Design of VHF system in J-PARC main ring

A beam power of 50 kW was attained for user operation by slow extraction from the J-PARC main ring (MR). One of the scheduled schemes to achieve the target beam power of more than 100 kW is increasing the beam intensity. As the beam intensity increases, the beam loss induced by the beam instability during the debunching process becomes enhanced. One solution is introduction of a dedicated cavity with the resonance frequency of 117.95 MHz. This very high frequency (VHF) is selected as the integer multiple of the beam revolution frequency. In addition, sinusoidal phase modulation is applied to the input RF power. The VHF system blows the longitudinal emittance up, leading to the suppression of beam instability. (author)