Truncation of the Beam in the Gaussian Tail by Using Convex Dipole Magnets

Abstract

In many ion beam applications, such as radioisotope production with high current proton beam and surface modification for industrial products, the ion beams are spread in order to reduce the localized dose rate and to obtain a uniform dose profile on the target surface [1,2]. For that purpose, a wobbler magnet system or a raster magnet system is widely used [3]. A wobbler magnet spreads the beams in circular shapes with a larger radius, and a raster magnet system spreads the beam into a large rectangular shape at the target surface. Usually, ion beams from accelerators have a Gaussian profile in the transverse direction, and there are many ions in the Gaussian tail. After a wobbler magnet system or a raster magnet system expands the beam, there are tails in the profile. In many cases, the beams in the tail are useless, and should be dumped to scrapers. As beam intensities from accelerators increase, which is a natural trend [4], the amount of the beam in the tail increases. For highenergy and high-current beam applications, however, it is not easy to install scrapers with enough cooling and radiation shielding. The conventional dipole magnets for a wobbler or raster system have a flat pole face to maintain the beam profile and to not increase the beam emittance. As Figure 1(a) shown, in a dipole magnet with a flat pole face, the beams do not intersect. Because the magnetic flux density is higher in the center of a dipole magnet with a convex pole face, the beam in the center can intersect with the off-axis beam on one side. The effect can be used to eliminate the beam tail. However, if we use a