Fast and Accurate Magnetic Field Shimming for A Compact Cyclotron

Isochronous magnetic field shimming was performed for a 10 MeV H^- cyclotron used for PET purpose. The shimming is de-composed into a sequence of separate small patches, and the isochronous field correction is approximated with the effects of a set of standard patches on the assumption that the effect of each small patch is proportional to its surface. The mapped magnetic field data is used to calibrate the B-H curve used for 3D TOSCA calculation to obtain a very precise FEM estimator with relative error less than 0.1%, thus the effect due to each standard patch is estimated precisely. Correction of the pole is found by solving a system of equations using the least square scheme. The shimming was performed and the measurement of the magnetic field was found in good agreement with the prediction, with error less than 2Gs; and the history RF phase slip within ± 4^° was achieved. The method proved to be of very high accuracy and thus no need of iteration.