Evaluation of higher harmonics generated in acceleration gaps during the high power beam acceleration at J-PARC RCS

The J-PARC 3 GeV Rapid Cycling Synchrotron (RCS) employs wideband RF cavities. During the high power beam acceleration, the acceleration gap voltage includes not only the driving harmonics, but also other multiple harmonic components due to the wake field. The low level RF control system for the RCS demodulates the harmonic components up to the fourth harmonic (h = 8) of the accelerator gap voltage. The recent beam tracking simulation studies suggested that the higher harmonic components above h = 8 may affect the longitudinal beam behavior during the high power beam acceleration. In this study, we measured the acceleration gap voltage during the 1 MW beam operation, and analyzed the higher harmonic components existing in it. Longitudinal beam tracking simulation is carried out to evaluate the effect of the higher harmonic components on the longitudinal beam behavior. The result of tracking simulation shows that the higher harmonic components deform the potential and bunch shapes at the end of the 1 MW beam operation. (author)