J-PARC RCS. Effects of nonlinear field components of injection bump magnets on circulating beam

In the J-PARC 3 GeV Rapid Cycling Synchrotron, four sets of injection bump magnets (pulsed dipole magnets) have been employed for multi-turn charge-exchange injection over a period of 0.5 ms (∼307 turns). The injection bump magnets include significant nonlinear field components due to their large ratio of gap length to core length. The nonlinear field components, which locally exist in the injection section not following the lattice super-periodicity, have a significant influence on the circulating beam during the period of multi-turn injection through excitations of high-order random resonances. In this paper, we discuss the detailed mechanism of emittance growth and beam loss caused by the nonlinear field components of the injection bump magnets, including its correction scenario. (author)