Emissive sheath dynamics in the aid-and-compete two-plasma mode of a hot-filament discharge

A cylindrical co-axial discharge with an inner radially emitting cathode is a common configuration for many plasma devices, such as the cylindrical magnetron or the hot filament discharge, which find useful applications in the plasma material processing industry and electric space propulsion. The physics of emissive sheaths around biased hot filaments also has implications for emissive probe measurements in plasma diagnostics.

We have numerically modelled the mode transitions of a hot-filament discharge in cylindrical geometry using the particle-in-cell code APEC1PIC : the Axisymmetric 1D3V (i.e. Radial) PIC-MCC code from the PEXPIC suite of electrostatic PIC solvers. We find that under the conditions of low energy deposition (electrostatic and kinetic), and sparse neutral density the discharge strikes as a two plasma mode. One plasma forms in the conventional upstream region through electron impact ionization of the neutral background. A second plasma, whose global effect on the discharge was not previously well understood, forms downstream through trapping of cold ions in the potential well of the filament's virtual cathode; a process enabled by ion-neutral charge exchange collisions. Three plasma sheaths intersperse the electrode gap - an emissive sheath between the hot filament and trapped-ion plasma, a double-layer sheath between the two plasmas, and an anode sheath between the upstream plasma and the outer wall.

Coupling between the two plasma is understood via an "aid-and-compete" model wherein the growth of one plasma enhances growth in the other, while simultaneously exhibiting plasma expansion dynamics antagonistic to each other. An approximate analytical version of the model also captures this duality in the two-plasma coupling. Outcome of the aid-and-compete effect are mode transitions and instabilities that transform the sheath-system back and forth between space charge limited (SCL), marginally space large limited (MSCL), classical, and inverse sheath anode glow modes (AGM); the last two being the possible steady states for a small radius filament. A periodic oscillatory solution is discovered by the taking the numerical experiment to a larger annular aspect ratio (thicker cathode).