Distribution transformation by using step-like nonlinear magnets

The uniform distribution of the beam on target is required in some high power beam applications to prolong the target lifetime or to improve the beam utilization. In this paper, a new method is proposed to transform irregular beam distributions in phase space into other kinds of distributions such as parabolic and fourth power in the transverse plane on target by using special nonlinear magnets. The nonlinear magnets can be designed similar to a dipole magnet, in which the emphasis is on the fringe field. The method is especially useful when dealing with beams with large emittance or/and with dense core and sparse halo. The simulations show that the peak current density in the defined irradiation footprint can be reduced significantly while avoiding cutting off important beam halo due to high beam power. The application of the method to a high beam power accelerator (the ESS project) is also presented, where the peak current density at the target is reduced by a factor of about three. The method of using standard multipole magnets is also discussed, and an application to obtain the fourth power distribution is presented.