PLASMA CONDITIONS FOR NON-MAXWELLIAN ELECTRON DISTRIBUTIONS IN HIGH-CURRENT DISCHARGES AND LASER-PRODUCED PLASMAS

Results from the standard quasilinear theory of ion-acoustic and Langmuir plasma microturbulence are incorporated into the kinetic theory of the electron distribution function. The theory is then applied to high current discharges and laser-produced plasmas, where either the current flow or the nonlinear laser-light absorption acts, respectively, as the energy source for the microturbulence. More specifically, the theory is applied to a selenium plasma, whose charge state is determined under conditions of collisional-radiative equilibrium, and plasma conditions are found under which microturbulence strongly influences the electron kinetics. In selenium, we show that this influence extends over a wide range of plasma conditions. For ion-acoustic turbulence, a criterion is derived, analogous to one previously obtained for laser heated plasmas, that predicts when Ohmic heating dominates over electron-electron collisions. This dominance leads to the generation of electron distributions with reduced high-energy tails relative to a Maxwellian distribution of the same temperature. Ion-acoustic turbulence lowers the current requirements needed to generate these distributions. When the laser heating criterion is rederived with ion-acoustic turbulence included in the theory, a similar reduction in the laser intensity needed to produce non-Maxwellian distributions is found. Thus we show that ion-acoustic turbulence uniformly (i.e., by the same numerical factor) reduces the electrical and heat conductivities, as well as the current (squared) and laser intensity levels needed to drive the plasma into non-Maxwellian states. These effects are large over a wide range of plasma temperatures and densities such as found in Z-pinch, laser-produced, or opening switch plasmas. The reduction in laser intensity thresholds for generating non-Maxwellian states is a result of microturbulence enhanced laser absorption, which can be observed at any laser intensity. Thus, if microturbulence is the cause of the heat flux inhibition, then the magnitude of the inhibition might be directly observed through the use of probe-laser beams. Finally, we calculate the laser intensities and the current densities needed to change the electron distributions in the presence of ion-acoustic turbulence. These calculations are carried out by assuming that the electrostatic fluctuations scale as the square root of the electron plasma parameter in a weakly microturbulent selenium plasma. The laser intensities calculated in this example are found to be only slightly in excess of those used in recently conducted experiments.