Formation of a large-area uniform ion beam using multipole magnets in the TIARA cyclotron

The formation of an ion beam with a uniform transverse intensity distribution based on a nonlinear focusing method has been studied at the azimuthally varying field cyclotron facility of Japan Atomic Energy Agency. The method enables us to perform uniform irradiation at a constant particle fluence rate over the whole area of a large sample. To obtain a large-area highly uniform beam on the target, we first smooth the intensity distribution of the beam from the cyclotron by multiple scattering and then focus the smoothed beam using octupole magnets. An efficient technique to enhance the scattering effect with a small energy loss is discussed. By collimating the tail of the smoothed beam before octupole focusing, a beam with a nearly uniform profile can be attained on the target. Moreover, we also show that a misaligned beam can be transformed into a uniform beam by adding sextupole focusing. The possibility of ultra-short-time and ultra-low-fluence irradiation has been demonstrated toward advanced applications in materials and biological science. The characteristics of the uniform beam produced by the nonlinear focusing method are discussed from the viewpoint of a uniform irradiation method as compared to other conventional methods.