Bismuth Hollow Cathode for Hall Thrusters

Abstract

B ISMUTH has several qualities that make it well suited for development as a Hall-thruster propellant. When compared with more conventional propellants such as xenon, bismuth holds significant advantages from both an energetics (lower ionization energy) [1] and cost standpoint. In addition, there are significant ground-test-facility cost savings, because bismuth does not require the use of cryogenic pumps. Unlike traditional propellants, bismuth is solid at room temperature; thus, the exhausted bismuth solidifies on the room-temperature vacuum chamber walls, and consequently the entire vacuum chamber becomes an effective pumping surface. With this in mind, operating a high-power bismuth Hall thruster would require only enough pumping speed to keep up with facility outgassing and minor vacuum leaks. However, there are some difficulties that need to be addressed when using a condensable propellant. Some of the issues include sustaining elevated temperatures for bismuth evaporation, regulating bismuth mass flow, mechanical limitations inherent with using refractory metal components, and bismuth plating of thruster and spacecraft components. Recently, there have been three new programs to develop bismuth Hall thrusters, with the first successful demonstration occurring in the spring of 2005 [2–4]. In this work, the bismuth thruster was operated using a xenonLaB6 cathode. The encouraging results of the bismuth thruster motivated a study to examine the feasibility of an all-bismuth system using a bismuth cathode. In addition to all of the physical and economical gains, it would be advantageous to incorporate a bismuth cathode to eliminate the need for multiple propellant supplies on an eventual flight unit. In 2005, a functioning prototype bismuth cathode was developed and a limited number of operating characteristics were reported [5]. The primary goals of the present research were to evaluate the operating characteristics of a bismuth LaB6 cathode at different mass flow rates, compare bismuth data with xenon and krypton performance, and to reduce the amount of power required for cathode operation.