On the electron sheath theory and its applications in plasma-surface interaction

Abstract

The electron sheath is a particular electron-rich sheath with negative net charges where plasma potential is lower than the biased electrode. Here an improved understanding of electron sheath theory is provided using both fluid and kinetic approaches while elaborating on its implications for plasma-surface interaction. A fluid model is first proposed considering the electron presheath structure, avoiding the singularity in electron sheath Child-Langmuir law. The latter is proved to underestimate the sheath potential. Subsequently, the kinetic model of electron sheath is established, showing considerably different sheath profiles in respect to the fluid model due to the electron velocity distribution function and finite ion temperature. The model is then further generalized involving a more realistic truncated ion velocity distribution function. It is demonstrated that such distribution function yields a super-thermal electron sheath whose entering velocity at sheath edge is greater than that prescribed by the Bohm criterion, implying a potentially omitted calibration issue in the probe measurement. Furthermore, an attempt is made to incorporate the self-consistent presheath-sheath match within the kinetic framework, showing a necessary compromise between realistic sheath entrance and the inclusion of kinetic effects. In the end, the consequent secondary electron emission due to sheath-accelerated plasma electrons in electron sheath are analyzed, providing a sheath potential coupled with the plasma and wall properties.