Innovative Operating Modes of Many Plasma Devices Enabled by Thermionic Surfaces with Inverse Sheaths

Abstract

Thermionic electrons are emitted from surfaces in a diverse array of low temperature plasma devices and also in magnetic fusion experiments. For almost a century since Langmuir's work [1], models of sheaths with electron emission assumed that the surface is below the plasma potential for all emission intensities. In recent works, we showed that under sufficient emission, surfaces rise above the plasma potential and form an inverse sheath [2]–[4] such that plasma ions are confined. In the inverse regime, major changes to the plasma occur which are not seen in conventional sheath regimes. In this presentation, we demonstrate properties of plasmas bounded by inverse sheaths using our newly developed continuum simulation codes in various 1D and 2D geometric configurations. Important applications across plasma science will be discussed. For example, numerous devices that rely on thermionic cathodes such as filament discharges, Hall thrusters, and thermionic converters, could be designed to operate in the inverse mode to minimize cathode sputtering and power consumption, thereby extending device lifetime [2]. We predict that thermionic divertor plates with inverse sheaths can cause extreme cooling of the nearby plasma which might be used to mitigate plasma-wall interaction problems (sputtering, melting, arcing) in tokamaks [3], [4]. Other recent results regarding inverse sheaths by us and other researchers will be reviewed with applications to emissive probes, negative ion sources, and capacitively-coupled discharges. Many past plasma experiments with thermionic surfaces that revealed discrepancies with conventional sheath theory that are now explainable by inverse sheath theory.