Control of beam halo formation through nonlinear damping and collimation

Abstract

This paper demonstrates that transverse beam halos can be controlled by combining nonlinear focusing and collimation. The study relies on one-dimensional, constant focusing particle-in-cell (PIC) simulations and a particle-core model. Beams with linear and nonlinear focusing are studied. Calculations with linear focusing confirm previous findings that the extent and density of the halo depend strongly upon the initial mismatch of the beam. Calculations with nonlinear focusing are used to study damping in the beam oscillations caused by the mismatch. Although the nonlinear force damps the beam oscillations, it is accompanied by emittance growth. This process is very rapid and happens within the first 2-3 envelope oscillations. After this, when the halo is collimated using a system of four collimators, further evolution of the beam shows that the halo is not regenerated. The elimination of the beam halo could allow either a smaller physical aperture for the transport system or it could allow a beam of higher current in a system with the same physical aperture. This advantage compensates for the loss of particles due to collimation. © 2005 The American Physical Society.