Multiharmonic rf feedforward system for beam loading compensation in wide-band cavities of a rapid cycling synchrotron

For the acceleration of very high intensity protons in the 3 GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC), the beam loading compensation in the rf accelerating cavities is implemented. The magnetic-alloy (MA) loaded cavities, which have a wide-band frequency response, are employed in the J-PARC RCS. The wake voltage in the wide-band MA cavity consists of not only a component of the fundamental accelerating rf, but also the higher harmonics. We developed a multiharmonic feedforward compensation system, which handles the most important three harmonics (h = 2, 4, 6). The function of the multiharmonic feedforward is described. Adjustments of the amplitude and phase patterns for the cancellation of the wake voltage during the acceleration period are not trivial. We developed the commissioning methodology of the feedforward. We describe the methodology in the cases without and with driving the accelerating rf. The commissioning of the systems for 11 cavities in the RCS was successful. We describe the commissioning results and the beneficial effects of the feedforward compensation in the beam operation.