Transformation of the beam intensity distribution and formation of a uniform ion beam by means of nonlinear focusing

Yuri, Yosuke; Yuyama, Takahiro; Ishizaka, Tomohisa; Ishibori, Ikuo; Okumura, Susumu

The transformation and uniformization of the transverse beam intensity distribution by means of nonlinear focusing were investigated systematically for better understanding of the nonlinear focusing mechanism. In the theoretical analysis, the change in the on-target beam radius was derived when the Gaussian beam is focused with the octupole force. In the particle tracking simulation, the octupole-force dependence of the beam uniformity was revealed. It was also shown that a more uniform distribution was formed by a combination of the octupole and dodecapole forces. The present results are practically helpful for the design and operation of a uniform-beam irradiation system using multipole magnets. In the experiment at the JAEA AVF cyclotron, large-area square-like and elongated ribbon-like uniform intensity distributions were actually achieved for different utilization by adjusting the nonlinear beam optics including two octupole magnets.