Evolution of a Maxwellian plasma driven by ion-beam-induced ionization of a gas

The ionization of gas by intense (MeV, kA/cm2) ion beams is investigated for the purpose of obtaining scaling relations for the rate of rise of the electron density, temperature, and conductivity of the resulting plasma. Various gases including He, N, and Ar at pressures of order 1 torr have been studied. The model is local and assumes a drifting Maxwellian electron distribution. In the limit that the beam to gas density ratio is small, the initial stage of ionization occurs on the beam impact ionization time and lasts on the order of a few nanoseconds. Thereafter, ionization of neutrals by the thermal electrons dominates electron production. The electron density does not grow exponentially, but proceeds linearly on a fast time scale tth=U/(vbρ dE/dx) associated with the time taken for the beam to lose energy U via collisional stopping in the gas, where U is the ionization potential of the gas, vb is the beam velocity, ρ is the gas mass density, and dE/dx is the mass stopping power in units of eV cm2/g. This results in a temperature with a slow time dependence and a conductivity with a linear rise time proportional to tth.