Modeling of the conductivity of a plasma created by beam-induced ionization of a gas

Summary form only given, as follows. The conductivity of a plasma created by ion-beam induced ionization of a gas has been analyzed. For the purpose of inertial confinement fusion driven by high current ion beams, it is expected that charge and current neutralization of the beam can be achieved by propagating the beam through a low pressure background gas. In recent experiments conducted at the Naval Research Laboratory, the net currents produced by the transport of intense (1 MeV, 1 kA/cm/sup 2/) proton beams through various gases in the 0.25 to 4.0 Torr range were measured. B-dot probes located at three positions along the propagation axis and placed outside of the beam envelope measured net currents of 2% to 8% of the injected beam current and indicated plasma return current decay times of hundreds of nanoseconds. Interferometric measurements indicate ionization fractions within the beam region of up to 5% of the background gas density. The plasma response is modeled according to the measured temporal evolution of the return currents. Timescales are such that significant plasma ion motion does not occur and therefore the plasma is considered to be a resistive electron fluid with an immobile ion component. The 2-1/2 dimensional (axisymmetric) hybrid fluid/particle code SOLENZ is used to aid the analysis. Preliminary results suggest that the return current decay can be well described by a classical Spitzer resistivity but that the conducting wall boundaries play an important role in the net current characteristics of the beam/plasma system. A model for the early time development of the plasma conductivity is being considered.