Booster subharmonic RF capture design.

The studies presented here indicate that a subharmonic system optimized using both 3rd and 12th subharmonic cavities (117.3 and 29.3 MHz) provides the minimum total gap voltage required to capture and accelerate a 15-ns linac macropulse at up to 10 nC with {+-} 1% energy spread. The simulations also show particle losses less than a part in 110,000 when including radiation damping, quantum excitation, and rf cavity beam loading. The only particle losses were found to be at injection due to phase-space mismatch. Optimization of the subharmonic capture idea requires knowledge of the gap voltage that can be achieved with an actual cavity. In general the lower the cavity frequency the harder it is to engineer a cavity that can sustain a given gap voltage without breakdown. Phase and amplitude control of both subharmonic and 352-MHz systems also needs to be carefully considered so that bunch purity is preserved and operational flexibility maintained. Phase stability is particularly important for the high power 352-MHz system, which preliminary simulation studies show needs 1 degree regulation at turn-on. Future optimization studies need to include amplitude stability tolerance studies for each ramped rf system at turn on