Emissive cathode immersed in a plasma: plasma–cathode interactions, operation and stability

Abstract

Thermionic emission from a polycrystalline tungsten emissive cathode immersed in a magnetized plasma column is investigated experimentally and numerically. Electrical and optical measurements of the cathode temperature show a highly inhomogeneous cathode temperature profile due to plasma–cathode interactions. The spatially and temporally resolved cathode temperature profile provides an in-depth understanding of the thermionic electron current, in excellent agreement with experimental data. The plasma-cathode coupling leads to a sharp and heterogeneous rise in temperature along the cathode, which can eventually lead to unstable cathode operation, with divergent current growth. A detailed thermal modeling accurately reproduces the experimental measurements, and allows to quantify precisely the relative importance of heating and cooling mechanisms in the operation of the cathode immersed in the plasma. Numerical resolution of the resulting integro-differential equation highlights the essential role of heterogeneous ohmic heating and the importance of ion bombardment heating in the emergence of unstable regimes. Detailed thermal modelling enables operating regimes to be predicted in excellent agreement with experimental results.

Model of non-equilibrium near-cathode plasma layers for simulation of ignition of high-pressure arcs on cold refractory cathodes

“…Note that hot thermionic cathodes are widely used for primary electron production in plasma sources, both at high and low pressures; e.g. recent work [38] and references therein. In some cases, the physics of the interaction of emissive cathodes with the plasma is substantially different from that in high-pressure arc discharges.…”

“…On the other hand, there are cases where the physics is similar to some extent. For example, this is the case of an emissive tungsten cathode immersed in a magnetized plasma column, which was investigated experimentally and numerically in [38]. The approximate models of plasma-cathode in high-pressure arc discharges, considered here, may be useful in such cases.…”