Feed-forward control of the amplitude and the phase of a high-power RF pulse based on the overdrive technique

We have developed a new feed-forward control system to precisely control the amplitude and phase of the pulsed RF power in an electron linear accelerator (linac) to make the accelerating field constant. Fast variations and ripples in the amplitude and the phase in RF pulses of some-microsecond duration are compensated for by applying anti-variation control signals to a variable attenuator and a phase shifter in the low-level system. The system makes use of the overdrive technique, which is commonly used in analog circuits, to speed up the slow response of the analog phase shifter, while the control signals are digitally processed; thus, the method is a hybrid of analog and digital techniques. Peak-to-peak variations in the amplitude and phase are reduced by these means from 11.6% to 0.4% and from 6.1°to 0.3°, respectively, in 7.6-μs-long RF pulses for the L-band electron linac at Osaka University.