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

TL;DR

This study combines experimental and numerical methods to analyze the temperature distribution, plasma-cathode interactions, and stability of a tungsten emissive cathode immersed in a plasma, revealing key factors influencing operation stability.

Contribution

It provides a comprehensive thermal model that accurately predicts cathode behavior and stability, highlighting the roles of heterogeneous heating and plasma interactions.

Findings

Temperature inhomogeneity affects cathode stability

Heterogeneous ohmic and ion bombardment heating are critical

Thermal modeling predicts operational regimes accurately

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.