Plasma neutralization models for intense ion beam transport in plasma

摘要

Plasma neutralization of an intense ion pulse is of interest for many applications, including plasma lenses, heavy ion fusion, cosmic ray propagation, etc. An analytical electron fluid model has been developed based on the assumption of long charge bunches (l/sub b//spl Gt/r/sub b/). Theoretical predictions are compared with the results of calculations utilizing a particle-in-cell (PIC) code. The cold electron fluid results agree well with the PIC simulations for ion beam propagation through a background plasma. The analytical predictions for the degree of ion beam charge and current neutralization also agree well with the results of the numerical simulations. The model predicts very good charge neutralization (<99%) during quasi-steady-state propagation, provided the beam pulse duration /spl tau//sub b/ is much longer than the electron plasma period 2/spl pi///spl omega//sub p/, where /spl omega//sub p/ = (4/spl pi/e/sup 2/sub p/ / m)/sup 1/2/ is the electron plasma frequency, and n/sub p/ is the background plasma density. In the opposite limit, the beam pulse excites large-amplitude plasma waves. The analytical formulas derived in this paper can provide an important benchmark for numerical codes, and provide scaling relations for different beam and plasma parameters.