Development of Optical Diagnostics for Bismuth Plasma Propulsion Devices

Summary form only given. As the goals of space flight push toward faster missions to the outer planets, issues of spacecraft mass, efficiency, and flight time become increasingly important. Electric propulsion systems in general have shown significant benefits in these respects over conventional chemical propulsion methods; however, state-of-the-art systems are still lacking in the efficiency and power handling required for the development of feasible nuclear-electric propulsion (NEP) missions to the outer planets. Present ion engine technology limits power to less than 10 kW, while such future missions would require several times that amount. In order to accommodate the requirements of mid- and far-term NEP missions, NASA recently selected a bismuth-fueled two-stage Hall thruster with anode layer (TAL) for further development. Current TAL thrusters have been demonstrated to perform at 25-140 kW and have the potential to scale up to greater than 500 kW for very high power NEP systems. Additionally, the bismuth propellant offers advantages in cost, availability, thrust efficiency, and spacecraft tankage fraction over the xenon propellant used in current ion thrusters. As the thruster is developed, there will be a need for non-intrusive diagnostic methods to optimize the geometry and operating conditions, as well as to make predictions of the potential for spacecraft contamination. Such optical diagnostic methods as laser induced fluorescence (LIF) and absorption spectroscopy for analyzing the velocity, energy, and number densities of BiI and BiII will be discussed. Candidate transitions, chosen on the basis of relative strength, hyperfine splitting data availability, and accessibility to tunable diode lasers, will be presented and their lineshapes modeled with respect to hyperfine splitting and broadening mechanisms. Experimental measurements of these transitions in both a bismuth microwave discharge (Evenson cavity) and a bismuth metal vapor lamp will also be discu- sed. Measurements will include both absorption and LIF data and will be used to validate the lineshape models of the candidate transitions, as well as to further advance the diagnostic methods for the developing thruster