The theory of final focusing of intense light ion beams

Abstract

Efficient transport of the high-power light ion beams of interest to inertial confinement fusion (ICF) experiments may require the use of large radius transport channels. The mismatch in the radii of the transported ion beam and the ICF target requires that the beam be focused following transport. A theoretical description of intense light ion beam focusing with a z-discharge plasma is presented. The theory is based on a Vlasov equation description of a fully charge- and current-neutralized ion beam, neglecting any angular momentum of the beam ions. An initial ion beam phase-space distribution with adjustable parameters is considered so that its effect upon the phase-space distribution at the focal plane can be analyzed. A numerical code is used to analyze the effect of angular momentum on the focusing properties of the final focusing cell. The theoretical analysis gives expressions for the focal length, for the discharge current required to produce a given degree of focusing, and for the radial number density profiles at the focal plane. The analysis of angular momentum indicates that the focal length and focusing current are not strongly affected even when beam ions have a significant amount of angular momentum.