Magnetic fields and beam optics studies of a 250 MeV superconducting proton radiotherapy cyclotron

A 250 MeV superconducting cyclotron for the proton radiation therapy was designed at the Michigan State University (MSU) for use at the Paul Scherrer Institut. This work was based on the conceptual design carried out at the MSU in 1994. The previous design was refined to finalize the magnet configuration and to optimally arrange cyclotron elements for the actual construction. The spiral angle of the pole was reduced, the new hill-edges and valley shims being introduced. The magnetic fields were highly isochronized using a least square fitting routine involving a schematic shimming scheme. The resulting reference field was adequate for the elaborate study of beam optics. The optics simulation predicted that extraction efficiency of above 80% was achievable for a beam with the initial phase width of 20°. The vertical deflector was investigated located in the central region to control the beam intensity with tracking of beam phase spaces. Some measurement results for the constructed cyclotron were found in a good agreement with those of the optics study.