Nonlinear charge and current neutralization for an ion beam pulse propagating through a background plasma

This paper demonstrates the possibility of quasi-steady-state transport of a finite-length ion beam through a chamber filled with plasma of arbitrary density. We conclude that, in principle, partially-current-neutralized equilibria exist in the reference frame moving with the ion charge bunch for arbitrary ratio of beam density to plasma density. The electric and magnetic fields generated by the ion beam are studied analytically for the nonlinear case where the plasma density is comparable in size with the beam density. Particle-in-cell simulations of current and charge neutralization agree well with analytical results. An important conclusion is that for long, nonrelativistic ion beams, with length much longer than the beam radius and the plasma neutralization length, which is the ratio of the beam velocity to the electron plasma frequency, the charge neutralization is, for all practical purposes, complete even for very tenuous background plasmas. Current neutralization is usually much weaker than charge neutralization. As a result, the magnetic pinching force dominates the electric defocusing force, and the beam ions are always pinched during quasi-steady-state beam propagation through the background plasma.