Phenomenological Model Describing Orificed, Hollow Cathode Operation

ORIFICED, hollow cathodes serve as sources of electrons in a number of plasma producing devices. In ion thrusters they are used as a source of electrons for both the main discharge region and the neutralizer. Recent experiments1'2 have provided such important physical data regarding hollow cathode operation as plasma properties within the cathode, insert temperature profiles, internal cathode pressure, and emission current density profiles over a wide range of cathode operating conditions. The physical understanding gained through these experiments has led to the development of a simple phenomenological model describing physical processes which take place inside the hollow cathode. Contents The experimental studies1'2 noted above showed that, for a cathode operating on mercury vapor at discharge currents of a few amperes, typical plasma conditions found in the vicinity of the orifice plate were a plasma density of a few times 1014 cm~ 3, a plasma potential of ~8 V, and an electron temperature of -0.8 eV. The experiments also showed that -85% of the total electron discharge current came from the last few millimeters at the downstream end of the insert where typical emission temperatures of ~ 1000°C were measured for discharge currents of a few amperes. The simultaneous measurement of emission current and emission surface temperature in these studies provided sufficient information to allow calculation of the emissive surface work function. That the calculated value of this work function was reasonable for the materials being used supported the underlying assumption on which the calculations were based; namely, that the surface emission mechanism for the cathode is that of field-enhanced, thermionic emission. The following physical description of orificed, hollow cathode operation is based on the results of these studies. The cathode orifice maintains a high neutral density inside the cathode by restricting the propellant flow and provides a current path to the downstream discharge. The electrons that exit through the cathode orifice are produced within the cathode both by surface emission and by volume ionization. As indicated in the schematic of Fig. 1, the surface electron emission comes uniformly from a narrow band (f«2 mm) on the insert downstream end. The electrons are produced at the insert surface by field-enhance d, thermionic emission (the very strong electric field is a consequence of the very dense plasma which produces a very thin plasma sheath across which the plasma potential drop occurs). The electrons Presented as Paper 81-0746 at the AIAA/JSASS/DGLR 15th