Bismuth Hall Thruster Plume Impact Study

Because of its condensable nature Bi may cause unacceptable spacecraft contamination when used as a Hall thruster propellant. Experiments are proposed to determine the potential for bismuth deposition on spacecraft surfaces. The proposing team’s extensive experience with bismuth technology has uncovered surprising bismuth/surface interaction kinetics that will greatly complicate deposition measurements: based on evidence observed over three years of bismuth thruster operation the proposing team has discovered that the largest source of bismuth contamination during ground-testing is attributable to facility effects. A Phase I research effort is described to assess and minimize the role of facility wall interactions in ground-test measurements of bismuth deposition rates. Plume properties will be measured in a 2-kW Bi thruster via Faraday probe, RPA, and ExB probe. Deposition rates will be quantified through QCMs. Careful control techniques utilizing both an electrically biased QCM as well as a collimated QCM staring into a patented “bismuth trap” will allow identification of facility-induced deposition rates separately from those that would occur in space. A Phase II program is described for building and testing a 20-kW bismuth thruster for contamination studies on a flight-relevant device. The possibility of using a bismuth plume for defensive counter-space is considered.