NICA/MPD Meeting Report Finalversion, Amended TMT-27/10/06

NICA/MPD meeting report Finalversion, amended TMT-27/10/06

Round Table Discussion II

Searching for the mixed phase of strongly interacting matter at the JINR Nuclotron

Nuclotron facility development

NICA/MPD Project

6-7 October 2006

JINR, Dubna

Participants

Proponents:A. Sissakian, A. Sorin (Physics program), S. Bogomolov (Ion source group), I. Meshkov (Accelerator group I), A. Kovalenko (Accelerator group II),V. Nikitin (Detector group I), A. Malakhov (Detector group II)

Members of expert referee committee: N.N. Alekseev (ITEP, Moscow, Russia), D. Dinev (INRNE,Sophia,Bulgaria), S.V.Ivanov (IHEP, Protvino, Russia), T.Katayama (Tokyo Univ., Japan),A.N. Lebedev (PI RAS, Moscow, Russia), P.Senger (GSI, Darmstadt, Germany), P.Spiller (GSI, Darmstadt, Germany), T.M.Taylor (CERN, Geneva, Switzerland)

(External experts absent from the meeting but having contributed written comments on the proposal: L.V.Kravchuk (INR RAS, Moscow, Russia), E.B. Levichev (INP RAS, Novosibirsk, Russia),D. Moehl (CERN, Geneva, Switzerland), R. Ostojic (CERN, Geneva, Switzerland), N. Xu (LBNL, USA))

Report of the Expert Referee Committee

Executive summary

The committee of external expertswelcomes the initiative of JINR, Dubna in its proposal for aprogressiveaccelerator-based program on the experimental study of heavy ion physics. When implemented, the new facility wouldenablescientists at JINR to perform cutting-edge experiments locally;the future facility at JINR could moreover be tailored to cover niches that will not be fully accessible elsewhere.Though relatively modest in scale and cost, such a facility would give a renewed dynamic to the Joint Institute on the world stage of particle and accelerator physics, and provide an invaluable resource both for attracting students into this field of activity and for the efficient training of Russian and member-state scientists in the fields of theoretical, experimental and accelerator physics, and of the associated technology.

The proposal that the Committee was asked to consider was the result of strong effort by JINR scientists over the last two months. The Committee congratulates the Institute for creating the impetus for this work and acknowledges the enthusiastic application of the groups involved. However, it is clearly not possible to put such an ambitious program together completely in so short a time, and there remains work to be done before giving the green light for the construction of an accelerator facility of this type. To achieve an average collider luminosity of 1027 cm-2s-1,considered to be the absolute minimum for the proposed experiment, it will be necessary to develop and install a number ofadditional elements into the accelerator chain, including a booster upstream of the Nuclotron and electron cooling for the Nuclotron beam. This equipment was mentioned in the proposals butcorresponding details and layout, as well as those of the storage rings and the interface with the experiment,require further study to prove feasability.

The Committee recommends that the physics program for the new facility be extended to include an outline of tasks beyond the initial 3 to 5 years of running the collider, as well as indications of a program for the fixed target physics potential that will become available with the facility. It recommends that the installation and commissioning of the accelerator equipment be staged to allow experiments to be made on fixed targets with the increasingly intense and energetic heavy ion beams as they become available. It recommends that thetwo accelerator groups and the two detector groups should merge into single groups to continue the preparatory work on the proposal. It also recommends that where appropriate the expertise of other laboratories, such as IHEP, ITEP, INR RAS and BINP, be called upon to help solve some of the problems related to the accelerator system and to collaborate on the design and manufacture of equipment.

The Committee endorses the intention of JINR to organize this program as a goal-oriented project. It would also advise the incorporation of intermediate goals corresponding to the stages, at which time stock should be taken as to progress towards the achievement of the final goal, and, if necessary, decisions taken as to the eventual modification of that goal. The first goal should be to achieve the design performance of the Nuclotron. The estimates of manpower requirements for the accelerator program appear to be correct; those for the experiment do not seem to allow sufficient manpower for simulation work.It is difficult to appraise the cost estimates at this early stage. For some equipment, notably the booster, it seems to be low, and the cost breakdown lacks entries for other essential items. It would be advisable to make a more complete cost breakdown, to be agreed upon internally at JINR, containing the major items of each system both in the proposed accelerator complex and for the experiment.

The Round Table provided an excellent opportunity not only for identifying the weak points in the proposal, but also for debating possible improvements, thus creating good conditions for generating a robust project proposal.

Introduction

The committee of external experts, hereafter referred to as the Committee, welcomes the initiative of JINR, Dubna in its proposal for a progressive accelerator-based program on the experimental study of heavy ion physics. While partially overlapping in its physics reach with other modern facilities such as RHIC and FAIR and thereby enabling scientists at JINR to perform cutting-edge experiments locally, the future facility at JINR could also be tailored to cover niches that will not be addressed elsewhere. Though relatively modest in scale and cost, such a facility would give a renewed dynamic to the Joint Institute on the world stage of particle physics, and provide an invaluable resource both for attracting students into this field of activity and for the efficient training of Russian and member-state scientists in the fields of theoretical, experimental and accelerator physics.

Findings

Physics Program

The physics program, which includes the measurement of rare probes such as charm observables and low-mass di-lepton pairs, is ambitious. To reach the goals of this program will be very difficult with the proposed collider concept (average design luminosity 1027 cm-2s-1, maximum energy SNN= 7 GeV), and with the proposed detectors. The low collision rate together with the low beam energy limits the physics program to the study of bulk probes such as the yields and phase-space distributions of hadrons including multi-strange hyperons. Di-electron measurements suffer from the low lepton momenta in the center-of-mass frame, and from a huge combinatorial background. Even for the measurements of bulk probes, the design luminosity of 1027 cm-2s-1 is a minimal requirement for competitive heavy-ion experiments at these energies. The search for critical phenomena requires to scan the beam energies, and to look for rapid changes of observables. It is, therefore, absolutely crucial that the design luminosity can be achieved comfortably over a wide range of beam energies, e.g. from SNN= 4 GeV to SNN= 7GeV.

The option of increasing the magnetic rigidity of the collider rings from 30 Tm to 48 Tm, mentioned in the accelerator concept of group 1, is attractive. The collision of ions at 5 AGeV (SNN= 12 GeV), equivalent to 70 AGeV on a fixed target, would open the perspective for charm measurements, as the production cross sections for J/ψ and D mesons in central Au+Au collisions increase by about two orders of magnitude when beam energy goes from 20 AGeV to 70AGeV.

If the accelerator R&D work does not convincingly demonstrate that the design luminosity of the collider can be reached comfortably, a fallback option could be a fixed target experiment with heavy-ion beams having energies of up to about 10AGeV, possibly using one of the collider rings as an accelerator as sketched later in this report.

NICA/MPDFacility

A major concern with the proposals is the lack of a clear roadmap to achieve the declared average luminosity of 1027 cm-2s-1. This luminosity is regarded as being the bare minimum for a collider with 2.5 GeV/u on 2.5 GeV/u beams of mass number around 200 to produce viable conditions for a physics experiment. In order to achieve this performance it will be necessary to raise the injection energy of the Nuclotron, i.e. incorporate a booster to take the ions from 5 MeV/uup to about 50 MeV/u. The machine mentioned in the proposal of Accelerator group IIis based on superconducting magnets with an advertised 5Hz operation and would require considerable further study to be convincing. Another problem is that the Nuclotron has a ratherpoor vacuum, and even if the pumping is massively improved it would seem to be absolutely vital that the booster be carefully designed to provide exceptionally good vacuum. A static residual gas pressure of the order of 10-12mbar (rather than the quoted 10-9mbar) will be required to achieve the satisfactory acceleration of low charge state heavy ions, due to the large cross section for charge exchange in collisions with residual gas ions. Another item that will be essential in the quest for high intensity is a beam cooling system in the Nuclotron and/or in the collider. An electron cooler is mentioned in the proposal, but it has yet to be demonstrated that such cooling in the Nuclotron can work with low charge state heavy ions. A solution may be to use an electron cooler with fully stripped ions at an energy of 500MeV/u in the collider ring.

There is general agreement that the proposed EBIS source would be appropriate for the project, on condition that the scaling law of available ion intensity is proportional to the cube of the strength of the magnetic field of the solenoid, up to 6 tesla. This performance is essential for the success of the whole scenario. The idea of using the poly-cylindrical cavity (PCC) option (rather than an RFQ) for the next stage was questioned. The most critical issue of the proposed linac system is that of space charge repulsion defocusing effects in the lowest energy section. The designed peak current reaches 5 mA, which is almost an order of magnitude larger than that in the presently operating heavy ion RFQ linac. It is obvious that the longitudinal and transverse focusing forces, whether the type be RFQ or PCC, must be carefully included at the design stage. In Russia several institutions are at the forefront in the development of RFQs and other alternative typesof linac, including the development of simulation codes. We suggest that JINR start collaborative work with other institutions on the design of the linear accelerator system.

The expected realistic (as opposed to theoretical) efficiencies, giving rise to beam losses at each stage of the chain of acceleration, were not presented. There was no mention of how to handle beam losses in the collider, and how the experiment could be effectively shielded.The internally-cooled superconductor of the type used in the Nuclotronis a viable option for fast-cycling use, but it is less obvious that it would be the right choice for the magnets of the collider, which should be capable of producing a high field of excellent quality. Both of the accelerator groups propose rings of rather small circumference. The accelerator and detector groups do not yet appear to have discussed the issues of geometry of beam crossing and local focusing. The RF does not figure in the layout, and there seems to be little space for additional equipment that may be requested in the future, such as Siberian snakes for polarization. With regard to beam dumping, we were told at the meeting of an idea to extract though the incoming beam transfer lines, but this will clearly need more study.

The collider experiments are evidently at a very early stage of conception. The Committee tended to favor the second proposal, based on the use of TPCs and featuring a completely open geometry. The advisability of using TRD in the detector was seriously questioned, however.

As regards cost, it is difficult to make a useful judgment at such an early stage of conceptual design. A number of identifiably costly items were absent from the lists and others, for example the cost of the booster, seemed to us to be obviously very optimistic.The overall order of magnitude is possibly correct, but it can be expected that in going from such a conceptual design to a technical design the cost estimate would increase by at least 50%. It is also important to state both currency and time (e.g. in “2006” roubles) for projects that span several years, so that inflation can be taken into account for future payments. With such adjustments to the costs, the stated manpower requirements for the accelerator appear to be about correct. For the experiments we would expect that simulation studies will require more effort than is currently foreseen.

The timescale presented is matched to the requirement for the declared collider experiment to produce results before competing experiments elsewhere, but the viability of the project depends on several as yet unproven issues of accelerator theory and technology. To solve these problems in a timely fashion will require a clear, systematic and sustained effort. A roadmap describing the steps that such an effort will need to follow was not presented.

Recommendations

The collider option as such calls for a well-defined, competitive physics-based motivation yet to be formulated with more arguments. As it is apparent that achieving a luminosity of 1027 cm-2s-1 is crucial for the success of the proposed experiments, it would be useful to look into the possibility of an enlarged or backup program, based perhaps on the use of polarized beams, that could usefully exploit less intense beams. It is most important that at no stage should implementation of the collider option preclude fixed-target heavy-ion experiments. Colliding beam and fixed target physics studies must be run in parallel. Indeed, the first step should be to improve the performance of the Nuclotron and to provide the maximum possible energy and intensity for heavy-ion beams to be used for fixed target experiments. In short, it is recommendedto put a strong effort into enlarging the scope of experiments that can be performed takingrealistic expectations of performance of the accelerator facility, and iterating according to the outcome of the accelerator R&D.

The booster appears to be essential. The multi-turn injection process into the booster and the particle transfer from booster into the Nuclotron require detailed consideration with computer simulation in order to verify that the expected beam intensity is feasible. This study should also address the space charge problems. It will be important to make realistic estimates for beam losses at each stage of the chain of acceleration.

In order to reach the required luminosity the use of beam cooling is absolutely essential. The implementation of cooling in the proposed accelerator chain must be studied in detail and appropriate tests made to verify its viability.The factor giving the most cause for concern in the collider specification is the statement that the injected intensity in a collider ring is stably guaranteed as 2.5 109 per bunch, or 5 1010 per ring, with good emittance and momentum spread. In the proposed scheme, it is assumed that the Nuclotron could provide this number of ions at each acceleration cycle. Is this a reasonable assumption, even if the booster is included in the Nuclotron chain? If the intensity of the injected beam is reduced by one order of magnitude, the luminosity will be reducedby two orders of magnitude. It is quite likely that the intensity of fully stripped (or almost fully stripped) uranium ions in the Nuclotron will be found to be lessafter careful investigation of the many factors of beam loss in the process of Nuclotron acceleration. The safest way to avoid this problem would be to include an accumulation function of (fully stripped) ions somewhere in the accelerator chain, such as is done for LEIR at the LHC. An accumulator ring between the Nuclotron and the collider could, for example, serve this purpose. This ring could be operated to provide fully stripped ions up to the space charge limit byincorporation of an electron cooler.

At this stage of the study it would be sensible to make the collider ring as large as is reasonably possible for installation in the hall, taking into account the shielding requirements as well as those of the insertions for the experiment, the RF and eventual ancillary equipment that may be needed (e.g. for polarizing the beams). Having empty straight sections is not costly, but having to squeeze as yet unforeseen equipment into a small space can be very costly – and perhaps impossible.