Demonstration of beam loading compensation system for discrete beam with comb-like structure in proton linear accelerator

Abstract

In the Japan Proton Accelerator Research Complex (J-PARC) linear accelerator (LINAC), certain macro pulses of a beam with a frequency of 25 Hz have a discrete comb-like structure (called an intermediate pulse), which is a typical period of 0.815 ns and a width of 0.456 ns. The ability to change of the beam structure has been a useful tool in adjusting beam current and in the various beam studies. However, the digital feedback (DFB) and feedforward (DFF) system controlling the radio frequency (RF) field in the acceleration cavity in LINAC has not been able to accommodate this change. Further, the momentum of the injection beam within the macro pulses varies, which can cause beam loss in the downstream accelerators and beam transport lines. Therefore, in this study, we established a chop pulse gate signal distribution system and its signal that contains information on the pattern of the intermediate pulses is sent to each DFB and DFF system. In contrast to the current system, which compensates the beam loading in an approximate rectangular shape without considering the intermediate pulses (FF_BEAM), we developed the beam loading compensation system by the same structure as the discrete beam by the external chop pulse gate signal; it was labeled FF_BEAM_chop. The experiment was carried out in the 324-MHz stations using a beam current of 25 mA, which is half the current of the usual beam. When the beam pattern was changed via thinnings where it means reduction of the number of the intermediate pulses, the FF_BEAM_chop method significantly suppressed the decrease in the amplitude and phase stabilities compared to the FF_BEAM method. When the widths of the intermediate pulses were changed, the amplitude and phase stabilities generated by the FF_BEAM method deteriorated, but those generated by the FF_BEAM_chop method exhibited almost no deterioration. This demonstrates that FF_BEAM_chop is advantageous for the beam loading compensation of the discrete beam. The adaption of this demonstrated FF_BEAM_chop can be expected to suppress the momentum variation within the macro pulse, to reduce beam loss, and to help the beam commissioning.