Chromatic correction for CSNS/RCS and nonlinear effects of chromaticity sextupoles

The rapid cycling synchrotron (RCS) of the China Spallation Neutron Source (CSNS) accumulates an 80 MeV injected beam, and accelerates 1.561013 particles to the design energy of 1.6 GeV, and extracts the high energy beam to the target. In order to keep low beam losses in the ring, it is necessary to control the chromaticity and to minimize the dependence of the optical properties on beam momentum spread. Three chromatic correction schemes are proposed and compared for CSNS/RCS in this paper. The combined effects of sextupole field and space charge are investigated by using the code SIMPSONS. The tune spread produced by chromaticity can be apparently reduced by using chromaticity sextupoles. However, the chromaticity sextupoles are non-linear elements, and can introduce an amplitude-dependent tuneshift. The combined effects of sextupole field and space charge can induce some beam loss.