Study of Induced Eddy Current Effects on Field Homogeneity for Fast Ramped Bending Magnets

An investigation of induced eddy current effects on the field homogeneity in fast ramped bending magnets was presented in this paper. The transverse field homogeneity and integral field homogeneity were calculated based on two-dimensional/three-dimensional finite-element analysis. The results showed that the transverse field homogeneity and integral field homogeneity were less than 0.05% after the field optimization with static analysis. The fast ramped bending magnet is used to deliver a proton beam for a superconducting proton therapy facility. The energy of the proton beam is adjusted by a fast degrader device and it will require all the magnets in the beam line to follow the energy changes of the degrader as fast as possible. During the energy change stage, induced eddy currents will appear in the beam tube. Then, the transient finite-element analysis is presented at different current ramp rates k and beam tube thicknesses b. The induced eddy currents in the beam tube were analyzed to research the eddy current effects on field quality. It shows that the field quality will be influenced more obviously with increased current ramp rate or beam tube thickness. The maximal field decrease in the midplane caused by eddy currents in the beam tube is the main reason for field quality disturbance. The relationship between the field decrease and current ramp rate k can be expressed simply with a mathematical formula. For different situations, the effects caused by eddy currents on field quality may be different. But the maximal field decrease in the center of the magnet is the same.