Bismuth Hall Thruster Spacecraft Interactions Study

Numerous gases may be ionized and accelerated in Hall Effect Thrusters. Among all the candidates, condensable bismuth has significant advantages including easy storability, higher thrust to power and higher efficiency. However, bismuth accumulation could adversely impact solar arrays, sensors, and other devices. This problem appears solvable by proper shielding, but that needs to be demonstrated. Therefore, Busek proposes to characterize in detail both the near and far field plumes produced by our existing experimental bismuth Hall thrusters and also characterize bismuth-surface interactions by measuring sticking and accommodation coefficients and deposition rates for numerous spacecraft materials. The goal is to use this dataset to construct and validate a bismuth plume-spacecraft interaction model. Busek will use a variety of instruments. Our subcontractor, PlasmaSolutions, will deliver an improved version of its Phase I overhang probe, as well as a compact optical interferometer, and will assist Busek in characterizing the plume, including determining the ratio of molecular to atomic bismuth. Additionally, an ExB probe and an ElectroStatic Analyzer will be purchased from Plasma Controls. The data obtained will quantify the problem and also enable accurate numerical plume/spacecraft interaction models, leading to a possible near-term flight demonstration. BENEFIT: The data that result from the Phase I and II efforts will allow condensable propellant thruster to be integrated with critical DoD spacecraft, enhancing spacecraft mobility. Bismuth thrusters represent a major advance because the peak T/P may be much higher than possible with xenon propellant. Today, typical high performance xenon HET’s generate 50 to 75 mN/kW of input power at an Isp between 1,000 and 3,000 sec, providing better thrust/power performance than any other electric propulsion system. A 10-kW class Bismuth thruster could have 1.3 to 1.5X better thrust/power and also higher efficiency than available from existing xenon HET’s. The proposed Phase II program will measure the plumes produced by laboratory model thrusters, and much of the data generated will feed back into the designs. More importantly, the data collected in the Phase II program will enable accurate numerical plume/spacecraft interaction models, leading to the potential use of a bismuth thruster on a near-term, high power, orbit raising mission such as DARPA’s Fast Access Spacecraft Testbed (FAST). Commercial applications include orbit maintenance and orbit raising of ComSats and other satellites. The high available thrust/power would significantly decrease the lag time between launch and operational status. High power bismuth thrusters could also be used for interplanetary missions. Bismuth also makes sense from an environmental standpoint; like xenon, bismuth is non-toxic, but bismuth is abundant while xenon, which has many terrestrial applications, is scarce.