Controlling Sheath and Plasma Properties due to Effects of Electron-Induced Secondary Electron Emission

Plasma-surface interactions in the presence of strong secondary electron emission (SEE) have been theoretically studied in the context of numerous plasma applications such as diverters, surface discharges, dusty plasmas, plasma thrusters, and plasma processing. The electron flux to the wall is determined by the electron velocity distribution function (EVDF) and by the sheath potential, which are determined by ambipolar constraints consistent with the wall properties. A traditional fluid description of the plasma-surface interaction suggests that electron emission from an electrically floating wall greatly reduces the sheath potential drop and, thereby weakens thermal insulating properties of the near-wall sheath leading to the enhancement of electron energy losses on the wall. However, in a typical low-pressure gas discharge, the electron mean-freepath can be large compared with the characteristic size of the discharge, and therefore, the electron motion can be almost collisionless. Recent PPPL kinetic studies of a collisionless plasma slab bounded by dielectric walls with SEE predicted a strongly anisotropic, nonmonotonic EVDF, which is depleted in the loss cone. This EVDF reduces the electron wall losses compared to Maxwellian plasmas, as shown in Fig. 1 [13]. Sheath oscillations occur due to coupling of the sheath potential and non-Maxwellian electron energy distribution function when there are intense electron beams emitted from the walls [2]. In a bounded plasma where the electrons impacting the walls produce more than one secondary electron on average no classical Debye sheath or space-charge limited sheath exists. Ions are not drawn to the walls and electrons are not repelled, as shown in Fig.1(c). Hence, the plasma electrons travel unobstructed to the walls, producing extreme particle and energy fluxes [1]. Strong dependence of the wall potential on SEE allows for active control of plasma properties by judicious choice of the wall material.