Nonlinear aspects of Landau damping in computer simulation of the microwave instability

Abstract

Computer simulation with the multi-dimensional particle-in-cell code SCOP-RZ is applied to the study of nonlinear saturation phenomena of the resistive microwave instability of a coasting high-current ion beam. Due to the option of a fast Poisson-solver employing r-z geometry the code is particularly suitable for long beams with strong space charge effects due to non-relativistic energy. It is shown here that for currents far above the Keil-Schnell limit the instability due to a broad-band resonator near the cut-off frequency saturates at a relatively low level without severe degradation of the momentum distribution, provided that the resistive part of the impedance is small compared with the reactive (space charge dominated) part: The final distribution shows Landau damping due to a thin stabilizing tail towards smaller momenta.