J-PARC加速器のビーム結合インピーダンスの影響と対策

抄録

High-intensity proton beams generate large electromagnetic fields, the effects of which are expressed as the wake fields W(t) in the time domain and the beam coupling impedances Z(ω) in the frequency domain. The fields affect the beam energy (beam loading etc.), the collective beam instabilities in longitudinal and transverse (horizontal or vertical or both) directions, and component heating. The coupling impedances were estimated in advance to the construction and beam instabilities were calculated (impedance budget). The ceramic ducts in the magnets of RCS (Rapid Cycling Synchrotron), kickers in RCS and MR (Main Ring Synchrotron), and stainless-steel vacuum ducts in MR were considered as large impedance sources. Stretched wire methods were also performed for the ceramic ducts and kickers. Transverse instabilities were observed both in RCS and MR under low chromaticity operation. The transverse instabilities in RCS mainly driven by the FX kickers are controlled by chromaticity and betatron tune. Recently kicker impedance reduction by diode and resister units is also implemented. The transverse instabilities in MR mainly driven by the resistive wall of the vacuum ducts and the kickers are controlled by chromaticity and an intra-bunch feedback system. Electron clouds have also been investigated in RCS and MR.