Simulation study of halo formation in breathing round beams

We study halo formation from cylindrical beams propagating in a uniform focusing channel, Of particular interest here are the breathing-mode oscillations excited by an initial beam-size mismatch, We develop a one-dimensional space-charge code which is simple but powerful in exploring the halo properties of breathing beams. After giving a brief description of two particle-core models, we apply the developed code to three different types of nonlinear phase-space distributions. Based on a number of multiparticle simulation runs, we show several interesting results useful for the design considerations of high-power linear machines. In particular, the intensity of halo current as well as the maximum extent of halos are self-consistently evaluated with the different sizes of initial mismatch and beam density. We then find that the halo extent normalized with the initial root-mean-squared beam size is only weakly dependent on the tune depression and that the halo intensity appears to increase with the degree of mismatch. It is further demonstrated that the beam core in phase space is roughly stable and, thus, most halo particles remain outside the core boundary. We also see that it is, in principle, possible to scrape halos, e.g., by means of a multicollimator system.