Intense ion beam propagation in a reactor sized chamber

We consider the physics of the ballistic transport of intense ion beams in a heavy ion fusion reactor chamber filled with low pressure Flibe gas. We consider first a single beam envelope model and show via a simple case that emittance growth is an issue in the chamber as well as in the accelerator. We develop a model for the neutralization of beam space-charge by the electrons produced by gas ionization by the beam and derive an expression for the evolution of the neutralization factor as the beam propagates into the chamber. We then extend the envelope model from a one species beam to a beam of ions of several charge states by considering the entire beam as a set of subbeams (one for each charge state) each described with coupled envelope equation. The fully electromagnetic PIC code BPIC was used to investigate the behavior in greater detail. A parametric study of the sensitivity of the final spot radius at the target versus the ion beam stripping and gas ionization cross-sections (which are characterized by large uncertainties) shows that, in the studied regime (Hylife-II parameters), the accessible window of cross-sections for ballistic transport in the chamber through neutral Flibe gas is eventually small. The temperature evolution for each species and the emittance growth for the entire ion beam was studied for a typical scenario and indicates that a fair amount of the initial electric potential energy carried by the beam as it enters the chamber is converted into temperature and transverse emittance. The high temperature of the ionization-produced electrons prevents a full charge neutralization of the ion beam as it approaches the target. It is shown that focusing a beam array or pre-ionizing a fraction of the background gas may help in reducing the focal spot.