CERN-SPSC-2004-024
SPSC-M-722 30 August
2004
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
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
6.3
A megaton water Cherenkov
detector 85
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
8.1
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
10.1 Recommended accelerator R&D.. 132
10.2 Recommended detector R&D.. 133
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 10 Event rates at the exit of the
straight sections in a Neutrino Factory. Note the scale. 23
Figure 12 Principle of the measurement of the
muon EDM.. 24
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 20 The Superconducting Fragment
Separator (Super-FRS) at the
FAIR facility. 34
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 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 38 : Conceptual layout of the EURISOL
facility. 68
Figure 48 UNO Detector Conceptual
Design. 86
Figure 49 Conceptual Design of Hyper
Kamiokande. 87
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 58 Dm2 atmospheric and
sin2q23 determination at T2K by assuming
present 101
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 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 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 16 List of participants: 142
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.
- CERN is the largest particle physics
laboratory in the world, with a history of building and operating accelerators
at the high-energy frontier, like the SPS proton/antiproton Collider, LEP and,
soon, LHC. Along that logic, CERN is now actively pursuing R & D of
CLIC,
- CERN has also a successful tradition of
addressing simultaneously diverse physics issues (fixed target experiments,
neutrinos, radio-active ions, anti-protons) exploiting at best the
capabilities of its accelerator complex. Therefore, the management, with the
support of the Council and the Scientific Policy Committee, is willing to plan
the future in that respect as well.
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:
- The discovery of neutrino oscillations has
opened a new window of exploration, unique in several ways. Measured mass
splittings and mixings of neutrinos have been already providing the first
experimental data on physics at higher energy scales. Discovery of leptonic CPV promises
insight into the most fundamental asymmetries of the universe. The accelerator
neutrino community in Europe, now organized also in the BENE network, will do
all efforts to maintain a leading role in the sector of accelerator neutrino
experiments beyond the CNGS. Its present plans should emerge clearly and in
detail.
- The manifestation of the strong and weak interaction in the atomic
nucleus can be rigorously investigated through nuclei with anomalous
proton/neutron ratio.
NuPECC estimates the size of the European community to
be over one thousand scientists and has prioritized two second-generation
facilities with complementary physics reach: GSI-FAIR, based on the In-Flight
separation (IF) method, and EURISOL, based on the Isotope Separator On-Line
(ISOL) method. Among the potential sites for hosting a future high-intensity
EURISOL facility, CERN has unique possibilities of synergy with a neutrino
programme. This workshop should help clarify its extent and determine whether
other physics programmes could also benefit
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
S
S, 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).
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 ® ℓ+X ¹ N ® ℓ-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.
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)
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:
- the increased brightness of the PS beam that will be welcome for the luminosity upgrade (the space charge limit in the PS is raised to the equivalent of 4´1011 protons per LHC bunch within the nominal emittances).
- an extended flexibility in the choice of bunch spacing,
- the simplification of the LHC filling scheme and the “comfortable” performance margin that should increase reliability and integrated luminosity.
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:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 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 |
~2 ´1013 /s
|
~8 ´ 1011
/s |
dc |
|
RCS |
1.0
´
1012 |
4.1 ´
1010 |