A prototype 7.5 MHz Finemet(R) loaded RF cavity and 200kW amplifier for the Fermilab Proton Driver

A 7.5 MHz RF cavity and power amplifier have been built and tested at Fermilab as part of the proton Driver Design Study. The project goal was to achieve the highest possible 7.5 MHz accelerating gradient at 15 Hz with a 50% duty cycle. To reduce beam loading effects, a low shunt impedance (500Ω) design was chosen. The 46cm long single gap cavity uses 5 inductive cores, consisting of the nanocrystalline soft magnetic alloy Finemet, to achieve a peak accelerating voltage of 15 kV. The 95cm OD tape wound cores have been cut in half to increase the cavity Q and are cooled from both sides using large water-cooled copper heat sinks. The prototype cavity has a shunt impedance of 550Ω, Q=11, and is powered by a 200kW cw cathode driven tetrode amplifier. Both cavity and amplifier designs are described. Results from recent cavity tests coalescing beam in the Fermilab Main Injector are also presented