A Low Shunt Impedance 53 MHz RF System for RF Stacking in the Fermilab Antiproton Accumulator

In the Fermilab antiproton Accumulator Ring, injected antiprotons are to be adiabatically captured and rf stacked at the edge of the stochastic cooling stack tail. Also large antiproton bunches which have been removed from the cooled core by low-frequency rf unstacking are to be pre-bunched at 52.813 MHz prior to bunch-to-bucket transfer to the main Ring at 8 GeV. A 100 kV 53 MHz rf system for accomplishing these two tasks is described. Because of the low momentum spread beam to be stored in the ring, a shunt impedance limitation must be imposed on all devices in the ring to prevent microwave instability. Also vacuum pressure in the range 10-10 T must be maintained. This report will describe how a normally high Q rf structure has been modified to meet these requirements.