A Novel RF Cavity Tuning Feedback Scheme for Heavy Beam Loading

The beam-cavity instability threshold as given by the Robinson1 criterion for the open loop case is often improved by phase, amplitude and tuning loops. The improvement obtained is, however, limited by cross coupling as the transfer functions vary with the relative beam loading. A more powerful scheme is fast additive feedback around the final amplifier stage. With this scheme beam loading several orders of magnitude above the Robinson threshold have been achieved in the CERN ISR and AA in the absence of fast tuning loops. A novel scheme to include a tuning loop insensitive to relative beam loading is presented, and first test results from the application of this scheme to the CERN PS booster second harmonic cavity are given.