Numerical Simulations of the Partially Ionized Gas in a 100-A LaB<sub>6</sub> Hollow Cathode

Abstract

Numerical simulations of a hollow cathode with a lanthanum hexaboride (LaB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) emitter operating at 100 A have been performed using the 2-D Orificed Cathode (OrCa2D) code. Results for a variety of plasma properties are presented and compared with laboratory measurements. The large size of the device permits peak electron number densities in the cathode interior that are lower than those established in the NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) hollow cathode, which operates at a 7.3× lower discharge current and 3.2× lower mass flow rate. The maximum electron current density also is lower in the LaB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> cathode, by 4.2×, due to the larger orifice size. Simulations and direct measurements show that at 12 sccm of xenon flow the peak emitter temperature is in the range of 1630°C-1666°C. It is also found that the conditions for the excitement of current-driven streaming instabilities and ion-acoustic turbulence (IAT) are satisfied in this cathode, similarly to what was found in the past in its smaller counterparts like the NSTAR cathode. Based on numerical simulations, it has long been argued that these instabilities may be responsible for the anomalously large ion energies that have been measured in these discharges as well as for the enhancement of the plasma resistivity. Direct measurements of the turbulent spectra and confirmation of the presence of IAT in this cathode have now been completed. Interpolation of the measured anomalous collision frequency based on slightly different operating conditions than the one in the numerical simulations suggests good agreement with the computed values.