ms » ge; 003,1% EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH · . , com .gtk 3 6 J O CERN - PS DIVISION CERN LIBRARIES, GENEVA llIlllllIllllllllllllllllllllll °"““`””’““” "`“’ CERN-PS-96-003 STOCHASTIC COOLING F. Caspers Abstract Examples of stochastic cooling (S.C.) systems which are operational in several accelerator laboratories are discussed. The status of new systems under construction (GSI Darmstadt, KFA Jiilich) is outlined in the subsequent chapters. For bunched beam stochastic cooling (BBSC) the results of tests on large machines like the CERN SPS and FNAL TEVATRON are compared with data obtained from smaller rings (AC and LEAR). This technique (BBSC), if found to be applicable reliably to large rings, may be considered for future projects at BNL and DESY. Since technological issues play an important role a separate chapter gives a status of the UHV compatible materials, microwave amplifiers, signal treatment and the possible impact of high temperature superconductors for stochastic cooling. A chapter on theoretical aspects outlines aperture limitations of microwave pick-ups for high Y-beams. Projects focus in particular on S.C. in the RIKEN RI machine for special radioac-tive ions. A new technique, the "Optical Stochastic Cooling", has a potential for beam cooling provided it can be made operational and reliable. Paper presented at the Workshop on Crystallization and Related Topics Erice, Italy, 11-21.11.1995 Geneva, Switzerland 24/1/% OCR Output STOCHASTIC COOLING F. CASPERS PS Division, CERN 1211 Geneva 23, Switzerland ABSTRACT Examples of stochastic cooling (S.C.) systems which are operational in several accelera— tor laboratories are discussed. 'I`he status of new systems under construction (GSI Darmstadt, KFA Jiilich) is outlined in the subsequent chapters. For bunched beam stochastic cooling (BBSC) the results of tests on large machines like the CERN SPS and FNAL TEVATRON are compared with data obtained from smaller rings (AC and LEAR). This technique (BBSC), if found to be applicable reliably to large rings, may be considered for future projects at BNL and DESY. Since technological issues play an important role a separate chapter gives a status of the UHV compatible materials, microwave amplifiers, signal treatment and the possible impact of high temperature superconductors for stochastic cooling. A chapter on theoretical aspects outlines aperture limitations of microwave pick ups for high y·beams. Projects focus in particular on S.C. in the RIKEN RI machine for special radioactive ions. A new technique, the "Optical Stochastic Cooling", has a potential for beam cooling provided it can be made operational and reliable. Introduction The stochastic cooling technique has been shown to be operational for nearly 20 years in the ISR and the experimental cooling ring ICE at CERN. Since then it has been sucessfully used in several machines at CERN, in the INS (Tokyo) and at FERMILAB and has contributed to the discovery of the W and Z particles by its ability to increase phase-space density beyond a factor of 10’ in several steps. Originally applied to antiprotons where it really became one of the workhorses for p accumulation and storage, it can now be considered for a wider range of applications such as protons and heavy ions. The theoretical aspects for coasting beam stochastic cooling are rather well understood and in line with the technological progress. The bandwidth of the S.C. systems has increased by more than one order of magnitude to the several GI-Iz we have now. Despite its complexity S.C. was found to be a reliable tool for phase-space reduction of "warm" particle beams. However, for cooling of "cool" beams to "co1d", electron cooling appears more favorable. 1. Review of Existing Systems 1.1. Stochastic Cooling in the AAC The peak performance and the operational values (1990) compared with the design data are shown in Table 1. The daily stacking rate is the average during stacking periods and takes into account the fluctuations of the PSB, PS and AAC machines' (AAC = Antiproton Accumulator Complex). OCR Output Table 1 - AAC Performance 1988 1989 and afterwards DESIGN I OPERATION I OPERATION I PEAK Production beam (ppp) | 1.0 x 1013 | 1.35 x 1013 I 1.45 x 101 Repetition period 2.4 s 4.8 s 4.8 s Yacm (5/p) 10.0 x 106 I 5.7 x 106 I 5.4 x 106 I 5.8 X 10· 5/pmse 5.0 X 107 I 4.9 X 107 I 7.0 X 107 I 7.7 X 107 Stacking rate (1010/h) 7.5 3.6 5.3 6.13 (13.10.90) Daily production (1011) | 10 6.0 8.5 11.5 Daily stack. rate 3.3 4.4 5.16 (1010/h) Stack. intensity max. I Lg X 1012 I 0_g5 X H)12 I Log X 1012 I 1.31 x 101 (9.10.89) TI‘&HSVCI'SC €II1.1tta!1CCS I 1_2n 2_3n 2_3n 95% of beam Total efficiency 50% 63% 91% 93% The 1-3 GHz AC stochastic cooling system in which moving pick-up and kicker electrodes accompany the beam as it s11rinks, was identified as a critical item in achieving the design performance of the AAC, even when using a 4.8 s instead of a 2.4 s cycle as originally plarmed‘. This cooling system is subdivided into three hequency bands (1 1.65,1.65-2.4 and 2.4-3.1 GHz). It consists of 9 pick-up and kicker tanks each operating in one of the frequency bands and in Ap/p as well as in the transverse plane. Despite having cryo·pick-up structures and preampliiiers the available power (about 10 kW for CW signals) limits the gain to values below optimum during most of the cycle. Several improvements were introduced during 1988 and 1989. Additional cryo-cooling of the pick-up loops and combiner boards. Periodic filters (power saver Hlters) for certain betatron systems to reduce the noise power transmitted to the kickers. Two-stage momentum cooling filters. A dynamic phase correction as function of pick-up and kicker positions. Additional power for band 3 (2.4- 3.1 GHz). Improved pick-up and kicker movement versus time. The first of these improvements was the most expensive and complex, but also the most effective. By lowering the temperature from 100 K to 30 K the thermal noise was OCR Output reduced by 4 dB, and acceptable transverse emittances and efficiencies were obtained after 2.4 s. But also the 4.8 s cycle was improved (Table 2). Table 2 - AC stochastic cooling performances 4.8 s 2.4 s Env | e (4 eVs) (mm.mrad) | (4 eVs) | (mm.mrad) May 88 ~151t I 68% 301t 35% May 89 411: 92% 13R 70% Improvements were also needed for the AA precooling system (momentum and vertical): A second frequency band was added: 1.6-2.4 GHz (initially only 0.8 to 1.6 GI·lz). Preamplifiers with better noise figures. Damping of propagating TE-modes in the vacuum chamber by ferrite material (to avoid coupling to the stack ) Nevertheless the precooling and the stack-tail systems are not sufficiently fast to digest the 5 with adequate efficiency for the 2.4 s cycle, while the precooling and the stack tail systems do not limit the efficiency with a 4.8 s cycle. As for the AC, the AA precooling gain is limited below the optimum condition by thermal noise and available power. The cheapest way to obtain sufficient gain and speed would be to add cryogenic cooling to the AA precooling pick-ups. Even so, barely 10% could be gained in overall daily production rate, since the number of production cycles per hour available from the PS using 2.4 s cycles only increases by a factor of typically 1.5 to 1.7 and not 2 due to other users (LEAR, SPS, East HALL, LEP). For this reason (and others such as budget and manpower) the cryogenic pick-ups in the AA were abandoned and hence also the 2.4 s cycle operation. In the years following the improvements (until end of 95) stacks around 10*2 have regularly been obtained and stacking efficiencies beyond 80%. Presently the stacking rate is limited to 3.5 x l0"’/h due to modifications on the proton beam and due to a reduced yield (magnetic horn) of 4.5 x IO" 1.2. Stochastic C00ling at LEAR The LEAR stochastic cooling systems consist of two chains, each working in all three planes, to cover two ranges of particle momentum in the machine. The high momentum system can be adjusted for a momentum range from 200 MeV/c to 2 GeV/c (protons and antiprotons) corresponding to a variation of particle velocity B = vlc by a factor of 4.35. The low momentum system is adjustable from 105 MeV/c to 61 MeV/c’. Figure 1 illustrates the configuration used in 1993. Over the years (beginning in 1986) the position of stochastic cooling pick-ups and kickers has been changed together with with the type of OCR Output coupling structure. Initially the longitudinal cooling systems had ferrite ring pick-ups (24 rings each) and kickers of the same type. These ferrites were outside the vacuum (outgassing) on a section of a ceramic vacuum chamber. With these units the usable frequency range extended from 30 MHz to 200 MHz showing a rather poor phase response (dispersion). However, at that time only a momentum cooling (high energy) range from Ap/p = 0.6% was required. _\_i mlow m *'“¤95 KCV·‘2 -·»·»~ G - Kcv-zi l KCM`u:¤\`\ Q ·°» g!,,,, xcm-zi i X2 1 .