Electron-distribution-function cutoff mechanism in a low-pressure afterglow plasma - art. no. 016401

A model is developed for self-consistent simulations of transient phenomena in a low-pressure afterglow plasma. The model is based on the nonlocal app roach which allows a kinetic description of the plasma decay under nonquasi stationary conditions. Such conditions arise when collisions (mainly electr on-electron) are not sufficient for the electron distribution function (EDF ) to follow changes in the self-consistent electric fields and the ion dens ity once the power is turned off. As a result, collisions cannot provide th e electron and ion particle balance by allowing electrons to flow out of th e potential well. A cutoff mechanism is suggested that provides such a bala nce during the transient period-from the glow, stationary plasma to the qua sistationary, afterglow plasma. This mechanism is essential for determining correctly the self-consistent wall potential (and hence the energy of ions impinging upon the wall surface) and other parameters, such as diffusion c ooling, which is the most important cooling mechanism at low pressures. The se phenomena are modeled using the time-dependent nonlocal electron Boltzma nn equation with a nonlinear electron-electron collision operator. A numeri cal treatment is made by extending Rockwood's method for finite-difference discretization of this operator in the total energy formulation. The model calculates self-consistently the temporal evolution of the nonlocal EDF and the electric potentials in the plasma and the wall sheath. Strongly non-Ma xwellian EDF's are predicted and it is observed that, depending on plasma c onditions, the transient period maybe rather long, of order of the ambipola r diffusion time, lower pressures resulting in longer transient times. The proposed approach can be applied to model self-consistently pulsed plasmas during both the power-on and power-off periods, including the breakdown per iod.