EVOLUTION OF A MAXWELLIAN PLASMA DRIVEN BY ION-BEAM-INDUCED IONIZATION OF A GAS

The ionization of gas by intense (MeV, kA/cm(2)) ion beams is investig ated for the purpose of obtaining scaling relations for the rate of ri se of the electron density, temperature, and conductivity of the resul ting plasma. Various gases including He, N, and Ar at pressures of ord er 1 torr have been studied. The model is local and assumes a drifting Maxwellian electron distribution. In the limit that the beam to gas d ensity ratio is small, the initial stage of ionization occurs on the b eam impact ionization time and lasts on the order of a few nanoseconds . Thereafter, ionization of neutrals by the thermal electrons dominate s electron production. The electron density does not grow exponentiall y, but proceeds linearly on a fast time scale t(th)=U/(upsilon(b) rho dE/dx) associated with the time taken for the beam to lose energy U vi a collisional stopping in the gas, where U is the ionization potential of the gas, upsilon(b) is the beam velocity, rho is the gas mass dens ity, and dE/dx is the mass stopping power in units of eV cm(2)/g. This results in a temperature with a slow time dependence and a conductivi ty with a linear rise time proportional to t(th). (C) 1996 American Is titute of Physics.