 /year

ISOLDE flux

1.75   [1215]

1.9   [1350]

(μA)   [pulses/hour]

PS East area spills

1.5

1.3

106 /year

nToF flux

1.7

1.5

1019 pot/year

 

Having considered the possible solutions, the following recommendations are made:

·         In the short term, define in 2004 and start in 2005 three projects: (i) a new multi-turn PS extraction, to reduce beam loss and activation, (ii) an increased intensity in SPS for CNGS (implications in all machines), (iii) the reduction from 1.2 s to 0.9 s of the PSB minimum repetition time.

·         In the medium term, design of a new linac (“Linac4” [Garoby04]) for the replacement of Linac2, with the goal of preparing for a decision of construction at the end of 2006.

·         In the long term, prepare for a decision concerning the optimum future accelerators by pursuing the study of a Superconducting Proton Linac [Spl_web], [Vretenar00] and by exploring alternative scenario for the LHC upgrade.

 

The estimated performance resulting from the implementation of the short and medium term measures is shown in Table 4. Numbers without parenthesis can be obtained by giving the highest prioritys to CNGS. Conversely, the numbers in parenthesis are achieved by giving priority to COMPASS and limiting CNGS to its basic request. These improvements, and especially Linac4, should allow reaching the “ultimate” luminosity presently foreseen in the LHC by providing a proton beam with the “ultimate” characteristics.

Table 4 Proton flux in 2010 after implementation of the improvements recommended by the HIP WG.

 

Estimated flux

Basic user’s request

 

CNGS flux

7.5 (4.5)

4.5

1019 pot/year

COMPASS spills

3.3 (5.6)

7.2

105 /year

ISOLDE flux

6.4   [2240]

1.9   [1350]

(μA)   [pulses/hour]

PS East area spills

1.5

1.3

106 /year

nToF flux

1.6

1.5

1019 pot/year

4.1.2       Long term possibilities

Decisions for the long term (beyond 2010) have to take into account (i) the need to replace the aging accelerators, (ii) the plans for upgrading the LHC and (iii) the future physics programmes. Some scenarios have already been proposed for the LHC upgrade, and, for example, the interest of a new 1 TeV injector replacing the SPS has been mentioned [Bruning02] which would probably have to be coupled with a new 50 GeV synchrotron replacing the PS. However, a detailed study is still needed to compare the different possibilities and draw all conclusions.

Concerning the future physics programmes, the HIP working group has already identified a number of possibilities envisaged by nuclear and neutrino physicists that could be satisfied with a multi-MW/ few GeV proton driver like the SPL. Since then, requests for physics with kaons and muons have shown the potential interest of a multi-MW proton source at 30-50 GeV. Although, in principle, all these requirements can be simultaneously satisfied, the needed resources are rather large and the consequences so important for the complex that priorities have to be established, at least to plan for a staged realization.

Assuming that accelerators are replaced not only to improve reliability but also the characteristics of the beam (energy, intensity, brightness…), the replacement of a given machine should be coupled with the change of its injector(s). Therefore the decision process begins at the low energy end and progressively covers all the energy range. The possibilities and the benefits for the different families of users are shown in Table 5. The comments and numbers are indicative and subject to evolution. The various schemes will require further and detailed studies before realistic proposals can be made.

 

Table 5 Possible improvement to the accelerator complex

(“RCS”=Rapid Cycling Synchrotron, “HEP”=High Energy Physics, “mMW”=multi-MW, “SC”=Superconducting)

Present accelerator

Replacement accelerator

Improvement

INTEREST FOR

LHC upgrade

Neutrino physics beyond CNGS

Radio-active ion beams beyond ISOLDE

Physics with kaons and muons

Linac2

Linac4

50 ® 160 MeV

H+ ®  H-

+

0

(if alone)

0

(if alone)

0

(if alone)

PSB

2.2 GeV RCS for HEP

1.4 ® 2.2 GeV

10 ® 250 kW

Brightness ´ 2

+

0

(if alone)

+

0

(if alone)

2.2 GeV/mMW RCS

1.4 ® 2.2 GeV

0.01 ® 4 MW

Brightness ´ 2

+

+++

for super-beam and beta-beam

+

(too short beam pulse)

0

(if alone)

2.2 GeV/50 Hz

SPL

1.4 ® 2.2 GeV

0.01 ® 4 MW

Brightness ´ 2

+

+++

for super-beam and beta-beam

+++

0

(if alone)

PS

50 GeV SC PS for HEP

26 ® 50 GeV

Intensity  ´ 2

Brightness ´ 2

++

0

(if alone)

0

+

50 GeV/5 Hz RCS

26 ® 50 GeV

0.1 ® 4 MW

Brightness ´ 2

++

++

0

+++

SPS

1 TeV SC Synchrotron

0.45 ® 1 TeV

Intensity  ´ 2

Brightness ´ 2

+++

?

0

+++

Linac2

Linac4 is a necessary first step, whatever the choices for the other machines, because it is designed to be compatible with the most demanding applications. For the higher energy accelerators, the choice is more open and depends upon the physics programmes.

PSB

If a second generation ISOL-type facility has to be hosted at CERN, the SPL is the ideal solution, which can also be used for all scenarios of neutrino physics (super-beam, beta-beam and neutrino factory). The SPL would also be an outstanding replacement of the PSB for the following accelerators serving high energy physics experiments. Once the precise goals of such a multi-MW/ few GeV driver are defined, the possibility of an RCS-based solution should be analysed and compared with the SPL.

If no experimental programme is approved that needs such a beam power at a few GeV, the most economical solution is to build a small size RCS able to fill the PS or its successor in 4 or 8 pulses. Most beam pulses would be available for other users, which could be of interest for radio-active ion production, although not at the level required by EURISOL (~ 200 kW of beam power instead of 4 MW) [Eurisol_web].

PS

The low energy of the PS beam presently limits the SPS performance and the situation will be worse if the SPS is replaced with a superconducting synchrotron reaching 1 TeV. The present estimate is that the successor of the PS should deliver beam at approximately 50 GeV. If a multi-MW beam power is needed at that energy, a Rapid Cycling Synchrotron has to be considered. It would be a very challenging machine, surpassing the most ambitious synchrotron presently in construction in Japan [Jparc_web]. Moreover, it would probably lack the flexibility of the present PS which would then have to be maintained in operation for the needs of heavy ions for LHC and slow ejection to the East area.

If multi-MW of beam power at 50 GeV is not needed, a synchrotron using superconducting magnets could replace the PS. The key technological item for such a synchrotron will be a fast pulsing superconducting dipole reaching a field of 4 to 6 Tesla in 2 to 3 seconds. The magnets in development for the needs of SIS100 at GSI would be of interest for that application [Fair]. The new process of multi-turn ejection which minimizes beam loss should be used.

SPS

Injection at 1 TeV in the LHC would drastically ease operation with the present magnets, open some interesting possibilities for upgrading the luminosity beyond the ultimate value, and would even be necessary for an energy upgrade [Bruning02]. Other users could also be interested, provided the new machine is capable of slow ejection. The developments taking place in GSI for the superconducting magnets of SIS300 could be exploited [Fair].

LHC

Extracted proton beams at 7 TeV are a potential field of investigation. The physics case should however be properly analysed.

 

 

 

4.2      The Superconducting Proton Linac (SPL) and its potential

(R. Garoby, W. Scandale)

4.2.1       Description

The SPL is a multi-GeV, multi-MW (typically 2.2 GeV/4 MW) linear proton accelerator. The basic characteristics of the first Conceptual Design [Spl_web], [Vretenar00] are summarized in Table 6. Operating at 50 Hz, it will be used both as a high-performance injector for the PS, replacing the PSB, and as a high-power proton driver for other physics applications, possibly complemented with an accumulator and a compressor rings.

 

Table 6 Main SPL parameters (CDR 1)

Type of ion

H-

 

Kinetic energy

2.2

GeV

Mean current during the pulse

13

mA

Beam duty cycle

14.0

%

Mean beam power

4

MW

Pulse frequency

50

Hz

Pulse duration

2.80

ms

Number of H- per pulse

2.27

´ 1014

Bunch frequency

352.2

MHz

Chopping duty cycle

61.6

%

Successive bunches/No. of buckets

5/8

 

Norm. r.m.s. transverse emittance

0.4

p mm mrad

Longitudinal r.m.s. emittance

0.3

p deg MeV

 

The low energy part, up to 160 MeV kinetic energy, is equipped with room temperature accelerating structures and makes extensive use of LEP RF equipment at 352 MHz. It is called Linac4 because it is proposed to be first employed to replace the present Linac2 [Garoby04]. Its characteristics in both modes of operation are outlined in Table 6.

In the initial design of the SPL, based on the quasi-exclusive use of LEP RF hardware, acceleration beyond 160 MeV took place in a 550 m long superconducting linac section which brought the beam kinetic energy up to 2.2 GeV. The schematic layout of this version of the SPL is presented in Figure 24 A second Conceptual Design is in preparation and will be published in 2005. It will take into account the results from the HARP experiments and the outcome of recent developments in superconducting RF. Instead of the 352 MHz LEP cavities, new bulk Niobium cavities will be used at 704 MHz. For the same beam power and cost, the machine will be more compact or, if a higher energy is necessary, it could fit inside the same footprint.

Table 7 Linac 4 specifications.

 

Phase 1 (PSB)

Phase 2 (SPL)

 

Maximum repetition rate

2

50

Hz

Source current

50

30

mA

RFQ current

40

21

mA

Chopper beam-on factor

75

62

%

Current after chopper

30

13

mA

Pulse length (max.)

0.5

2.8

ms

Average current

15

1820

mA

Max. beam duty cycle

0.1

14

%

Number of particles per pulse

0.9

2.3

´ 1014

Transv. emittance (rms, norm.)

0.28

0.28

p mm mrad

Longitudinal emittance (rms)

0.15

0.15

p deg MeV

 

Figure 24 : SPL schematic layout (CDR 1 design for the superconducting part).

4.2.2       Applications

The SPL is ideal as a proton driver for a second generation ISOL facility (“EURISOL”) [Eurisol_web] and for all scenarios of neutrino physics (super-beam, beta-beam and neutrino factory) [Muons_web]. It would also be a top class replacement for the PSB. For the superbeam option an accumulator ring is needed to shorten the beam pulse from 2.8 ms to 3 ms. For a neutrino factory, a second synchrotron has to be built to reduce bunch length to 1 ns rms. In the proposed layout on the Meyrin site, these rings are placed inside the ISR building and connection with the existing machines re-uses existing tunnels (Figure 25).

Figure 25 : Proton driver complex on the CERN site.

4.2.3       Other benefits

With the SPL replacing the PSB, the LHC will benefit from:

Limited profit is expected in terms of intensity per pulse from the PS and SPS, because these machines will already be close to their ultimate capabilities after the installation of linac4.

Most of the SPL being superconducting, it is conceivable to lengthen the beam pulse and increase the beam power by upgrading the electrical and cooling infrastructure. If necessary more high power users could then be accommodated.

4.2.4       Planning

In a context of renovation of the injector complex, it makes sense to begin with the low accelerators because (i) the low energy machines are the oldest, (ii) beam brighness is defined at low energy and (iii) the following accelerators can be based on a high performance/state of the art injector. The choice of the future machine that will replace the PSB has to be made early.

The realization of the SPL is split in three phases, in increasing order of beam energy, cost and benefits. An indicative planning highlighting the key dates is given in Fig. 3.3.

In the first phase the performance of the pre-injector, up to 3 MeV of kinetic energy, will be investigated. A test stand equipped with an RFQ accelerator has been funded and is presently under preparation, with the goal of operating with beam during the year 2007.

In the second phase, Linac4 will be built to replace Linac2, and increase by a factor of two the intensity and brightness of the PSB beam. Linac4 is a useful first step, whatever the choices for the other machines, because it is designed to be compatible with the most demanding applications. Developments for Linac4 [Hippi_web] are taking place with the support of the European Union and of the International Science and Technology Center (Moscow). CERN management confirmed recently its commitment to decide in 2006, with construction starting in 2007. The setting-up with beam could then take place in 2010 and operation for physics could begin in 2011 with immediate benefits for LHC and ISOLDE.

Figure 26: Indicative multi-year planning for the full SPL project.

 

In the third phase, the full SPL would be built. The decision on its construction will depend upon the future physics programmes at CERN and upon the needs of the LHC upgrade. Considering that finalization and setting-up of the SPL imply interruption of the proton beams for one year, it is logical to plan it during the shutdown for LHC upgrade which is estimated to be in 2014. To match this date, the decision of construction has to be made in 2008.


4.3      The Rapid Cycling Synchrotron options

(C. Prior)

Two criteria have been identified and generally accepted as essential to the definition of the type of machine known as a proton driver:

C1: the need for high beam power, usually within the range 1 to 4MW, possibly achieved via a phased upgrade scheme;

C2: for the neutrino factory application, the ability to produce high intensity short bunches of protons, with a representative time duration of 1–2 ns (rms). For a neutrino superbeam, a pulsed beam with a duty factor of 10-4 is sufficient.

To put these requirements in context, it is useful to consider the list of existing and proposed proton sources shown in Table 8 and see the extent to which they meet the criteria C1 and C2. The most powerful pulsed machines in operation output approximately 0.15MW and those at the construction phase are generally aimed at £ 1MW, with peak power expected to be reached after several years of progressive commissioning and development. Furthermore, not all of these will satisfy the criterion C2 easily. The step to ³4MW should not be under-estimated, and meeting full proton driver specifications will be an extremely challenging task.

Table 8 Parameters of Existing and Proposed Proton Sources

 

4.3.1          ISIS

 

Of the machines listed in Table 1, ISIS at the Rutherford Appleton Laboratory (RAL) is the world’s most powerful source of pulsed neutrons and is approaching almost 20 years of successful operation. The accelerating system comprises a 70MeV H- linac injecting via an Al2O3 stripping foil into an 800MeV proton synchrotron. Each pulse consists of two bunches of approximately 100 ns duration, directed onto a tantalum target at a repetition rate of 50 Hz, where a variety of experiments are carried out for condensed matter research.

Although far below the levels of performance required in a proton driver, several special features have nevertheless been built into the ISIS accelerators and are of importance for next-generation designs. Injection of H- is essential in order to build up the required beam intensity within an acceptable transverse emittance. To keep stripping foil temperatures to reasonable levels, phase space painting is included via vertical orbit beam bumps, and to trap and accelerate as many protons as possible, the total RF voltage in the synchrotron is varied according to a prescribed programme during the cycle.

Dampig of the  head-tail instability can be done by changing the vertical tune during injection, and, indeed, variable tune throughout the cycle is an important feature of ISIS and all high intensity machines. Of more interest is the synchrotron’s resistance to the e-p instability thought to be caused by the so-called electron cloud phenomenon. At high intensities, protons can interact with residual gas to release electrons which are first trapped within the bunch and then released from the tail to hit the vacuum chamber walls. Secondary electrons are released that can in turn interact with successive proton bunches, possibly producing a cascade effect leading to severe beam loss. Such behaviour is believed to limit intensity in the Los Alamos Proton Storage Ring (PSR). The absence of any such problem at ISIS is now thought to be due to RF shields built into the dipoles and quadrupoles, which effectively suppress the secondary emission [Bellodi04] . However, this conclusion is still speculative and an intensive R&D programme is in progress to resolve the issue.

Even without the e-p instability, ISIS still shows a loss of about 10% of the beam, generally below 80 MeV, and the bunches of 100 ns duration, while suitable for neutron production, cannot easily be compressed to the 1 ns levels required for a neutrino factory  . A phased upgrade path to a full proton driver is however under study and is described in §4.3.

4.3.2          SNS and ESS

 

Many of the ISIS features were incorporated in the design for the European Spallation Source (ESS), which in turn provided a template for the US neutron source SNS. The ESS remains a paper study [ESS03]  while the SNS is currently under construction at Oak Ridge, Tennessee. Both have full energy linacs feeding into accumulator rings, producing pulses of the order of 1 µs at their spallation targets. The ESS design also includes a 2 ms long-pulse option. However, both have had to confront the need for a very low loss system, high beam power and high intensity achieved through charge exchange injection and proton accumulation. Elaborate collimation systems are incorporated into the linacs and rings. Special features have been included, such as fast beam choppers in the linacs at energies of 2–3MeV and achromatic arcs in the transfer lines from the linacs to the rings, to facilitate low loss H- injection. Phase space painting during injection is used in both cases, with the ESS also carrying out energy ramping in the linac-ring transfer line and ring RF steering during accumulation. The ESS design also attempts to reduce problems caused by high intensity by splitting the protons between two rings stacked on top of one another.

Neither ESS nor SNS conforms to the strict definition of a proton driver, however. Although both meet the high beam power criterion, bunch compression could only be achieved through the use of an additional ring, where several megavolts of RF would be required.

The enterprising J-PARC project currently under construction at Tokai-mura in Japan, is the first proton driver proper to be built, albeit at the lower end of the beam power range (see Table 1). Based on rapid cycling synchrotrons (RCS), the complex contains a 3 GeV ring producing proton pulses for spallation neutrons and a 50GeV ring in which bunch compression may be carried out. A dedicated neutrino factory is being planned with muons, generated from a pion target by the proton driver, being accelerated in a series of fixed-field, alternating gradient (FFAG) rings before decaying to neutrinos in a dedicated storage ring. Further details and a status report of this project are given in chapter 3.1.

4.3.3       Ideas for RCS-based Proton Drivers

Whether the scenario adopted is a full energy linac with accumulator ring (such as SNS) or a synchrotron-based system (such as ISIS or J-PARC), the lessons that have been learnt from the last ten years of study can be summarized as follows:

• To achieve a beam power of several megawatts requires a careful balance of repetition frequency and energy

• Building up the intensity in the ring through H- charge exchange injection is a demanding task. Stripping foil temperatures must be controlled and beam dumps are required for unstripped H- and partially stripped H0 ions. Studies for the ESS suggest that a suitable choice of the injection beam energy can minimise problems arising from different H0 excited states [ESS03] . Injection conditions (including orbit bumps, RF voltages and optical Twiss parameters) must be chosen to maintain uncontrolled beam losses below the accepted level of 0.01%. The transverse distribution of the accumulated beam must be as uniform as possible to avoid high peak current and consequential space charge problems.

• The separate operations of compressing the proton bunches to nanosecond (rms) durations and particle accumulation impose entirely different demands on the lattice beam optics. To devise a system which can meet both requirements is extremely diffcult. The CERN solution is to use a separate compressor ring, based on a simple FODO lattice, in which the accumulated bunches circulate for only ~7 revolutions while being compressed by a total of about 8MV of RF voltage. RCS scenarios attempt to balance the operations of accumulation, acceleration and compression more evenly between systems of rings, and the peak RF requirements can generally be kept lower.

• At such high levels of longitudinal bunch compression, the beam current is high (~1000 A). Space charge will affct the lattice parameters (b-functions, tune, transition energy) and the non-linear optical behaviour needs to be carefully studied.

• Bunch compression is facilitated if the original longitudinal emittance is small. Injection at a lower energy might be preferable for such machines.

4.3.4       Design models

Models developed at the Rutherford Appleton Laboratory extend these ideas. A linac accelerating H- ions to an energy in the range 150–200MeV provides a suitable combination of parameters to accumulate bunches with a (normalised) longitudinal emittance of about 1 eV.s in a compressor ring. Acceleration to top energy can then be divided between two synchrotrons, the first with a lattice optimised for injection and low-loss particle accumulation and the second designed for final bunch compression before transfer to the target. The RAL designs show a method of doubling mean radii and halving frequencies that reduces the demands on fast cycling dipole magnets and eases the burden on the high gradient RF accelerating cavities.

The models all have a common linac developed from designs originally formulated for the ESS [ESS03] . Through an experimental programme at RAL, positive steps have been taken to devise a high current (50–60 mA) H- ion source with a lifetime of approximately 10 weeks or longer. This is based on the ISIS Penning source and, after extensive research with the aid of EU support, is probably the best of its kind available today. From the ion source the beam is matched by the Low Energy Beam Transport system (LEBT) into a radio frequency quadrupole (RFQ), bunched and accelerated to 2.5 MeV. The fast beam chopper [Cla02]  which follows is a crucial component of the structure, without which low-loss ring accumulation would be impossible. As the frequency of the RFQ is likely to be 234.8MHz, the gap between micro-pulses is of the order of 2–3 ns during which a kicker field has to rise to deflect up to 30% of the micro-pulse train to a beam dump. Operation is at an harmonic multiple of the ring revolution frequency. An intensive R&D programme has been set up aimed at full beam tests within five years (finances permitting). Some support has been obtained from the EU within the Framework Programme 6 (FP6) and the merits of the RAL chopper design will be compared with an alternative model under study at CERN.

Beam passing through the chopper is accelerated in a drift tube linac (DTL) to an energy of 90MeV and undergoes a triple frequency jump to 704.4MHz before being raised to 180MeV in a side-coupled linac (SCL) [Gerigk04] . This work also receives funding from the EU under FP6.

4.3.5          5 GeV, 4MW Model

 

The HARP experiment (see chapter 5.2.2) will indicate the optimum driver energy for maximum pion yield for a neutrino factory. Pending the results, models covering a range of energies have been developed.

The first (Figure 27) has a top energy of 5 GeV and is site-independent. The synchrotron system consists of two stacked booster rings of mean radius 32.5m each taking two bunches of 2.5 × 1013 protons to an energy of 1.2 GeV at a frequency of 50 Hz. The injection period per ring is 0.2 ms and the rings are filled immediately after one another. A simulation of the injection into longitudinal phase space is shown in Figure 6. Although chopped, the beam does not initially fit completely inside the stable phase space bucket, but particles outside are gathered up in the non-adiabatic process as voltages and magnetic fields change and acceleration begins. Momentum ramping and RF steering are used to assist trapping and keep beam loss, which is mainly from scattering in the injection foil, below 0.01%. Over 160 injection turns, the average number of foil traversals per circulating proton is low, keeping temperatures down and probably allowing use of an ISIS-type Al2O3 stripping foil, though carbon, with a higher sublimation temperature, would be a safe, alternative choice.

 

Figure 27 Rapid Cycling Synchrotron models at 5 GeV (left) and 15 GeV (right)

Extraction from both synchrotrons has to be at the same energy, which means on the upward part of the accelerating cycle for the second ring and the downward part for the first. All four bunches - which are 100 ns in duration - are transferred to one of the main synchrotrons, which have twice the radius (65 m) and operate at half the frequency, 25 Hz at harmonic number h = 8.

The beam is accelerated to 5 GeV over 20 ms, during which time the boosters reset and operate again so that as the first main ring is extracted, the second main ring is filled. Extracting alternately in this way restores the 50 Hz cycle. Bunch compression is achieved by choosing the main ring to have g = 6.5, only very slightly greater than  g = 6.33 at top energy. Towards the end of acceleration, the bunches are then almost frozen longitudinally. An additional 500 kV of RF at harmonic number h = 24 are brought into play and this is sufficient to compress the bunches to 1 ns (rms). The process is accompanied by an increase in momentum spread, which requires non-linear optics to be considered, but a sextupole scheme has been devised to compensate for these high order effects. Further details are given in [Prior00]  and [Prior00a] .

 

 

4.3.6          15 GeV, 4–6MW Model

 

A 15 GeV driver has also been designed on the same principles, with the main rings specifically of a size to fit within the CERN ISR tunnel. The booster synchrotrons have mean radius 50 m, h = 3 and output 50 ns bunches at 3 GeV and 25 Hz, after accumulation and acceleration from the 180MeV H- linac. The main rings have radius 150 m, and each compresses 6 bunches to 1 ns (rms) with a peak RF voltage of 1.7MV at harmonic number 36. Space charge tune shifts are of the order of -0.02. Figure 27 shows a possible layout, with the boosters neatly fitted inside the main synchrotrons. For simplicity, all but one of the transfer lines between the rings have been suppressed. The model is nominally designed for 4MW output but is capable of being upgraded to 6MW by increasing the number of bunches.

4.3.7       4.1.3 30 GeV, 4MW Model

Adapting parameters in a different manner, a 30 GeV slow cycling synchrotron at 8 Hz has also been designed for the ISR tunnel [Aut01] . The scenario uses the same 180MeV H- linac and 180 achromat described above, and has a booster accelerating two proton bunches to 2.2 GeV at 50 Hz. Batches of 8 bunches (1014 protons in total) are accelerated in the main ring to 30 GeV, which requires a total of 3.8MV of peak RF voltage for the 1 ns rms bunch compression. Such a machine would not only provide an alternative to the CERN SPL but could also inject directly into the CERN SPS above transition energy. It might also be considered as a possible replacement for the elderly CERN PS.

4.3.8       ISIS Upgrades

In its present configuration, the machine is limited by space charge to an intensity of 2.5×1013 protons per pulse. A combined h = 2, h = 4 RF system is being installed which, by stretching the stable areas of longitudinal phase space, may allow up to 50% more beam to be injected without any change in the transverse tune effects [Prior93] . The relative phases beween the RF harmonics have to be carefully controlled throughout the cycle but the general principle is well-tried and should provide additional benefits for neutron scatterers for a fairly modest cost. A second target station is also being built, to operate at 10Hz and, by taking one in five pulses from the existing 50 Hz target, will effectively absorb the extra beam power.

The second phase of the upgrade is to replace the ageing ISIS linac with the 180MeV linac described above. At this energy, space charge levels at injection are halved, and initial studies suggest that the synchrotron could output 0.4MW of beam power, which the present target could probably just about withstand.

Beyond this, the only practical method of increasing the power of the facility is by raising the energy through the addition of a second synchrotron. Studies have been carried out to enlarge the 8 GeV ring developed for Fermilab to a mean radius of 78m, three times ISIS’s 26 m. This ring could then operate in two modes. Taking ISIS’s pulses by a simple bucket-to-bucket transfer, the beam could be accelerated to 3.5GeV to produce 100 ns bunches at an energy suitable for a 1–1.5MW, 50 Hz spallation neutron source. This is a fairly costly project since a new target would be required. In the machine’s alternative mode, one in three ISIS pulses could be accelerated in the main synchrotron to 8 GeV at a frequency of 16.67 Hz (the other two pulses being directed to a beam dump). Experiments of nanosecond bunch compression

could be carried out, studies of lattice and beam behaviour near transition would be possible, and the beam could provide the means for neutrino factory pion target tests. Further development would entail the construction of a new booster, effectively replacing the current ISIS accelerator. This could fit inside the 78m ring, thus allowing ISIS to continue operation with relatively little disruption to users. The booster is based on stacked 1.2 GeV, 50 Hz synchrotrons fed by the 180MeV H- linac via the achromat of Figure 27. The main synchrotron, operating at 25 Hz, would be upgraded to 2.5MW and would provide an enhanced neutron facility at 3 GeV with further scope for neutrino factory tests at 6 or 8 GeV.

The final phase of the upgrade programme would be to build a second main synchrotron, stacked on top of he first, so as to recoup the overall 50 Hz frequency as explained above. This would provide a 4–5MW proton driver in the full sense of the term and provide a dual neutron/neutrino facility that would fit comfortably on the RAL site.

4.3.9       Proton Driver R & D

R&D is of course of vital importance, and the following representative (but incomplete) list of topics is adapted from the Snowmass working group’s report [PDWG01] .

(1) The requirements from the H- ion source are a current of 60–75 mA, 6–12% duty cycle, and a normalised rms emittance enrms » 0.2 p µrad.m. The lifetime should be at least 2 months. The RAL ISIS ion source is the only source realistically likely to meet these demands in the immediate future.

(2) Work is required on radio frequency quadrupole linacs (RFQ) at frequencies in the range 200 to 400MHz with currents up to 100 mA. 99% transmission efficiency is a goal and the unit must have its higher order modes suppressed.

(3) A fast beam chopper is essential, with rise time<_2 ns. Materials and configurations meeting the thermal demands imposed on the chopper beam dump need to be studied, and a means for handling 5–10kW of beam power should be assessed.

(4) Funnelling may be required to double the linac current, particularly in full-energy linac/accumulator ring systems. The ESS design contains a 20MeV funnel and there are plans to build and test this at RAL at some future date [Prior99] .

(5) R&D is required for high efficiency and high reliability RF sources, such as inductive output tubes (IOT) and multi-beam klystrons.

(6) In the area of linac diagnostics, studies should be carried out on: (a) noninvasive H- beam profile measurements, using for example laser wire, ionization and fluorescent-based techniques; (b) on-line measurements of beam energy and energy spread; (c) halo monitors, especially in superconducting systems; (d) longitudinal bunch shape measurements.

(7) The EU FP6 HIPPI contract covers work on high-gradient low and intermediate b superconducting cavities and spoke cavities. Much will be learnt from the SNS experience in cryogenics.

(8) There is an active group currently analysing space charge problems, in particular exploring fast accurate codes and devising and carrying out benchmarking tests (see [Prior03b] ).

(9) Experimental studies of ring lattices are desirable, for example to explore the higher order dependency of   gt  on Dp/p, tune shifts and space charge.

(10) Injection foil lifetime and stripping effciency need to be investigated and experiments on the lifetime of H0 excited states as a function of magnetic field and beam energy should be carried out. Studies of the efficiency of slow extraction systems would also be of interest.

(11) There is an active international collaboration trying to understand the electron cloud problem. Here too codes are being developed to incorporate an increasing range of physical effects. A benchmarking programme is under way but needs to run in parallel with the experimental programme proposed at ISIS and ongoing studies at the Los Alamos PSR and CERN.

(12) Ring beam loss, collimation and radiation protection issues are of high priority. 3D code development is required, and engineering aspects of collimation and beam dumps should be investigated. The efficiency of beam-in-gap cleaning systems will benefit from SNS experience. Collimation with resonant extraction could, with interest, be explored.

(13) Diagnostics need to be developed to measure beam parameters during ring injection, for example beam position monitors over a large dynamic range for turn by turn measurements, and equipment for fast, accurate, non-invasive tune measurements.

(14) Covering a range of different options, studies of RF in the ring could profitably be carried out to develop:  low frequency (~5 MHz), high gradient (~1MV/m) RF systems, some with ~50% duty cycle; (c) high voltage (>100 kV) barrier bucket systems; (d) transient beam loading compensation schemes (e.g. for low-Q magnetic alloy (MA) cavities).

(15) Synchrotron magnets with combinations of different harmonic fields need to be designed and tested. At RAL and Fermilab, for instance, a magnetic field variation

B(t) = B0 - B1 cos(2pft) + B2 sin(4pft)

is proposed with B2 chosen to help minimise the peak RF voltages needed for acceleration. Suitable power supplies need to be developed, and their cost effectiveness taken into account.

(16) Since most proton driver rings are likely to include inductive inserts to reduce the effects of high space charge levels during bunch compression, a formal R&D programme would be desirable covering aspects of both theory and practice (programmable inserts, inserts with large inductive impedance). An experimental programme is planned for the Fermilab Booster and the J-PARC project.

4.3.10  Conclusions

While there is much work to be done, it appears that there are no insurmountable difficulties to the construction of a successful proton driver. By sharing the challenging aspects over different parts of a machine — chopping in the low energy part of the linac, halo control in intermediate energy stages and the achromat, acceleration and bunch compression in separate rings, and doubling the rings to reduce space charge as necessary — a synchrotron-based scenario provides a feasible and cost-effective solution for future high power needs. With the support of organisations such as the European Union through Framework 6, good progress is already being made in the design of high intensity linacs with energies up to 200MeV, and further backing will be requested for synchrotron development in the near future. A model has been devised for a slow-cycling replacement for the CERN PS, and the ISIS upgrade plans, which progressively develop an existing facility into a machine for both high energy physics and condensed matter studies, look particularly interesting.

 

5        Facilities around the intense proton source

5.1      Additional installations for a neutrino physics facility

(A. Blondel, S. Geer, H. Haseroth, A. Rubbia)

5.1.1       Overview

Several neutrino physics facilities have been discussed as shown in Figure 8.  The beta-beam is quite different and will be discussed in section 5.4. The Neutrino Factory based on a muon storage ring is being investigated in the US, Japan and Europe since quite a few years. A "Neutrino Superbeam" is a conventional neutrino beam from p decay and is very similar to the front end of the Neutrino Factory. This section concentrates on what is needed behind a proton driver for a Superbeam, then a Neutrino Factory. The overall layout is repeated in Figure 28; as can be seen in Figure 29, a suitable arrangement could be found at CERN.

 

Figure 28  Schematic layout of a Neutrino Factory

Figure 29 Possible layout of a Neutrino Factory on the CERN site

5.1.2       The Neutrino Factory

New accelerator technologies offer the possibility of building, not too many years in the future, an accelerator complex to produce and capture more than 1020 muons per year.  It has been proposed to build a Neutrino Factory [nufact] by accelerating the muons from this intense source to energies of several GeV, injecting the muons into a storage ring having long straight sections, and exploiting the intense neutrino beams that are produced by muons decaying in the straight sections. The decays:   and    offer exciting possibilities to pursue the study of neutrino oscillations and neutrino interactions with exquisite precision.

To create such an intense muon source, a Neutrino Factory requires an intense multi-GeV proton source capable of producing a primary proton beam with a beam power of 1~MW or more on target. This is just the proton source required in the medium term for Neutrino Superbeams. Hence, there is a natural evolution from Superbeam experiments to Neutrino Factory experiments.

Neutrino Factory designs have been proposed in Europe [Aut99], [Gru02], the US [MuColl] [StudyI][StudyII], and Japan [Japnufact]. Of the three designs, the one in the US is the most developed, and we will use it as a example in general with a few exceptions. The Neutrino Factory consists of the following subsystems:

  1. Proton Driver.  Provides 1-4 MW of protons on a pion production target. For the neutrino factory application the energy of the beam is not critical, since it has been shown that the production of pions is roughly proportional to beam power. In the low energy region, this statement may need to be substantiated with e.g. the Harp measurements (see section 5.2 ). The time structure of the proton beam has to be matched with the time spread induced by pion decay (1-2 ns); for a linac driver, this requires an additional accumulator and compressor ring. For the superbeam however, the energy of the protons directly impacts on the resulting neutrino beam energy from pion decay which is typically 5 to 10% of that of the incident protons.
  2. Target , Capture and Decay. A high-power target sits within a 20T superconducting solenoid, which captures the pions. The high magnetic field smoothly decreases to 1.75T downstream of the target, matching into a long solenoid decay channel. A design with horn collection has been proposed at CERN for the neutrino factory, with the benefit that it can be also used for a superbeam design. The advantage of the horn that it sign-selects the pions and muons is compensated by the fact that in a neutrino factory design one could contemplate to accelerate both signs of muons, thus doubling the available flux. 
  3. Bunching and Phase Rotation. The muons from the decaying pions are bunched using a system of rf cavities with frequencies that vary along the channel. A second series of rf cavities with higher gradients is used to rotate the beam in longitudinal phase-space, reducing the energy spread of the muons.
  4. Cooling.  A solenoid focusing channel with high-gradient 201 MHz rf cavities and either liquid-hydrogen or LiH absorbers is used to reduce the transverse phase-space occupied by the beam. The muons lose, by dE/dx  losses,  both longitudinal- and transverse-momentum as they pass through the absorbers. The longitudinal momentum is replaced by re-acceleration in the rf cavities.
  5. Acceleration. The central momentum of the muons exiting the cooling channel is 220 MeV/c. A superconducting linac with solenoid focusing is used to raise the energy to 1.5 GeV. Thereafter, a Recirculating Linear Accelerator raises the energy to 5 GeV, and a pair of Fixed-Field Alternating Gradient rings using quadrupole triplet focusing accelerate the beam to at least 20 GeV.
  6. Storage Ring. A compact racetrack geometry ring is used, in which 35% of the muons decay in the neutrino beam-forming straight section. If both signs are accelerated, one can inject in two superimposed rings or in two parallel straight sections.

This scheme produces over 2 ´ 1020 useful muon decays per operational year. The European Neutrino Factory design is similar in general to the US design, but differs in the technologies chosen to implement some of the subsystems. The Japanese design is very different, and uses very large acceptance accelerators rather than a system that reduces the phase-space occupied by the muons so they fit within the more limited acceptance of a more normal acceleration scheme.

5.1.3       The proton Driver

High power of the proton beam is a challenge in terms of beam losses, which can yield undesired activation of the machine components making hands-on maintenance impossible. In the CERN scheme with an H- linac with charge exchange injection into an accumulator ring the stripping foil needs very close attention. A common problem of all proton drivers is the production of very short bunches in order to reduce finally the energy spread of the muons with a scheme called “debunching” amongst linac experts (“phase rotation” for neutrino people).

5.1.4       The target

For a high power target there are many areas of application in neutrino physics, studies of rare processes initiated by muons, studies of materials with neutron beams from a spallation source, the accelerator production of tritium, accelerator transmutation of waste, accelerator test facilities for fusion reactor materials and many others.

The main problems are the survival of components against melting/vaporization, the survival of components against beam-induced pressure waves (in the case of pulsed proton beams), the survival of components against radiation damage.

Massive solid targets (or rotating-wheel targets), typically water cooled, have been used in most applications with not more than 1-MW beam power. But for beam powers in excess of 1 MW such passive solid targets become very problematic in view of the challenges mentioned above. This has led to consideration of flowing liquid targets: mercury, molten lead, molten Pb/Bi, etc.

 

      

Figure 30 Experiments with liquid mercury jets. On the left is seen a jet exposed to a beam of 4 1012 protons of 24 GeV at BNL; the jet explosion begins long enough after the impact. On the right is shown the behaviour of the jet inside a high magnetic field; the jet is able to penetrate the intense magnetic field, and Eddie currents smoothen it. 

The usual liquid target systems still require solid-walled containment vessels and beam windows that isolate the target region from the rest of the accelerator complex. An example of such a design within a horn is given in  Figure 31. Experience has shown that if a liquid target is confined inside a metal pipe in the region of the interaction with a pulsed proton beam, then the beam-induced pressure waves can cause pitting (associated with cavitation during the negative-pressure phases of the waves) and possible failure of the solid wall. Such concerns indicate that it would be preferable to have a flowing liquid target in the form of a free jet, at least in the region of interaction with the proton beam. In a recent workshop for "High-Power Targetry for Future Accelerators" in Ronkonkoma it was stated that Targets for 1 MW machines exist (but unproven) and that there is no convincing solution for the 4 MW class machines.

Rotating solid targets, granular targets, liquid metal targets, e.g. Hg have been considered at several labs. Tests with Hg were done at BNL and CERN. Tests with Hg jets injected into high magnetic fields were done by CERN at Grenoble. (Figure 30).

A number of valuable results were obtained, like the measurement of the radial velocity of the dispersal of the Hg jet as a function of the proton beam intensity and the observation that the Hg dispersal is largely transverse to the jet axis and that there is no visible manifestation of jet dispersal before 40 ms. At Grenoble the stabilizing effect of the jet when injected into a magnetic field was observed. There is now a proposal [target-exp] to the Isolde and nToF Committee for an experiment at CERN in the TT2a tunnel using a Hg jet with proton beam AND magnetic field supported by RAL, CERN, KEK, BNL and Princeton University, which would allow very good progress in the understanding of the basic mechanisms important for the design of multi MW targets.

5.1.5       Pion capture

For the pion capture different schemes are proposed. In the US a Solenoid with 10-20 Tesla is being considered (lifetime >>1 year), whereas at CERN the collection with magnetic horns is explored. A magnetic horn would be needed to select either positive or negative pions. Present estimates give a possible lifetime of 6 weeks. HARP results are needed to optimize the proton driver energy, the target and the collection device.

Figure 31  Possible layout of a Hg jet target and a horn. On the right, a prototype horn built at CERN  [Gilardoni]

5.1.6       Cooling and phase rotation

Phase rotation in the CERN scheme is achieved with rf cavities operating at 88 MHz [Lombardi]. The American scheme [Study II] was using induction linacs. Now a 200 MHz rf capture system is being worked on. In both cases one lets the muon beam generated via the very short (1 ns rms) proton bunch spread out in the longitudinal direction and use the corresponding time-position correlation to correct the energy of the muons with a time-varying electric field. To perform cooling, the beam is sent through liquid hydrogen absorbers, reducing the transverse and longitudinal momenta. Subsequent reconstitution of the longitudinal momentum occurs with RF cavities. Basically the cooling channel is a linear accelerator with (liquid hydrogen) absorbers. The cooling channel will be fairly long and expensive, hence the interest in “ring coolers”, where cooling is done over many revolutions.

Ionization cooling involves many new technologies, in particular operation of high gradient RF cavities in high magnetic field, and in the vicinity of hydrogen absorbers. In order to assert the performance that can be achieved in a real channel, the MICE experiment (Figure 32) is being prepared at the RAL (UK). Liquid hydrogen absorber prototypes have been already operated at Fermilab and the first 200 MHz cavities with Be windows is being built in Berkeley, while prototypes of the tracker and detectors are operated in UK, Japan Italy and CERN [MICE].

Figure 32 Layout of the MICE experiment. 

5.1.7       Acceleration

The acceleration of muons should proceed in several steps and be very fast. After an initial linear accelerator "Recirculating Linear Accelerators" (RLAs) are investigated, as normal synchrotrons are too slow and the decay losses of muons would not be tolerable (the muon’s life time is only 2.2 ms). RLAs are a good compromise between cost and speed. For the acceleration 200 MHz sc cavities, sputtered at CERN, are tested at Cornell.

Another interesting proposal might be mentioned here: the possible use of a rapidly pulsed synchrotron, which seems feasible by making use of the fairly low repetition rate, at least in the US scheme.

The use of FFAGs is also being investigated, after the successful operation of proton FFAGs in Japan. These machines have a large acceptance, both in longitudinal and transverse phase space, hence cooling may not be needed and the acceleration can be fast due to fixed magnetic field. [Keil04]

5.1.8       Decay ring

The decay ring has long straight sections to produce the well directed neutrino beams. The geometry is quite flexible, but in order to achieve high precision on the flux it is best to use a race-track or triangle geometry to ensure muon precession. This allows measurement of the beam energy and energy spread with great precision and ensures that the average polarization is zero. The optics can be designed in such a way as to ensure a beam divergence of less than 0.2/g, and the necessary diagnostics (at least a beam current monitor, a polarimeter and a measurement of beam angular divergence) can be accommodated. [Keil00], [ECFAreport].        

5.1.9       Muon Colliders

Some time ago regarded by some people as science fiction, it must be noted that the advances in cooling theory and technology are so impressive as to consider this type of machine as a real possibility in the future opening the "High Energy Frontier" to leptons.

5.1.10  Progress on Neutrino Factory design

An impressive Neutrino Factory R&D effort has been ongoing in Europe, Japan, and the U.S. over the last few years, and significant progress has been made towards optimizing the design, developing and testing the required accelerator components, and significantly reducing the cost. To illustrate progress in cost reduction, the cost estimate for a recent update of the US design [APS04] is compared in Table 9 with the corresponding cost for the previous “Study II” US design [Study II]. It should be noted that the Study II design cost was based on a significant amount of engineering input to ensure design feasibility and establish a good cost basis. This engineering step has not yet been done for the updated design, but the new cost estimate is based on experience from the Study II work. The conclusion is that the latest design ideas are expected to lead to very significant cost reductions, although more work must be done to establish a reliable new cost estimate.

Neutrino Factory R&D has reached a critical stage in which support is required for two key international experiments (MICE and Targetry) and a third-generation international design study. If this support is forthcoming, a Neutrino Factory could be added to the Neutrino Physics roadmap in less than a decade.

Table 9 Comparison of unloaded Neutrino Factory costs estimates for the US Study II design and for the latest updated US design. Costs are shown including or not including the Proton Driver and Target station in the estimates. The New design cost estimate has not yet benefited from the level of engineering effort included in the Study II work. Table from Ref. [APS04].

 

All

(M$)

No Proton Driver

(M$)

No Proton Driver &

No Target station  (M$)

 

Study II

 

1832

 

1641

 

1538

 

New / Study II (%)

 

67

 

63

 

60



The scientific case for pursuing Neutrino Factory R&D is strong. The encouraging technical progress in Neutrino Factory R&D over the last few years has been matched by progress in building the level of international collaboration needed for the next step, and preparing proposals for the critical R&D experiments. All of this has been accomplished with very limited funding. The next steps require an increase in funding, but to a level which is still modest considering the nature of the enterprise. If a Neutrino Factory is to remain a viable option for the future it is important that MICE, the Targetry experiment, and a third-generation international design study are supported. If this is the case, we have much to look forward to.

 

5.2      Hadro-production experiments

(M. Apollonio, A. Blondel)

5.2.1       Introduction

The construction of a neutrino factory or a superbeam requires optimisation of target material,  collection scheme and proton energy. Present studies are based on simulation codes for pion production which show large discrepancies, both in p/K yield and (pL,pT) distributions. This reflects the poor experimental data, based on old experiments covering small acceptances, few materials and few incident proton energies, and the lack of a good phenomenological description of  low energy hadronic interactions. The situation calls for a new generation of dedicated hadronic experiments as integral part of the neutrino physics programme.  The E910 experiment at BNl took data in the late 1990’s for proton energies between 6 and 24 GeV and first results were presented recently [BNL91004]. The MIPP experiment at Fermilab [MIPP] is presently being commissioned for proton energies between 15 and 125 GeV. The HARP experiment at CERN is the only one to cover the low energy range of the baseline SPL option (protons of 2.2 GeV kinetic energy (~3 GeV/c  momentum) [Vretenar00] where it is crucial to understand the p+/p- ratio as well as the rate of K± and K0 production.

5.2.2       The HARP experiment

HARP [HARP-proposal] was proposed in 1999 as a hadro-production experiment whose goals are:

·         The optimisation of the p+(p-) yield in view of a neutrino factory or a superbeam

·         The calculation of beam fluxes for other experiments, K2K [K2K03] and MiniBooNE [MiniBoone]

·         The improvement of the present knowledge about atmospheric neutrino fluxes

·         The input for hadronic Monte Carlo generators

The first and second points will be developed in the following.

Figure 33 The HARP experiment

Figure 33 shows a layout of HARP; the experiment, located in the PS T9 East Hall at CERN, collected about 420 million events in 2001-2002, with a distribution of beam particle and energy and targets shown in Table 10 at a high DAQ rate (2.5 kHz, ~106 events/day). The detector can be ideally decomposed into a Large Angle Region (covered by a TPC and several RPCs inside a 0.7 T solenoidal magnetic field) and a Forward Region (covered by a spectrometer and a series of detectors for particle identification).

Table 10 Summary of the HARP data taking campaign (2001-2002). Many materials have been tested ranging from H to Pb, at proton momenta covering an entire decade (1.5 to 15 GeV/c). Some special targets (like the K2K and MiniBooNE replicas and cryogenic targets) have been thoroughly studied.

5.2.3       Optimisation of the neutrino factory (and super-beam): large angle

The aim is to reach a precision of 5% in the pion yield (and p+/p- ratio). It should be stressed that such measurements will be also extremely useful for a super-beam.  At all energies most of the pions are produced at high angles but this is especially true for the low enery of SPL.  For this reason the use of Large Angle Detectors (TPC and RPCs) are of paramount importance. Presently the TPC calibration and correction of various distortions and cross-talk is underway. A campaign of calibrations using cosmic rays, radioactive sources (83Kr and 55Fe) and data was pursued [HARP03].  This calibration program allowed:

·         The equalisation of gains and mapping of dead pads

·         A first evaluation of the correction for  cross talk in the readout planes

·         The first determination of the dE/dX as a function of p

·         An improvement in pT resolution

Some of these results are summarized in fig. 2 and fig. 3 (left and right). These results will be verified using the well-known elastic scattering processes of protons (pions) on hydrogen target at low momenta (3 GeV/c).

 

Figure 34 Transversal momentum resolution as a function of pT, as  measured by the TPC.

Figure 35 (left) dE/dX as a function of p, showing different particle populations (p,m and protons); (right) energy peaks for the radioactive sources used to calibrate the detector: (a) overall picture with 55Fe and 83Kr. (b) Fe peaks (at 5.9 and 3.0 keV) and Kr peaks (the main one being at 41.6 keV). This case is obtained using equalised pads.

 


5.2.4       The K2K and MiniBoone case: the forward analysis

This important physics case has been chosen as the subject of our first analysis.  In the K2K experiment [K2K03], one of the largest systematics in the n oscillation parameters comes from the uncertainty on the far/near ratio, which depends on the p-production model used. The pion flux from KEK can be monitored and checked against simulation of the beam down to neutrino energies of 1 GeV, while for lower energies there is no experimental information. Unfortunately the oscillation effect that K2K is meant to measure takes place somewhere between 0.5 and 0.75 GeV. This translates into a (p,q) distribution for parent pions which is well covered by the HARP forward detectors (see fig. 4).

Figure 36 (p,q) distribution for pions in the K2K case as described in the text. The parameter space is well within the reach of the forward HARP detectors.

 

Figure 37 (left) momentum distribution, integrated over q, for secondary pions from 12.9 GeV/c protons impinging onto a 5% l Al target. (right) angle distribution, integrated over p, for the same sample. Vertical axis is in arbitrary units [Cervera04].

 

A special program with an Al K2K replica target has been followed by HARP; at present the collaboration is strongly focussing into this subject, aiming at the calculation of the pion cross section in a range of momenta pp<8 GeV/c and of emission angles qp<300 mrad.

Preliminary results of this analysis, based on an Al thin target at 12.9 GeV/c, have been already made public [Cervera04], and are summarized in Figure 37, in the form of shape distributions for p and q. Albeit still missing of a real determination for the systematic error they represent our first measurement of the pion yield as a function of momentum and emission angle.

5.2.5       Conclusions

The optimization of future neutrino facilities requires good understanding of  pion production by various beams and targets. The HARP experiment was assembled very rapidly and took an extensive data set. The experiment and its analysis require great care, but the experiment should be able to fulfill its goals. The measurement on pion and kaon yields at the low energy of SPL will be important for the decision on this accelerator.

 

5.3      EURISOL : a new nuclear physics facility

(Y. Blumenfeld, P. Butler, A. Müller)

During the past two decades, progress in nuclear physics has been largely fuelled by the development and improvement of radioactive ion beams. The two main methods used to produce such beams are called projectile fragmentation and ISOL. In the former a high energy heavy ion beam impinges on a thin target and a large array of fragments is produced. The isotopes of interest are selected by a fragment separator and the resulting beam  transported to the experimental areas. This method, used at GANIL, GSI, the NSCL/MSU and RIKEN, is particularly efficient for a large variety of species with short lifetimes and delivers high energy beams with relatively modest resolution qualities. The ISOL method, in use at CERN/REX-ISOLDE, GANIL/SPIRAL and TRIUMF, uses a driver accelerator (p, d or heavy ions) and a thick target. The nuclei are produced at rest, diffuse and effuse out of the target before being ionised and then accelerated in a post-accelerator. Beams of high quality but modest energy are produced. The efficiency of such a system depends on the diffusion and effusion times and certain elements, such as the refractory elements, are particularly difficult to produce due to their chemical properties.

Technologies are now being developed, which should allow for improvements of orders of magnitude of the intensities of radioactive beams. A vast physics program has been identified, which is extensively discussed in other talks of this workshop.  This led NuPECC to propose the construction of two ‘next generation’ RIB infrastructures in Europe, i.e. one ISOL and one in-flight facility. The in-flight machine would arise from a major upgrade of the current GSI facility, while EURISOL would constitute the new ISOL facility.

An RTD program, for a preliminary design study of EURISOL, was coordinated by GANIL and J. Vervier, and implemented under the auspices of the EU 5th framework program. The result is a preliminary design report [EURISOL] which outlines a concept for a future EURISOL facility (fig. 1). The driver accelerator would be a super-conducting CW proton LINAC, of energy 1 GeV and power 5 MW, with additional capability of accelerating deuterons and possibly heavier ions with A/Q = 2. Several target stations would be built, including a fission target with a liquid mercury converter allowing for the use of the full 5 MW beam power, and targets receiving directly the approximately 100 KW of proton beam for production of lighter or neutron deficient isotopes. The post accelerator would be a super-conducting heavy-ion LINAC with a maximum energy of 100 MeV/nucleon. The maximum energy is somewhat arbitrary and for a linac is defined by cost; higher energies could be achieved at CERN by exploiting its existing synchrotron accelerator chain or new accelerators required for the beta-beam facility.

In EURISOL there will be several experimental areas devoted to physics at different energies: fundamental physics, nuclear astrophysics, nuclear structure and nuclear reaction studies. Among the experimental equipment necessary, one can cite ion traps and high precision spectroscopy set ups for very low energy beams (similar to present ISOLDE equipment); a variety of high resolution and/or large acceptance magnetic spectrometers; an innovative gamma-ray tracking array (i.e. the AGATA concept); high granularity charged particle and neutron detectors, a 4P charged fragment detector, and a fragment separator for production of nuclei very far from stability through secondary fragmentation.

With the wide diversity of scientific disciplines and individual experiments being served by the facility, various multi-user installations (such as at the present ISOLDE) are needed, requiring the design of a beam switchyard that allows parallel operation. 

Typical intensities in particles per second would be 1013 for 132Sn, 1011 56Ni, 5 1013 6He, and 5 1012  for 18Ne.The total cost of such a facility was estimated at 600 M€, including buildings but excluding manpower. The large production of 6He and 18Ne would make EURISOL an attractive source of unstable nuclei for a b-beam installation, as outlined in chapter 5.4.

Figure 38 : Conceptual layout of the EURISOL facility.

 

 

The roadmap towards EURISOL includes three main aspects:

--The vigorous scientific exploitation  of current ISOL facilities : EXCYT, Louvain, REX/ISOLDE, SPIRAL

--The construction of intermediate generation facilities : MAFF, REX upgrade, SPES, SPIRAL2

--The design and prototyping of the most specific and challenging parts of EURISOL in the framework of the Eurisol design study (EURISOL_DS) proposed in the sixth framework program.

 

In close contact with the nuclear physics and neutrino communities,  a design study proposal  (EURISOL_DS) was submitted in March 2004, with the aim of performing detailed engineering oriented studies and technical prototyping work for the future EURISOL facility, which would be coordinated by GANIL and G. Fortuna. This design study proposal includes 21 participating institutions from 14 European countries, as well as 21 other contributing laboratories from Europe, North America and Asia who will provide their expertise on specific technical points.   The total cost of the design study would be 33 M€, of which 9.16 M€ is requested from the EU, the remainder being provided by the participating institutions.

The work is to be subdivided in 11 tasks (the laboratories leading the tasks are indicated in parentheses) grouped under 4 topical subjects. Several of these tasks include a large effort of technical prototyping as specified:

•Physics, beam intensity and safety

–Physics and instrumentation (Liverpool)

–Beam intensity calculations (GSI)

–Safety and radioprotection (Saclay)

•Accelerators :

 

–Proton accelerator design (INFN Legnaro)

–Heavy ion accelerator design (GANIL)

–SC cavity development (IPN Orsay): prototyping of SC cavities and multipurpose cryomodule

•Targets and ion sources :

 

–Multi-MW target station (CERN) : prototyping of mercury converter

–Direct target (CERN) : Several target-ion source prototypes

–Fission target (INFN Legnaro) : prototyping of UCx target

•Beam properties :

 

–Beam preparation (Jyväskylä) : prototyping of 60 GHz ECR source

 

–Beta-beam aspects (CERN)

 

Several synergies have been identified, in particular with the HIPPI JRA and the BENE network of the CARE Integrated Infrastructure Initiative. This workshop has represented an excellent opportunity to start implementing these synergies. 

EURISOL, FAIR and RIA

As outlined in 3.3, the other major “next generation” nuclear facility in Europe, FAIR, will embrace research programmes in hadron and nuclear physics, atomic and plasma physics that are complementary to EURISOL, using intense beams of heavy-ions and anti-protons. In the overlapping area of radioactive ion beam physics EURISOL and FAIR will provide secondary beams in different domains of energy and beam quality, and therefore offer different experimental conditions. The EURISOL facility provides very high secondary-beam intensities for many species and beam properties (continuously variable beam energy of good definition, high purity and small emittance), which are well adapted to highly elaborate experimental approaches. EURISOL will also provide radioactive ion beams of > 100 MeV/u, that will fragment to the most exotic nuclei. The fragmentation in-flight technique used at FAIR is most interesting for very short-lived nuclei in the vicinity of the drip-lines and/or for RIB production at very high energies. FAIR will also provide cooled, stored beams of longer-lived exotic nuclei.

The American approach is to combine the features of in-flight and ISOL methods into one facility: the Rare Isotope Accelerator (RIA). The primary driver of RIA will be a linac that accelerates protons to 900 MeV and heavy ions to 400 MeV/u. For ISOL its beam power (of the order of 100 KW) is much smaller than EURISOL (5 MW) while its maximum heavy-ion energy is much less than FAIR (1.5 GeV/u).  A core element of RIA is the use of gas catcher to stop fragmentation products prior to post-acceleration.  This would offer chemistry independent ISOL beams for long-lived (ms) radionuclides but space-charge effects will limit the secondary beam intensity.

5.4      A neutrino beta beam facility at CERN

(M. Lindroos)

Figure 39 Beta-beam base line design, partially using existing CERN accelerator infrastructure (parts in black).

 

 

The proposed beta-beam facility  [Zucchelli] can be divided into two parts, a low energy part stretching up to 100 MeV/u and a high-energy part for further acceleration and ion stacking and storage in the decay ring, serving as neutrino source. This division is logical as the low-energy part corresponds to the requirements for an ISOL-type radioactive beam factory as proposed and promoted by the European Nuclear Physics community. The high-energy part, serving the neutrino physics community, would be one of several users of such a radioactive ion beam facility and would consequently share the cost and operation of the low-energy part with other physics applications.

The radioactive ions 6He and 18Ne will be produced in an ISOL system using the proposed Superconducting Proton Linac (SPL) as a driver. Following production, the ions will be fully stripped, bunched and accelerated with a linac to approximately 100 MeV/u. Further bunching will be achieved by multi-turn injection into a Rapid Cycling Synchrotron (RCS), followed by acceleration to 300 MeV/u before injection into the Proton Synchrotron (PS). The beam will then be accelerated in several bunches to PS top energy, transferred to the Super Proton Synchrotron (SPS) and accelerated to the desired top energy. Finally, the ions will be transferred to the decay ring where they will be merged with the already circulating bunches through a longitudinal stacking procedure.

Several bottlenecks exist in this process, not least the bunching at low energy, space-charge limitations in PS and SPS, decay losses along the accelerator chain and the longitudinal stacking procedure at high energy in the decay ring.

5.4.1       Ion production

The flux at the detector depends on the average energy of the neutrinos at rest as this determines the focusing of the neutrino beam. A further constraint is set by the decay losses in the accelerator chain that increase with shorter life-time and another aspect to consider are the decay products that could create long lived contamination in the low-energy part. All constraints together point towards two isotopes of particular interest, 6He to generate electron anti-neutrinos and 18Ne for electron neutrinos.

Both species can be produced in large quantities through the so-called ISOL method. The helium isotope is best produced in a BeO target using a very intense primary proton beam of a few GeV, impinging on a so-called neutron converter. For the neon isotope, spallation in a MgO target with a less intense proton beam, hitting the target material directly, is the method of choice. Due to the use of converter technology typically ten times more helium than neon isotopes can be produced.

5.4.2       Ionization, bunching and pre-acceleration

The ions can be transported away from the ISOL target directly in gas form since the chosen elements are noble gases. Alternatively, a high efficiency (for noble gases) mono-charge ECR source, close to the target, can be used to transport the singly charged ions using classical beam transport. In either case the beam has subsequently to be ionized and bunched for further acceleration in the injector chain.

Efficient bunching (<20 ms pulse length) and full stripping of a high-intensity beam can be achieved using a high-frequency 60 GHz ECR source.

Once fully stripped, the ions are first accelerated in a linac to increase their lifetime. The acceleration of high-intensity radioactive ion beams to ~100 MeV/u using a linac has already been studied within the EU-financed study EURISOL. This study is planned to continue as design study within the 6th EU framework programme.

5.4.3       Stacking

The energy of the beta-beam neutrinos will be in the range of atmospheric neutrinos. As the time structure of the neutrino beam mirrors the one of the ions circulating in the decay ring, the beam has to be concentrated in as few and as short bunches as possible to permit efficient background suppression in the detector. Four bunches, each 10 ns long, were chosen for the base line design.

A new scheme of longitudinal stacking has been proposed for the beta-beam. It uses asymmetric bunch pair merging, which relies on a dual-harmonic rf system to combine adjacent bunches in longitudinal phase space such that a fresh, dense bunch is embedded in the core of a much larger one with minimal emittance dilution. The fact that only the central part of the residing bunch is affected results in a net increase of the core intensity. The surrounding “older” ions are pushed out, towards the bucket separatrix, where the “oldest” ions will eventually be lost. Asymmetric bunch pair merging has recently been demonstrated in the PS.

5.4.4       Intensity

Starting from the production rates for 6He and 18Ne at the ECR source, and taking into account only beta-decay losses, the beam intensities along the accelerator chain can be calculated. Table 11 quotes the estimated production rates at the source, the beam intensities at extraction from the synchrotrons in the injector chain and the average circulating beam intensities in the decay ring for the beta-beam baseline scenario, assuming 16 Hz operation of the RCS and 8 s cycling time of the SPS and the complete injector chain. The number of batches required to fill the downstream machine is also indicated.

 

 

Table 11: Ion intensities for 6He and 18Ne operation along the accelerator chain for the beta-beam base line scenario (only beta-decay losses are taken into account).

Machine

6He ions

extracted

18Ne ions

extracted

Batches

Source