Design and beam dynamics validation of a spiral FFAG accelerator for CSNS-II

Abstract

The fixed-field alternating gradient (FFAG) accelerator, with its compact structure and the combined advantages of high energy from synchrotrons and high beam intensity from cyclotrons, offers unparalleled benefits in accelerating protons, heavy ions, and short-lived particles such as muons and unstable nuclei. The China Spallation Neutron Source plans to establish an FFAG accelerator with a diameter of 20 meters at the end of the negative hydrogen linac for various purposes, including experiments in nuclear physics and medical applications. For this design, a scaling scheme using spiral magnets as the basic focusing structure is investigated to provide the proton beam with kinetic energy ranging from 300 MeV to 600 MeV. We present the design results for the scaling FFAG accelerator and discuss the optimization of the cell tune. A detailed beam dynamics verification and discussion of the proposed FFAG accelerator lattice are presented using the ray-tracing code Zgoubi, including off-axis optical characteristics, large amplitude transverse motion, and full-cycle longitudinal acceleration. The FFAG accelerator is demonstrated to provide a large transverse and longitudinal acceptance for the tracking beam in the designed lattice. A new simulation code based on Python for the study of FFAG accelerator beam dynamics has been developed, and the corresponding verification results are also presented.