Beam loss caused by edge focusing of injection bump magnets and its mitigation in the 3-GeV rapid cycling synchrotron of the Japan Proton Accelerator Research Complex

Hotchi, Hideaki  ; Tani, Norio  ; Watanabe, Yasuhiro  ; Harada, Hiroyuki  ; Kato, Shinichi; Okabe, Kota  ; Saha, P. K.   ; Tamura, Fumihiko   ; Yoshimoto, Masahiro  

In the J-PARC 3-GeV RCS, transverse injection painting is utilized not only to suppress space-charge induced beam loss but also to mitigate foil scattering beam loss during charge-exchange injection. The space-charge induced beam loss is well minimized by the combination of modest transverse painting and full longitudinal painting. But, for sufficiently mitigating the foil scattering beam loss, the transverse painting area has to be further expanded. However, such a wide-ranging transverse painting had not been realized until recently due to beta function beating caused by edge focusing of pulsed injection bump magnets. This beta function beating additionally excites random betatron resonances, causing significant extra beam loss when expanding the transverse painting area. To solve this issue, we newly installed pulse-type quadrupole correctors to compensate the beta function beating. This paper presents recent experimental results on this correction scheme, while discussing the beam loss and its mitigation mechanisms.