Using bismuth in place of gases such as xenon for Hall thruster propellant could potentially offer both physical and economical gains. As research continues to develop Hall thrusters that are fueled with bismuth, it will become advantageous to maintain one propellant supply rather than multiple supplies for the anode and cathode. The recent development of a bismuth Hall thruster at Michigan Tech, operated using a xenon LaB 6 cathode, provided a motive to explore the feasibility of developing an

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... entire bismuth system. This paper provides a background on the development and operation of a bismuth vapor LaB 6 cathode. Comparisons of operating parameters are provided for the cathode running on xenon and bismuth propellants along with a description of the mass flow technique used. Complications in determining and controlling the mass flow rate are presented as well. Nomenclature A = cathode orifice area (m 2 ) k = Boltzmann constant (m 2 ·kg/s 2 ·K) m = atomic mass (kg) m & = mass flow rate (kg/s) P v = vapor pressure (Pa) T = temperature (K) I. Introduction ISMUTH has many attributes that make it well suited for development as a Hall thruster propellant. Attractive physical characteristics follow from the atomic properties of bismuth. Bismuth, with an atomic mass of 209 amu, is significantly more massive than the more traditional xenon (131 amu). The large, heavy atoms thus have a lower neutral diffusion velocity and a larger electron-impact cross-section, resulting in a greater probability of ionization and increased propellant utilization. Not only is the ionization probability greater for Bi than Xe, but the energy cost-per-kg of mass flow to create a bismuth plasma is only 37% that of Xe: Bismuth's first ionization level is 7.3 eV, resulting in an ionization cost of 0.035 eV/amu, compared to xenon's