Study on the fringe field effect of quadrupoles for CSNS MEBT

Background Medium energy beam transport (MEBT) of the China Spallation Neutron Source matches the beam from RFQ to DTL. It is found in the beam commissioning that the fringe field effect of quadrupoles in MEBT has a great influence on matching.
Purpose The aspect ratio of the quadrupoles is 1.67, which induces a strong fringing field effect, and the hard-edge model used in the design needs to be improved.
Methods The slicing model was performed, and the accuracy of the quadrupole model was improved. Furthermore, to use the improved model online, based on the premise that the integral field of the quadrupole magnet remains unchanged, an equivalent transfer matrix was worked out to make the beam parameters consistent with the slicing model in the simulation.
Results and conclusions In the machine study, with the new model, the beam parameters at the RFQ exit were measured, which is much closer to the RFQ design value. The MEBT lattice was redesigned based on the equivalent transfer matrix. With the new matched MEBT, the transmission rate of DTL reached 99.5% in the beam commissioning, which is increased by a factor of 3%.