Propagation of neutralized plasma beams

Abstract

Beams of neutralized plasma will cross a transverse magnetic field, magnetized plasma, and neutral gas, by a combination of single particle and collective-plasma processes. At low density the beam will deflect; at higher density the beam will exhibit undeflected motion by the E×B drift or diamagnetic (high-beta) exclusion of the magnetic field. In the high-beta regime, where β>1 (β is the ratio of plasma-beam energy density to magnetic field energy density), beam magnetization occurs on a time scale orders of magnitude faster than the classical Spitzer time scale. Recent studies of fast magnetization suggest that transport processes in large ion gyroradius beams, y/ρi<1, are dominated by ion motion rather than electron motion as previously understood for small gyroradius beams, y/ρi>1, where y and ρi are the beam transverse width and ion gyroradius, respectively. Although quantitative aspects of the small gyroradius propagation physics theory are still valid, most notably the E×B drift, much work remains to fully understand fast magnetic diffusion processes, beam-trapping mechanisms, and long-range propagation limits.