Waves in expanding electronegative plasmas containing double layers

Abstract

Expanding electronegative (EN) plasmas have been previously observed, experimentally, to generate wave activity. Using a particle-in-cell (PIC) code we have investigated these waves in expanding EN plasmas containing a double layer (DL) between an upstream source region and an expanded downstream region. Oxygen reaction rates were used but modified to correspond more closely to experimental conditions. Under a subset of pressures, for which a DL existed, waves were observed traveling upstream in the expanded region, and growing in amplitude in the direction of travel. Both slow and fast waves were observed. The fast wave existed only over part of the slow wave pressure range. The PIC results were compared to both fluid and kinetic theory, both of which assumed axial uniformity. The results of a somewhat simplified fluid theory, ignoring fast wave coupling and collisions with the background gas, gave a remarkable result: if the theory predicted a slow wave instability for any axial parameters in the downstream region, the instability was observed in the simulation. Conversely, if no instability was predicted at any axial position, no instability was observed. More accurate kinetic calculations, including electron and ion Landau damping, and also collisional damping against the background gas, gave wavelengths and growth rates that were consistent with the PIC simulations, and with the fluid results. The kinetic theory also indicated that the fast waves were always stable but became weakly damped for conditions of unstable slow waves. We postulate that nonlinear and nonuniformity effects excite the fast waves.