Step-like field magnets to transform beam distribution at the CSNS target

The method of using step-like field magnets to obtain a uniform beam distribution at the CSNS target has been studied. This is important to prolong the lifetime of the target vessel and the proton beam window, and to improve the beam utilization. To be effective for the non-linear magnets, the phase advance between the magnets and the target should be close to an integer multiple of π. The derivations for the phase advance requirement are also given. The two different core beam emittances at the CSNS-I and the CSNS-II are used in the simulations. The simulations show that the method has a good flexibility and that the reduction on the maximum current density at the CSNS target by more than a factor of two can be obtained. The magnets also help to correct the beam center displacement at the target caused by the jitter or misfiring of the ring extraction kickers, and some simulation results are presented.