Faraday probes are a common plasma diagnostic used to determine the local ion charge flux of electric propulsion plumes. Standard practices, guidelines, and recommendations are provided for experimental methods and analysis techniques that aim to standardize community practices, to mitigate test environment effects, and to reduce systematic measurement error in order to improve plume predictions in the space environment. The approaches are applicable to time-averaged plasma properties in the near-field and far-field of electric propulsion plumes, with emphasis on Hall effect thrusters and gridded ion thrusters. Considerations for other electric propulsion technologies are provided, including electrosprays, arcjets, and electromagnetic thruster concepts. These test strategies are expected to increase the quality of comparisons between different thrusters and vacuum environments, thereby broadening the applicability of ground-based measurements and enhancing the fidelity for on-orbit predictions and modelin...

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Recommended Practice for Use of Faraday Probes in Electric Propulsion Testing

The electric propulsion community has been implored to establish and implement a set of universally applicable test standards during the research, development, and qualification of electric propulsion systems. Variability between facility-to-facility and more importantly ground-to-flight performance can result in large margins in application or aversion to mission infusion. Performance measurements and life testing under appropriate conditions can be costly and lengthy. Measurement practices must be consistent, accurate, and repeatable. Additionally, the measurements must be universally transportable across facilities throughout the development, qualification, spacecraft integration, and on-orbit performance. A recommended practice for making pressure measurements, pressure diagnostics, and calculating effective pumping speeds with justification is presented.

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Recommended Practice for Pressure Measurements and Calculation of Effective Pumping Speeds during Electric Propulsion Testing

A flight-model SPT-100 Hall thruster from the Fakel Experimental and Design Bureau was characterized with respect to performance and plume properties at The Aerospace Corporation and at L-3 Electron Technologies, Inc. over vacuum facility background pressures ranging from 2.3×10 -4 to 9.5×10 -3 Pa (1.7×10 -6 to 7.1×10 -5 Torr). Thrust and xenon mass flow rate measured at both facilities agreed to within the measurement uncertainties. Thrust decayed exponentially, while flow rate increased linearly with decreasing pressure. Using simple curve-fits for extrapolation, thrust, specific impulse, efficiency, and flow rate were projected to change by -4, -10, -14, and 7% respectively at zero pressure relative to 6.7×10 -3 Pa (5.0×10 -5 Torr). A decrease in net ion accelerating voltage and an increase in plume divergence were identified as contributors to decreased thrust with decreasing pressure. The amplitude of discharge current oscillations decreased linearly with decreasing background pressure, while the center frequency of the oscillations also decreased, from 26 to 16.5 kHz, but in non-linear fashion. Neutral density near the exit plane of Hall thrusters can be quite low, O(10 18 ) m -3 , such that background neutrals from the residual gas in the vacuum chamber may significantly influence ionization and conduction there.

The Effect of Background Pressure on SPT-100 Hall Thruster Performance

We describe the effects of finite pressure in a large vacuum facility on a magnetically shielded Hall thruster using internally and externally mounted cathodes. A numerical model of the pumping system was first used to guide the placement of an ionization gauge and an auxiliary propellant injector used for increasing the backpressure. The laboratory model H6MS Hall thruster was operated at 300 and 800 V discharge voltage, 2-20 A discharge current, and 0.6-9.0 kW discharge power at pressures calibrated for xenon from 2.4 to 100 uTorr-Xe. The hollow cathode was located in three positions: internally mounted on thruster centerline and two external mounts outside of the magnetic circuit. Performance variations exceed that expected from ingestion of background gas in all configurations but were still consistent with prior studies of unshielded Hall thrusters. Magnetic field strength, cathode location, and magnitude of discharge oscillations can greatly affect the apparent level of ingestion, which suggests that mechanisms other than ingestion of background gas affect the pressure sensitivity of the discharge. In particular, cathode location is shown to have a profound influence on the measured performance and stability. When the internally mounted cathode is used, the plasma response is shown to be much less sensitive to pressure changes than when an externally mounted cathode is employed. Our results suggest that high-power Hall thrusters using internally mounted cathodes minimize the effects of finite pressure on performance and stability, providing a path for reliably correlating ground tests with flight performance.

Finite Pressure Effects in Magnetically Shielded Hall Thrusters

The particle-based coupling between background gas flows in vacuum test facilities and neutral ingestion into Hall effect thrusters is investigated. An analytical model of the facility background flow environment is developed to accommodate facilities with different geometries and pump placements, as well as compute the ingested flow rate of background neutrals into a given Hall effect thruster. The ingested flow rates computed by the model are shown to predict previous empirical datasets taken using the 5 kW P5, the 6 kW H6, and the 1.5 kW SPT-100 Hall effect thrusters in different test facilities to within the experimental uncertainty. When compared to predictions generated assuming ingestion by the random flux of neutral particles, the ingested flow rates computed by the background flow model are shown to be 40 to 70% closer to the empirical measurements without requiring any semiempirical inputs. It is also shown that the neutral ingestion at a fixed facility pressure can vary by as much as 91%, sugge...

Background Flow Model of Hall Thruster Neutral Ingestion

2000-hour Wear-Testing of the HERMeS Thruster

The electric propulsion community has been implored to establish and implement a set of universally applicable test standards during the research, development, and qualification of electric propulsion systems. Existing practices are fallible and result in testing variations which leads to suspicious results, large margins in application, or aversion to mission infusion. Performance measurements and life testing under appropriate conditions can be costly and lengthy. Measurement practices must be consistent, accurate, and repeatable. Additionally, the measurements must be universally transportable across facilities throughout the development, qualification, spacecraft integration and on-orbit performance. A preliminary step to progress towards universally applicable testing standards is outlined for facility pressure measurements and effective pumping speed calculations. The standard has been applied to multiple facilities at the NASA Glenn Research Center. Test results and analyses of universality of measurements are presented herein.

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Michael W. Swiatek

Papers

Recommended Practice for Pressure Measurements and Calculation of Effective Pumping Speeds during Electric Propulsion Testing

2000-hour Wear-Testing of the HERMeS Thruster

A Step Towards Electric Propulsion Testing Standards: Pressure Measurements and Effective Pumping Speeds

Recommended Practice for Pressure Measurements and Calculation of Effective Pumping Speeds During Electric Propulsion Testing