Reflection of Electrostatic Solitary Waves within Pulsed Ion Beam and its Effects on Ion Beam Neutralization

Abstract

The neutralization of an ion beam pulse directly by electron injection excites the two-stream instability of neutralizing electrons and produces an electrostatic solitary wave (ESW) after the instability saturates. The ESW propagates stably and reflects back and forth in the potential well of the ion beam pulse. Through a two-dimensional Particle-in-Cell code, we numerically simulated the whole neutralization process of the ion beam pulse as well as the excitation and propagation of the ESW. It is found that the reflections of the ESW at the edges of the ion beam pulse cause a lot of neutralizing electrons to be thrown out, forming runaway electrons released in a pulsed manner. Each reflection of the ESW accelerates the reduction of the neutralization degree and brings a small disturbance to the ESW. The accumulative effect of multiple reflections results in rapid collapse of the ESW at the end. The reflections of the ESW at the head and tail of the beam pulse cause the ion beam to lose more neutralizing electrons than the slow attenuation of the ESW inside the ion beam.