Simulation of an Octupole Scanning Magnet for Spot Scanning in Proton Therapy

Current proton therapy scanning systems always use two independent dipole magnets for spot scanning in proton therapy. However, the space occupied by these two dipole magnets located after the final gantry bending magnets is very large and increases the overall size of the gantry. In order to construct a compact nozzle and decrease the size of the gantry, we decide to design an octupole scanning magnet to replace these two separate dipole magnets. The octupole scanning magnet, which is completely different from traditional octupole magnet, can generate rotating dipole magnetic field with the change of the loaded sinusoidal current phases. In this paper, we have finished the static optimization of an octupole scanning magnet model, including the length and shape of the poles, the diameter of the gap, and the shims on the pole edges, both in Opera two-dimensional (2-D) and 3-D. The corresponding relationship between the size of the gap and the good field region was also studied. The effect of eddy currents on magnetic field stability was also simulated in Opera 3-D. All the simulation results are presented in this paper.