Busek

Abstract: : Three different types of sub kilowatt class Hail thrusters tor on-board propulsion of small to mid size satellites are being developed at Busek. This paper describes their performance focusing on plasma behavior outside of the thrusters end contrasts the typical Hail thruster plasma with that encountered in MHD generators end accelerators researched in the past and familiar to the plasmadynamic community. A simple performance predicting analytical model. applicable to all sizes of thrusters also is presented. It adequately matches the measured thrust and specific impulse expressed in terms of primary electron loss parameter, and an overall voltage loss which includes the loss in the plasma bridge between the external cathode and the thruster. The external plasma plume was surveyed using a Faraday cup and an emissive probe to measure the beam current and the plasma potential. These measurements. together with the model support our visual observation that the plume. of a well performing thruster. forms at its center a highly conductive jet with sharply defined cone shaped boundaries. These boundaries were tentatively identified as an ion acoustic shock. The preliminary data and analysis indicate that the plasma downstream of the ion acoustic shock is at least partially responsible for the widely reported correlation between increasing test tank pressure and an increase in performance of Hall thrusters.

Abstract: In an effort to understand the technical issues related to running multiple Hall effect thrusters in close proximity to each other, testing of a cluster of four Busek BHT-200-X3 devices has begun in Chamber 6 at the Air Force Research Laboratory. Preliminary measurements have shown that the variations in the discharge currents of the four thrusters are synchronized, possibly due to cross talk through the thruster plumes. Measurements of plasma density, electron temperature, and plasma potential in the thruster plumes obtained using a triple Langmuir probe are presented. Anomalously high electron temperatures were recorded along the centerline of each thruster. Collisionless, magnetosonic shock waves induced by the ion-ion two-stream instability are proposed as a possible cause of the high temperatures. The unperturbed ion velocity distribution along the centerline of a Hall thruster is shown to be unstable and a simple geometric model is presented to illustrate the qualitative changes in plasma properties expected across the proposed shock. Estimates using this model show that relatively large changes in electron temperature are consistent with small changes in electron number density across a shock. Qualitative arguments are presented indicating that collisionless shocks are unlikely to form as a result of clustering multiple thrusters.

Abstract: : An axisymmetric hybrid-PIC model of the Hall thruster plasma discharge has been upgraded to simulate the erosion of the thruster acceleration channel, the degradation of which is the main life-limiting factor of the propulsion system. Evolution of the thruster geometry as a result of material removal due to sputtering is modeled by calculating wall erosion rates, stepping the grid boundary by a chosen time step and altering the computational mesh between simulation runs. The code is first tuned to predict the nose cone erosion of a 200 W Busek Hall thruster, the BHT-200. Simulated erosion profiles from the first 500 hours of operation compare favorably to experimental data. The thruster is then subjected to a virtual life test that predicts a lifetime of 1,330 hours, well within the empirically determined range of 1,287-1,519 hours. The model is then applied to the BHT-600, a higher power thruster, to reproduce wear of its exit ring configuration over 932 hours of firing. Though some optimized code features remain the same, others need adjustment to achieve comparable erosion results. Better understanding of the physics of anomalous plasma transport and low-energy sputtering are identified as the most pressing needs for improved lifetime models.

Abstract: Two flight-qualified clusters of four Colloid Micro-Newton Thruster (CMNT) systems have been delivered to the Jet Propulsion Laboratory (JPL) and subsequently delivered to ESA for spacecraft integration. The clusters will provide precise spacecraft control for the drag-free technology demonstration mission, Space Technology 7 (ST7). The ST7 mission is sponsored by the NASA New Millennium Program and will demonstrate precision formation flying technologies for future missions such as the Laser Interferometer Space Antenna (LISA) mission. The ST7 disturbance reduction system (DRS) is a payload on the ESA LISA Pathfinder spacecraft along with the European gravitational reference sensor (GRS) as part of the ESA LISA Technology Package (LTP). To achieve the nanometer-level precision spacecraft control requirements, each of eight thruster systems is required to provide thrust between 5 and 30 µN with resolution ≤0.1 µN and thrust noise ≤0.1 µN/vHz. Developed by Busek Co. Inc., with support from JPL in design and testing, the CMNT has been developed over the last six years into a flight-ready and flight-qualified microthruster system, the first of its kind. Recent flight-unit qualification tests have included vibration and thermal vacuum environmental testing, as well as performance verification and acceptance tests. All tests have been completed successfully prior to delivery to JPL. Delivery of the first flight unit occurred in February of 2008 with the second unit following in May of 2008. Since arrival at JPL, the units have successfully passed through mass distribution, magnetic, and EMI/EMC measurements and tests as part of the integration and test (I&T) activities including the integrated avionics unit (IAU). Flight software sequences have been tested and validated with the full flight DRS instrument successfully to the extent possible in ground testing, including full functional and 72 hour autonomous operations tests. In the summer of 2009 the cluster assemblies were delivered to ESA along with the IAU for integration into the LISA Pathfinder spacecraft. Spacecraft-level testing will include magnetics, acoustic, and thermal vacuum environmental testing with a planned launch and flight demonstration in April 2012. 1 2