Effect of low-frequency disturbance on the self-sustained discharge of hollow cathode

Abstract

The existence of low-frequency disturbance can affect the discharge stability of a hollow cathode, and may even lead to the extinction of the discharge. In this paper, a semi-transient algorithm based on the classical 0-D theoretical model of an orificed hollow cathode was developed to study the discharge stability of a hollow cathode under a continuous low-frequency electric field disturbance. A laboratory experiment using an orificed BaO-W cathode was carried out to confirm the validity of transient analysis results and to determine an appropriate time step in the analysis. The theoretical evolution trend of the discharge current is largely in agreement with the experiment for a time step value of 12.5 µs although there is a 7% error in the discharge duration prediction. This study has identified the role of two emission enhancement mechanisms in the failure process of a self-sustained discharge, namely the Schottky effect and the sheath funneling effect. The presence of the sheath funneling effect induces larger amplitude oscillations and a sharp drop in discharge parameters when the effect ceases to exist, and this significantly weakens the discharge stability. The influence of operating conditions and disturbance parameters on transient discharge characteristics was examined for the NASA NSTAR cathode. Within the parameter range that was studied, findings show that higher xenon flow rates or lower discharge currents provide optimal duration as a result of higher electron temperatures and denser plasma at the initial state. For a fixed operation point, the discharge duration was found to decline exponentially with disturbance amplitude and quadratically with disturbance frequency.

Plasma behavior of a low current orificed hollow cathode using krypton propellant for electric thruster applications

“…When the krypton propellant is used, the plume region no longer shows a 'spot' mode in the diode configuration, but forms a cone structure with diffuse plasma, which extends from the orifice and fills the cylindrical anode. Results from earlier studies demonstrate a consistent association between such a plasma distribution and the 'plume' operation of the hollow cathode [16,57]. The diffuse structure in the far field seems clearer for the kryptonfed cathode and a high-density plasma in the plume region may be due to the ionization and excitation events in the far field plume, which will be discussed in detail in the following section.…”