Dynamic variation of chromaticity for beam instability mitigation in the 3-GeV RCS of J-PARC

Saha, P. K.   ; Hotchi, Hideaki  ; Shobuda, Yoshihiro   ; Harada, Hiroyuki  ; Tamura, Fumihiko   ; Watanabe, Yasuhiro  ; Takayanagi, Tomohiro   

In order to mitigate beam instability in the RCS caused by the impedances of the extraction kicker magnets, an application of dynamic variation of the chromaticity has been studied. The beam instability, especially for a high intensity beam delivered to the MR is one of the main issues in the RCS because of maintaining a smaller beam emittance as compared to that for the MLF beam. In this research, such a beam instability has been successfully mitigated by introducing dynamic variation of the chromaticity by using bipolar Sextupole magnetic fields. The method includes a partial chromaticity correction at lower beam energy to reduce the chromatic tune spread required to minimize the beam loss and the beam emittance, while introducing an excess of chromaticity at higher beam energy to utilize larger chromatic tune spread to enhance the Landau damping for beam instability mitigation. The method has been established based on detail numerical simulations by using ORBIT code and the corresponding experimental studies. The method has already been successfully implemented for the RCS beam operation to the MR.