Pressure Map of a Facility as a Function of Flow Rate to Study Facility Effects

A neutral gas background pressure map of the Large Vacuum Test Facility (LVTF) at the University of Michigan is presented. The LVTF was mapped at a series of cold anode flow rates corresponding to P5 Hall thruster operating conditions of 1.5, 3.0, and 9.0 kW. The chamber pressure was mapped at nominal xenon pumping speeds of 140,000 and 240,000 l/s with a rake consisting of five calibrated Bayard-Alpert (BA) hot-cathode ionization gauges. The cold flow results were used to validate a full 3-D numerical model of the LVTF with a cold-flowing Hall thruster. This computational facility model was built with MONACO, a 3D unstructured direct simulation Monte Carlo method (DSMC) code that includes asymmetric features such as the gridded chamber floor and cryopumps. This investigation is intended to begin initial development of the facility effects characterization being pursued by the Plasmadynamics and Electric Propulsion Laboratory (PEPL) at the University of Michigan. The measured axial pressure profiles on the thruster centerline indicate that the plume expands to the facility background pressure in approximately 1.7 m. The plume expansion appears to be independent of anode flowrate and facility background pressure. The experimental and computational data exhibit the same general trends. However, there is a discrepancy of a factor ranging between 4 and 6 between the measured data and the computed results. This discrepancy is thought to stem from the choice of neutral-to-cryosurface sticking coefficient and a high computational cryosurface temperature. The next steps include modifying the model, making a similar cold-flow map of a NASA vacuum facility, and making a hot-flow map of the LVTF.