Enhancement of droplet ejection from molten and liquid plasma-facing surfaces by the electric field of the sheath**Substantial portions of this paper are adapted from section 4.1 of the first author’s recent PhD thesis.

Abstract

Maintaining the stability of a liquid surface in contact with a plasma is of crucial importance in a range of industrial and fusion applications. The most fundamental feature of a plasma-surface interaction, the formation of a highly-charged sheath region, has been neglected from the majority of previous studies of plasma-liquid interactions. This paper considers the effect of the electric field of the sheath on the ejection of micron-scale droplets from bubbles bursting at the liquid surface. A numerical simulation method, based on the ideal electrohydrodynamic model, is introduced and validated against the well-known Taylor cone theory. This model is then used to include the electrical effects of the sheath in simulations of bubble bursting events at a plasma-liquid interface. The results show a significant enhancement in droplet ejection at modest electric fields of between 10% and 20% of the critical field strength required for a solely electrohydrodynamic instability. This finding is in good qualitative agreement with experimental observations and its importance in a wide range of fusion and atmospheric-pressure plasma-liquid interactions is discussed. The inclusion of sheath physics in future studies of plasma-liquid interactions is strongly advocated.