Analysis of the Electron Distribution Function Inside of a LaB₆ Hollow Cathode

Langmuir probe measurements have been performed many times in hollow cathodes and remain one of the most commonly used diagnostic methods to determine electron temperature, density, and plasma potential. The objective of this project was to analyze which possible electron processes can also be determined using a single-wire Langmuir probe in combination with the Druyvesteyn electron energy distribution function (EEDF) method. The well-documented JPL NASA LaB6 hollow cathode using argon, without a heater and without an orifice, was chosen for this study. The probe tip was located at the backend of the plasma. The Druyvesteyn EEDF method resulted in three distinct electron population peaks, representing single ionization, secondary electron production due to ion bombardment of the low work function insert, and thermionic electron production. The electron temperature for almost all three peaks decreased slightly with increasing mass flow rate. The electron number density also decreased with mass flow rate; however, each population showed a different rate of decrease. The detection showed different populations for different locations in the upstream plasma sheath. The results of this investigation indicate that thermionic emission, secondary ion bombardment, ionization, and their associated electron extractions can be measured using the Druyvesteyn EEDF method.

I. Introduction

Current hollow cathode theory states that two major electron production processes are happening: ionization through electron-neutral collisions and thermionic emission due to energy input into the insert. Additionally, ions are known to bombard the insert surface during the self-sustaining mode. In the case where plasma is limited by walls, several electron processes can occur (see Fig. 1). Fig. 1.

Electron processes in a plasma.