No abstract

U SE of the vapor-fed hollow cathode both as plasma-bridge neutralizer and as the ion chamber electron source has been advantageous in mercury bombardment ion thrusters. 1^3 A hollow cathode differs from earlier types (e.g., refractory metal ribbon, or oxide magazine) in several important ways. For example, it is-a source of mercury atoms, as well as of electrons. Consequently, a very large pressure gradient exists just outside the hollow-cathode orifice as a result of the mercury vapor expanding into a region of low pressure. Also, electron injection is from a much smaller area than it is with other types of cathodes. The properties of the external discharge plasma obtained also differ, and it was necessary to introduce a baffle in the discharge chamber of the electron bombardment thruster to obtain good performance when the discharge was fed by a hollow cathode. 3 To investigate the nature of the hollow-cathode discharge, Langmuir probe measurements were made in a diode configuration with a plane circular anode placed opposite the hollowcathode. The hollow cathode assembly consisted of a 0.32-cm-outside-diam tantalum tube with a 0.1-cm-thick, 2% thoriated tungsten-alloy disk attached to one end by electronbeam welding. A 0.02-cm-diam orifice was cut in the center of the tungsten disk. A barium carbonate coated tantalum foil insert placed inside the tube facilitated starting the discharge. In operation, the cathode tube was electrically heated by a tungsten-rhenium wire wound around it and embedded in a flame-sprayed coating of alumina.The anode was a 14-cm-diam stainless steel disk. The distance of the cathode could be varied. The experiments were conducted in a 45-cm-diam vacuum bell jar. The typical pressure during the runs was on the order of 10~6 torr.

No abstract