Multiharmonic vector rf voltage control for wideband cavities driven by vacuum tube amplifiers in a rapid cycling synchrotron

Abstract

Beam loading compensation in the rf cavities is a key for acceleration of high-intensity beams in the 3 GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC). Since we employ wideband magnetic alloy rf cavities for the J-PARC RCS and the wake voltage contains several harmonics, multiharmonic beam loading compensation is required. The multiharmonic rf feedforward for the six most important harmonics is implemented in the existing low-level rf (LLRF) control system, which has been working fairly well for acceleration of high-intensity beams of up to 1 MW. However, we found degradation of the performance for compensation of the feedforward with very-high-intensity beams. The gain variation with output current of the tube amplifier affects the performance of the feedforward, since it has an open loop configuration. Also, there is a voltage waveform distortion caused by the distortion of the output current, which essentially cannot be compensated by the feedforward. Therefore, we consider employing the vector rf voltage feedback control for beam loading compensation, where the feedback is expected to work with the gain variation within the stability margin. A multiharmonic vector rf voltage control has been developed as the core part of the next-generation LLRF control system for RCS. Prior to the completion of the new system, the multiharmonic vector rf voltage control was tested with one of the 12 cavities. The details of the system configuration, commissioning methodology, and beam test results using very-high-intensity beams are described. Beam loading by the 1 MW equivalent beam in the cavity is successfully compensated.