Stochastic Momentum Cooling of a Low Energy Beam at TARN

Abstract

storage ring which accumulates the beam both in the longitudinal and transverse phase spaces from the injector sector focusing cyclotron. This storage ring has been mainly used to develop the accelerator technology related to the future project of high energy ion accelerator at INS. Details of the pe;formance of beam accumulation are presented elsewhere. In the meanwhile the stochastic momentum cooling has been also performed in this ring for the low energy 7 MeV/u RF stacked protons and c( particles in view of future application of such a technique to the cooler ring for the study of precise nuclear and atomic physics experiments. The stochastic cooling of a low energy beam has been successfully tried at LEAR, CERN where they cool antiprotons, such as 50 or 175 MeV. However recently still lower energy beam, for example 5 MsV, is required for the nuclear experiments with nearly stopped antiprotons. In contrast with high energy beam cooling, two major experimental conditions should be attained. Firstly the beam life due to the residual gas scattering should be much longer than the cooling time. At the average pressure of around lo-l1 Torr, the beam life of protons and a. particles are several thousands seconds which is one order larger than the present cooling time of several hundreds seconds. Secondly the special attention should be paid to obtain the high coupling and high shunt impedance of the pickup and kicker as the particle's velocity is as low as 12 % of the light velocity. In the present experiment a travelling wave type coupler with a helical inner conductor has been used with a sufficient performance. The cooling system including these couplers and a notch filter operates successfully, and experiments have been carried out to measure cooling time, final momentum spread, etc. by varying system gain and signal's transmission time. For the analysis of experimental data, we improved the Sa;h;rer's formulation of a Fokker-Planck equation. ' In the improvement, coherent correction of particle energy is expressed explicitly in terms of the difference of particle-signal's transmission time from time-of-flight of the particle. This paper describes the experimental results on cooling time, final momentum spread, etc., in compared with our theory. The hardware of the cooling system is presented elsewhere.5