Scanning magnets are applied to spot scanning procedures in proton therapy. Two separated dipole magnets are mostly used to deflect the beam in two orthogonal directions. Since the overall size of scanning magnets in beam axis is one main factor to determine gantry radius, it is desired to have a compact design. Recent years, a multipole scanning magnet has been proposed which can generate a circular magnet field with dynamic configuration of coil currents.
In this paper, the number of poles of the multipole magnet was chosen to be eight after some comparisons. The distribution of the transverse field of the octupole magnet was calculated and the influence of different shapes of pole tips on the quality of the magnetic field is analyzed in Opera-2D. The rotation performance of its magnetic field was also studied by changing the phase of current in coils both in Opera 2D and 3D. The quality of the integrated field was performed and the distribution of its relative harmonic field errors were calculated.

Harmonic field analysis, octupole scanning magnet, pole tip, proton therapy, static field calculation