Stochastic Cooling System for Antiproton Accumulation in the RESR

The main purpose of the RESR storage ring in the FAIR project is the accumulation of high intensity antiproton stacks, mainly for the HESR storage ring, but also for the deceleration in the NESR storage ring which provides antiprotons for low energy experiments in the FLAIR experimental area. The high efficiency of antiproton production required for high luminosity experiments in the HESR is achieved by combining the large acceptance Collector Ring (CR) with the accumulator ring RESR. The first ring after the antiproton production target (CR) can be designed for large acceptance and for operation of a dedicated stochastic pre-cooling system. Concerning the accumulator ring RESR, this concept results in moderate requirements for the acceptance needed for injection of the secondary beam and gives more freedom for the design of an ion optical lattice which supports the accumulation process. First considerations to use a barrier bucket system for the accumulation revealed problems at high intensity operation of the stochastic cooling system, when up to 1×10 antiprotons are accumulated. Consequently, it was decided to design a system for longitudinal accumulation similar to the one of the AA ring [1] at CERN or the FNAL Accumulator Ring [2]. The ion optical lattice of the RESR provides the large momentum acceptance necessary for the accumulation by coordinated longitudinal manipulations with the rf system and the stochastic cooling system. Main feature of the RESR lattice is a large flexibility in the choice of the transition energy (variable in the range 3.3 t 6.4 for a fixed position of the cooling pick-ups and kickers) which is achieved by tuning of the focussing magnets. Additional ring lattice requirements come from the stochastic cooling system. The electrodes should be installed in positions with small vertical beta function. Electrodes with a small vertical gap have a high sensitivity for the beam and a small extension of the field in radial direction. This allows precise control of the electric field with a radial distribution matched to the requirements of accumulation. The accumulation technique involves the injection of the pre-cooled antiproton bunches from the collector ring CR on an inner orbit with a momentum offset of !p/p = -0.8 % with respect to the central orbit. The fresh batch is accelerated by rf to the deposit orbit (!p/p = 0.0 %) close to the central orbit. On the deposit orbit, the combined action of two tail cooling systems acts on the beam driving the particles with an exponentially decreasing gain to an outer orbit. The particles finally end on the core orbit which has a momentum offset !p/p = +0.8 % relative to the central orbit. At the core position an intense stack of up to 1×10 antiprotons is built up and finally cooled by the core part of the stochastic cooling system. The tail cooling systems operate in the frequency band 1-2 GHz, the core cooling system in the band 2-4 GHz. For transfer, an adjustable fraction of the stack is decelerated with rf to the injection orbit and extracted to the subsequent ring. The effect of the stochastic cooling system on the antiprotons was studied with two different simulations codes using the Fokker-Planck equation in longitudinal phase space [3][4]. The main goal was the optimization of the system parameters for the required fast longitudinal momentum change. Important parameters are the geometry of the electrode system, the required rf power, special delays and filters to shape the correction signal. The effect of the various parameters on the cooling performance was studied and optimized with the simulation code.