Kinetic Modeling of Beam-Neutralization in a Background Neutral Gas

Ion beams are used in various engineering applications, such as particle accelerators, ion thrusters, and ion implantation devices. Ion beam space-charge neutralization by electrons has been a recent topic of interest where the use of an external electron emission source has been shown to prove ineffective because of the excitation of plasma waves in the beam[1]. In our previous work, electrostatic-solitary-waves (ESWs) were observed in 2D and 3D ion beams[2], and these waves have been shown to slow the process of beam neutralization in earlier work[1]. In this work, we plan to show how these ESWs can be limited in their amplitudes and even fully dissipated in a neutralizing ion beam when a background neutral gas is introduced in the vicinity of the beam. Using our newly implemented Monte-Carlo-Collision routine coupled with Particle-in-Cell (PIC-MCC), we will show how ion-neutral gas collisions and electron-neutral collisions affect the plasma waves in a neutralizing beam. We will quantify the lifetime of ESWs in the ion beam as a function of gas pressure and the loss of ion throughput current due to charge-exchange collisions. We will also show the dissolution of the virtual cathode and its effects on the plasma waves in the beam. The quantification in this work will provide estimates of how to avoid undesirable plasma waves when neutralizing an ion beam using a background neutral gas.