PRECISE CAVITY TUNING SYSTEM OF A LOW OUTPUT-IMPEDANCE SECOND-HARMONIC CAVITY AT ISIS

A very low output-impedance (~35 ohms) second-harmonic cavity system is being developed for high intensity proton accelerators [1]. The final amplifier is comprised of a grounded cathode scheme with a feedback loop from anode to grid. Due to the Miller effect, the grid voltage waveform is seriously distorted even if only a few percent of sub-harmonic or higher harmonic are mixed in the driver current. Such distortion is much enhanced by the beam loading. In order to eliminate the effect of this distortion upon the phase detector used to achieve precise cavity tuning, a swept bandpass filter was applied to the grid voltage at the phase detector input. Filter design details and the result of high power tests are reported.