Study on the fringe field and the field interference effects of quadrupoles in the Rapid Cycling Synchrotron of the China Spallation Neutron Source

The Rapid Cycling Synchrotron (RCS) of the China Spallation Neutron Source (CSNS) accelerates beam from 80 MeV to 1.6 GeV to strike a solid metal target to produce spallation neutrons with a repetition rate of 25 Hz. In this kind of high bunch intensity synchrotrons, to suppress the space-charge effect, the large acceptance is always required. For CSNS, with the design acceptance of 540 pi mm.mrad, the apertures of quadrupoles are quite large, which are comparable to the effective magnet lengths. In this case, the fringe field effect cannot be neglected. For some quadrupoles, the interval between the quadrupole and its neighbor magnet is comparable to the aperture of quadrupole, and the effect of field interference need to be considered. The fringe field and field interference effects have been studied, and by including these effects, the lattice was re-matched based on slicing model of quadrupoles to include these effects. After the re-matched lattice was applied to the beam commissioning of the RCS of CSNS, the measured twiss parameters are very close to the design parameters.