Gastaldi, Ugo

Kilian, K

Möhl, D

CERN Document Server

DECELERATION OF ANTIPROTONS FOR PHYSICS EXPERIMENTS AT LOW ENERGY (A LOW ENERGY ANTIPROTON FACTORY) K.Kilian Institut für Kernphysik, Germany U.Gastaldi, D.Möhl CERN, Geneva, Switzerland 1. Introduction Experiments using low energy antiprotons at the CERN PS/1/ and else¬ where/2/ are limited by the rates and the momentum resolution obtainable in stan-dard beams (Table 1). The aim of this paper is to work out the relative merits of a small 4 to 0.1 GeV/c antiproton decelerator synchrotron - similar to the "shuttle ring"/3/ proposed to speed up phase-space cooling of beams for the SPS. We conclude that even without cooling a large factor in intensity of low energy beams could be gained. If in addition (as proposed by Rubbia/3/ and first consi¬ derated in the present context by Skrinsky et al./4/) deceleration is combined with cooling, another large improvement in beam density would result. This could open quite new possibilities of fixed target experiments with sharply collimated beams. Furthermore one can hope for luminosities which make high precision —p and -A experiments/5/ with stored beams possible. 2. Antiproton production at the PS In order to compare a standard beam line and a decelerator, we estimate the production by 23 GeV/c protons impinging on a lead target (Fig. 1). As. usual, the rate is given per interacting proton. Here we normalize to a phase-space volume v = [1msr| × [±1%( p/p)] since for this choice the rate accepted by a magnetic beam line, which has constant v independent of momentum, is directly proportional to the reading on the curve. Fig. 1. Production of antiprotons at 0° on lead by 23 GeV/c protons/6/. The number of per interacting proton is normalized to 1 msr solid angle and ±1% momentum bite. Below 4 GeV/c the curve is estimated by "ki-nematic reflection"/7/. The straight line shows the expected phase-space density of antiprotons (v p2) in a deceleration syn-chrotron (without bunch rotation or phase-space cooling). 179 The values above 4 GeV/c in Fig. 1 are measured/6/. Since, to our know-ledge, there are no comparable low-energy measurements available, the correspon-ding branch of the curve is calculated assuming symmetry of the production as in the nucleon-nucleon centre-of-mass system. This approximation, known as the "reflection" method/7/, is exact for a hydrogen target. In the standard beam (Table 1), the are collected at an energy close to the working energy of the experiment. The expected yield is therefore given by the dashed line of Fig. 1. In order to get reasonable rates at low momentum one has to accept the largest possible phase-space volume. This results in well-known experi-mental problems : beam lines and spectrometers become bulky and expensive and tar-gets have to be large in volume since one has to accept big beam cross-sections, big divergencies and large momentum spreads. The detector systems have to match the large targets in size and complexity. The poor separation and the big target-and detector volumes can cause severe background problems. The lower the energy the more difficult it becomes to find a compromise between intensity and beam quality. In the decelerator, antiprotons are captured at high momentum where den-sity and rate are optimal. During deceleration the horizontal and vertical beam Table 1 : Some low energy beams Beam k4 CERN k18 Brookhaven C2,4 Target Beryllium (internal) Beryllium (internal) Beryllium (internal) Iridium (external) Production angle (mrad) 260 300 360 180 Proton Energy (GeV) 19.2 23 23 28 Protons on target 1011 7 × 1011 6 × 1011 1012 Solid angle ( ) 0.50 3.3 6.4 2.65 Momentum bite (±%) 1 1.5 1.8 1 flux per burst at : 1.2 GeV/c 220 1.0 GeV/c 64 0.9 GeV/c 1800 6000 0.8 GeV/c 16 0.75 GeV/c 7500 0.7 GeV/c 6.8 0.6 GeV/c 1.8 emittances Eh,Ev and hence √Eh Ev increase as 1/p. The same is true for the relative momentum spread. Hence, ideally, the phase-space density N/v will decrease like p 2/p 2 i n j e c t i o n. This is shown as the solid line in Fig. 1. Actually by RF gymnastics and/or by phase-space cooling, this loss of den­sity can be reduced or overcome. How­ever it is concluded from Fig. 1 that even without these techniques, the decelerator compares favorably at low momentum. 3. Some features of the decelerator The decelerator tailormade to our purpose is a, say, 4 to 0.1 GeV/c synchrotron with characteristics similar to the rings which have been proposed for the ISR/9/ or the SPS/3/ : large acceptance; powerful RF system for debunching (bunch rotation) and deceleration; possibly stochastic precooling and postcooling by electrons (as suggested by Rubbia) / 3 /. 180 Clearly the "shuttle ring"/3/ proposed as part of a dense antiproton source for the CERN SPS can be used for our purpose if extraction into an expe-rimental area is incorporated. Some typical parameters (from ref. 3) are given in Table 2. Note that this ring can deliver ~ 2-5 × 107 per PS pulse (1013 pro-tons) at all energies between 3.5 and 0.4 GeV/c. Although the density decreases during the deceleration procedure, the number of antiprotons remains constant because the acceptance of the ring is much larger than the acceptance of the injection system. As a second example we assume that the experimental cooling ring (ICE) now under construction at CERN is converted into a decelerator at some later stage. This implies the installation of a conversion target in the beam line, the addition of RF and ejection equipment to the ICE ring as well as "pulsed" operation T a b l e 2 Examples of a decelerator Machine "Primitive Machine" Optimized Machine "ICE" "upgraded ICE" "shuttle ring " lower ener-gy shuttle" Radius (m) 12 12 25 25 Momentum(GeV/c) 2.1-0.3 3.2-0.1 3.5-0. 15 3.5-0.1 Injected beam *: p/p ± 3‰ ± 30‰ Eh (mrad. mm) 12π 28π E v (mrad. mm) 30π 28π p/p after bunch rota­tion + 1‰ ± 6‰ Acceptance inside ring: p/p ± 3‰ ± 40‰ Eh (mrad. mm) 80π 230π Ev (mrad. mm) 45π 230π Conversion /p *) **) 0.7×l0-7 1.5×l0-7 25×l0-7 Solid angle accepted*) (msterad) 1.3 2 No. of protons/pulse used 2×10 1 2 1013? 10 1 3 (1 bunch) (5 bunches) (5 bunches) No. of /pulse at: 3 GeV/c - 1.5×l06 2.5×l07 2 " 1.5×105 " " 1 " 1.3×l05 1.l×lO6 " 0.45 6×l04 3.5×105 " 0.3 " 3.5×104 1.5×l05 1.1×l07 0.1 " -1.6×l04 1.2×l06 Transition energy γtr 0.97 4.5 RF voltage for bunch rotation U(kV) 20 250 1000 *)Maximum values, smaller beams injected for deceleration to lowest ener­gies. **) 6 cm tungsten target, efficiency η = 0.3, quadrupole matching. 181 of the magnets (cycle of 10-30 seconds because the magnets are not laminated). Injecting the produced from one PS bunch (with 2 × 10 1 2 p) the expected yield is 1.5 × 105 /pulse for momenta between 2.1 and 1 GeV/c, decreasing to  3 × 104 at 0.3 MeV/c. The RF system needed for bunch rotation and deceleration is relatively modest and in fact a considerable gain in yield seems possible with a more powerful (250 kV) RF system together with a very fast kicker which permits injection of 5 bunches. The two cases considered suffice to illustrate that even without cooling an optimized decelerator might deliver a clean beam (free of pions and other back ground particles) of 107 /pulse down to 0.3 GeV/c (106 /pulse at 0.1 GeV/c) and a rather "primitive" machine seems capable of, say, 105 /pulse at 0.3 GeV/c or 104 at 0.1 GeV/c. Clearly this would open new possibilities for many of the experiments /1,2/. A difficulty in the decelerator is beam control (low intensity) 4. Deceleration and cooling Stochastic cooling/10/ prior to deceleration can "compress" the phase space such that the low energy beam still fits into the aperture. This eliminates the p2 loss (Fig. 1) in density (intensity), and ideally all that can be captu-red into the aperture at high energy can be decelerated. The characteristics of a momentum and emittance cooling system designed according to the criteria of ref.10 are given in Table 3. One notes that at intensities up to 107 /pulse, cooling times in the range of 2 to 20 s are within the realm of possibility. T a b l e 3 Stochastic cooling prior to deceleration* Circumference/2π of ring R = 12 m, cooling at p = 3 GeV/o. Momentum and hori­zontal emittance cooling by : 10 pairs of pick-up electrodes, 10 accelerating gaps (50 each), 10 amplifiers (500 MHz bandwidth, 60 W RF power, 250 nA rms noise current each). Vertical emittance cooling by : 10 pairs of pick-up elect­rodes, 10 pairs of deflector plates, 10 amplifiers (500 MHz bandwidth, 60 W RF power, 250 nA rms noise current each). Number of particles assumed N 107 Momentum spread p / p ( ‰ ) 1 5 Gain parameter (fraction of error corrected per passage) g Momentum & Ehcooling 0.005 0.001 Ev cooling 0.0025 0.0025 Cooling time (lower limit), (sec) Momentum 2 10 Eh cooling 4 20 Ev cooling 15 15 *) The amplifier system assumed is much more powerful than the one to be used in the ICE where cooling will take several minutes. 182 Once the beam has been decelerated, electron cooling/11/ which is very efficient at low energy and which is less sensitive to the number of seems then the "natural" ingredient to collimate the low energy beam (size < 10 mm, p/p < 10 - 3). These dense and clean beams would resolve all the problems of the stan¬ dard beam discussed above. In addition, new types of experiments become feasible/4/ for instance scattering of stored beams on an internal jet target, colliding -p experiments/5/ or experiments with "overlapping" and p beams at low energy which might prove to be a very powerful tool to investigate antiprotonic atoms. As an example, Table 4 gives some parameters of a system of "overlapping storage rings" which consists of an antiproton and a separate proton ring. In the "overlapping mode"/5/, the two beams travel parallel in the intersection region. This set-up should provide for a precision study of the -p system at low energies by investigating e.g. resonances near the threshold, emission and reso­nance spectra of the -p atom, quasinuclear bound states and annihilation from known initial states. 5. Conclusion Deceleration and/or cooling open new and exciting possibilities for antiprotons physics at low energy. Deceleration alone could largely improve the conditions of experiments which are presently under way. Addition of cooling would make a new class of experiments feasible. The "shuttle ring - freezer ring" system proposed as source for the SPS could be used for our purpose if extraction into an experimental area is incorporated. Alternatively a relatively simple decelera-tor ring with characteristics similar to the experimental cooling ring ICE or some T a b l e 4 A 0.3 GeV/c -p overlapping storage ring system (some tentative parameters) Radius, R = 5 m, (each ring) Lattice functions in interaction point:βv βh=1m, αp = 3.5 m Number Np = 2×1012 Emittances: Eh Ev = 100π mrad. mm, p/p = ±2°/oo Beam size (2σ) in interaction point: ap = ±10 mm Antiproton parameters: Number N = 1011 Emittances: Eh Ev<50π mrad.mm, p/p<2°/oo Beam size (2σ) in interaction point: a < 5 mm Length of overlapping region: ±0.50 m Luminosity (unbundled): 2×1028 v' c cm-2 sec-1 Typical velocity spread (2σ) in c.m. system: v' 3×10-3 c 183 other 4 GeV/c synchrotron to which stochastic and electron cooling is added could serve low energy physics. Both cooling techniques have been tried experimentally, however extra-polations made in this paper extend over several orders of magnitude. In addition, beam control at the expected intensity of the decelerator will be difficult. References 1. J.V. Allaby et al. (editors), Experiments at CERN, CERN Scientific Information Service report RD/171-4050 (1976); (Experiments S 135, S 142, S 149, S 151 use low energy beams). 2. See e.g. : M. Alston-Garnjost et al., Phys. Rev. let. 35 (1975) p.1685. A.S. Caroll et al., Phys. Rev. let. 32 (1974) p.247. 3. C. Rubbia, CERN/NP Note 77-1, to be published. D. Cline, P. Mc Intyre, F. Mills, C. Rubbia, Fermilab TM 689 (1976). C. Rubbia, P. Mc Intyre, D. Cline, in Proc. of Aachen Neutrino Conf. (1976). F. Bonaudi, S. van der Meer and B. Pope, CERN/DG-2 (1977). 4. VAPP-NAP Group (presented by A.N. Skrinsky), Electron cooling and new possibi-lities in elementary particle physics, paper presented at the Internat. Conf. on High Energy Physics, Tiblisii, 1976. 5. T. Kalogeropoulos, BNL/CRISP 72-11, ISABELLE Project (1972). U. Gastaldi, CERN/EP/Int. Report 76-23 (1976) and CERN-p Note 13 (1977). P. Dalpiaz, CERN-p Note 06 (1977). 6. D. Dekkers et al., Phys. Rev. 137 (1965) p.962. 7. C. Rubbia, CERN/EP/Int. Report 76-13 (1976). M. Zeller, L. Eosenson and R.E. Lanou Jr., BNL Report 16000 (1970) p.193. 8. P. Kalmus et al., CERN 71-25 (1971). W. Brückner et al., Proposal CERN/EEC-75/42. G. Bassompierre et al., Nuclear Instr. Methods 40 (1977), p.251, and Beams at U.S. High Energy Laboratories, BNL Report 22160 (1976). 9. K. Hübner et al., CERN/ISR/Workshop/F-4 - PS/DL/Note 76-27 (1976). 10. L. Thorndahl, CERN/ISR-RF/75-55 (1975). S. van der Meer, CERN/ISR-PO/72-31 (1972). 11. VAPP-NAP Group (Seminar at CERN on March 24, 1977, by Y. Derbenev, I. Meshkov), CERN 77-8, and the references given therein. 184 

Deceleration of antiprotons for physics experiments at low energy: A low energy antiproton factory

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