Source of instability in the rapid cycling synchrotron of the China spallation neutron source

Abstract

The rapid cycling synchrotron (RCS) of the China spallation neutron source accumulates 1.56 \(\times\) \(10^{13}\) protons and accelerates them from 80 MeV to 1.6 GeV at a repetition rate of 25 Hz, delivering a 100 kW proton beam to the target. Despite predictions that no instabilities would occur at high beam intensity, an unexpected coherent oscillation of the beam was observed during the beam commissioning. To confirm and address the instability, a series of systematic measurements and studies were conducted, revealing that the issue is a transverse coupled bunch instability and identifying a narrow-band impedance with a low frequency in the RCS. Further bench measurements confirmed that ceramic chambers with RF shields are the source of this impedance, which is consistent with the analysis of the experimental data. Based on these findings, a tune curve during the ramping process and chromaticity optimization have been implemented to completely mitigate the instability in the RCS at 100 kW.