Anode position influence on discharge modes of a LaB6 cathode in diode configuration

摘要

A laboratory model of a 5 A-class cathode was experimentally studied in diode configuration with a disk anode. The core of the cathode is a flat disk lanthanum hexaboride (LaB6) insert. Electron emission is achieved using a heating element in direct contact with the insert. The paper reports the characterization of the LaB6 cathode operated at xenon mass flow rates between 0.4 and 1.0 mg s(-1) with discharge currents ranging from 2 A to 12 A. Apart from the operating envelope and discharge mode (spot versus plume) differentiation, the influence of anode position on cathode discharge mode was studied. For this purpose, the cathode was operated at 4 A and 0.6 mg s(-1) and at 10 A and 0.6 mg s(-1), while the cathode anode gap was increased from 20 mm to 70 mm. Both electrical and plasma parameters were collected and analyzed in order to highlight the main changes in cathode discharge when the cathode-anode gap was increased. Particular attention was paid to the identification of the discharge mode and mode transition based on spectral analysis of discharge current waveforms. It was demonstrated that an increase in the cathode-anode gap induces the discharge mode transition from spot mode, corresponding to lower gap values, to plume mode, corresponding to higher gap values. Changes in plasma property were also noticed, the cathode-anode gap increase inducing lower plasma density and higher electron temperature in the cathode plume. (C) 2018 Elsevier Ltd. All rights reserved.