Rapidly Tuned Buncher Structure for the los Alamos Proton Storage Ring (PSR)

In the PSR's short-bunch operating mode, accumulated beam currents are intense and change rapidly. The resonant frequency of the 503.125-MHz buncher used in this mode must be rapidly adjusted through a 100-kHz range to maintain the correct 90° phase relation between cavity voltage and beam current. Modulation rates are up to 3 kHz/μs. Each structure consists of two side-coupled buncher cavities, resonantly coupled to a ferrite-loaded tuner cavity. The needed frequency change Δf in the buncher cells is produced by a 50 × Δf change in the tuner, accomplished by varying a magnetic field applied to the ferrite perpendicular to the rf magnetic field. Fast modulation of this bias is provided by a low-inductance ferritecore magnet excited by a special function generator. The resonantly coupled multicavity structure configuration allows buncher and tuner cells to be independently optimized for their specific functions. This paper describes the buncher design, ferrite selection, and test results from a prototype ferrite-loaded tuner cavity. The tests have demonstrated the tuning scheme's feasibility, showing that the necessary 5-MHz range can be attained with only 12% of the tuner cell filled with ferrite, and that losses in the ferrite are small throughout this frequency interval.