Insert Heating and Ignition in Inert-Gas Hollow Cathodes

Abstract

An extensive 2-D computational model of the fundamental ionization and transport processes that occur inside electric propulsion hollow cathodes has been developed over the last few years. The computed charged-particle fluxes from the plasma to the internal cathode surfaces have been used as input to a rudimentary hollow-cathode thermal model also developed recently. It is shown that, in hollow cathodes with a very small diameter orifice, the plasma density peaks inside the orifice and that the cathode is heated primarily by the orifice plate which is, in turn, heated by the plasma inside the orifice and along the orifice plate. As the orifice diameter increases the peak plasma density moves upstream of the orifice with ions and electrons contributing to the heating of both the orifice plate and the insert. In hollow cathodes with a very large diameter orifice the plasma extends along much of the insert, the plasma density peaks well within the insert region, and the cathode is heated primarily by ion bombardment of the insert. By solving a 2-D axisymmetric boundary value problem, it is also shown that when a voltage is applied between the ends of a long cylindrical channel the potential in the absence of the plasma falls almost by one order of magnitude with each radius downstream of the entrance. The implication is that cathodes with orifices of large length-to-radius ratio will be harder to ignite.