Evaluation of dispersion functions derived from fluctuation of the bending magnetic field of a synchrotron

The purpose of this research was to evaluate a dispersion function derived from fluctuation of the bending magnetic field of a synchrotron (BM-derived dispersion) for correcting positional drift of the irradiated beam in synchrotron-based particle beam therapy systems. At the exit of a synchrotron, BM-derived dispersion is proportional to the dispersion function derived from acceleration frequency (RF-derived dispersion), and the proportionality constant of the two types of dispersions depends only on the momentum compaction factor of the synchrotron and the Lorentz factor of the extracted beam theoretically. The two types of dispersions were measured at a carbon-ion therapy facility, and the measured proportionality constant agrees with the theoretical value at accuracy of 16%. Correcting the BM-derived dispersion at the isocenter effectively reduces the variation of the irradiating-beam position caused by momentum variation due to the magnetic aftereffect.