' NSTAR Ion Thruster Plume Impacts Assessments

Tests were performed to establish 30-cm ion thruster plume impacts, includingplume characterizations via near and far-field ion current measurements, contamination, and sputtering assessments. Current density measurements show that 95% of the beam was enclosed within a 22° half-angle and that the thrust vector shifted by less than 0.3° during throttling from 2.3 to 0.5 kW. The beam flatness parameter was found to be 0.47, and the ratio of doubly charged to singly charged ion current density decreased from 15% at 2.3 kW to 5% at 0.5kW. Quartz sample erosion measurements showed that the samples eroded at a rate of between 11 and 13 ]am/khrat 25° from the thruster axis, and that the rate dropped by a factor of four at 40°. Good agreement was obtained between extrapolated current densities and those calculated from tantalum target erosion measurements. Quartz crystal microbalance and wimess plate measurements showed that ion beam sputtering of the tank resulted in a facility material backflux rate of -10 !_/hr in a large space simulation chamber. Nomenclature Jtot Total ion beam current, A data available for 30-cm xenon thrusters operated at R Radius in beam, m the NSTAR power levels of between 0.5 and 2.3 kW x coordinateof beam center-of-pressure This paper reports on preliminary measurements of y y coordinateof beam center-ofpressure NSTAR ion thruster plume properties and impacts performed todemonstrate diagnostics to be used in the 0 probe rake angle final thruster integration assessments. Introduction Previous studies of ion thruster plume impacts have focussed on either thrusters using mercury propellant The objective of the NASA Solar Electric or low power, small diameter xenon thrusters. Propulsion Technology Applications Readiness Byers, 2 reviewing results for mercury propellant (NSTAR) program is to flight-qualify and thrusters, separates ion thruster plume impacts into demonstrate a 30-cm diameter xenon ion thruster those arising from the ion beam, low-energy plasma system for primary propulsion applications. 1 In resulting from charge exchange collisions, neutral addition to thruster performance and lifetime propellant, and non-propellant effluxes. While • demonstrations, satisfying the program objectives neutral mercury deposition can be quite deleteriousto requires a complete assessment of the thruster plume spacecraft surfaces, neutral xenon has no such effects impacts on spacecraft subsystems and functions, and so is not directly considered in this study. Other , Potential plume impacts include sputtering, studies have demonstrated diagnostics for the beam contaminant deposition, surface charging and current density,3-5 doubly to singly charged current discharging, and communications signal degradation, density ratio,6 and charge-exchange phenomena,6,7 While ion thrusters have been the subject of extensive all of which are requiredto establishplume divergence ground and space testing, there are currentlyno plume and impacts. Results of these studies have been used to establish spacecraft integration requirements. 8,9 2.3 and 0.5 kW were 2.5 x 10-3 Pa and 8 x 10-6 ton', For instance, once the ion beam is characterized, respectively. "exclusion zones" can be estabfished to prevent sputteringof spacecraft surfaces. The doublycharged ion density impacts internal thruster erosion, thrust Diagnostics and Experimental loss factors, 10 and directly affects spacecraft Configurations integration because the ions are accelerated to twice the energy of the singly charged ions, and so can do Ion Current Density greater damage than the singly charged ions. An Near-field ion current measurements were made using understanding of the relationship between thruster a 1-cm2 area molybdenum probe rotated across the operating parameters and the doubly to singly charged EMT centerline 2-cm downstream of the thruster on ion current ratio is necessary to ensure that the centerline. Because of the dished grids, this distance thruster is not operated at unreasonable conditions, increased to 4.3-cm at the outer edge of the grids. Ion beam parameters are also used to establish The probe was biased to -34.2 V with respect to the thruster loss mechanisms.10 tank ground and the probe current was measured using a 1-kohm shunt. All near-field measurements were This paper reports on the development of beam made in the smaller vacuum facility, and should not diagnostics and preliminary results of ion beam have been affected by the higher facility pressure as diagnostics for the NSTAR 30-cm ion thruster, the minimam charge-exchangemean-free path is 4-m Following a brief description of the thrusters, at the highest facility pressure. This mean-free path facilities, and diagnostics, the results of beam current was evaluated using the cross-sections given in Ref. density measurements, doubly to singly charged ion 13. current densities, sputtering, and contamination measurements are presented. The impact of the test Far-field ion current densitymeasurements were made facility on the measurements is also discussed, by rotating a 1.4-m-radius rake of eight Faraday Finally, a summary of the major conclusions is probes located 3-m from the thruster exit plane given, through the plume. Far-field ion current measurements were made every 0.25 degrees as the probe rotated 365 degrees. The small overlap (5 Thrusters and Vacuum Facilities degrees) was used to ensure full circumferential coverage. A similar technique has been described Two thrusters, a Functional Model (FMT) and an before.5 The eight Faraday probes were separatedby Engineering Model (EMT) thruster were used to 0.2 m and each probe consisted of a 0.5-inch diameter measure the plume properties. A schematic of the molybdenum disc with a 0.3-cm wide molybdenum EMT is shown in Fig. 1. Differences between the guard ring separated from the main probe by a gap of FMT and EMT are minor, and include an increase in 0.05--era. Both the probes and guard rings were accelerator grid thickness from 0.38 to 0.51 nun and a biased to -9.5 V with respect to tank ground for the decrease in the grid compensation from 0.3% to measurements, and testing revealed that the results 0.2%. Other changes are described in Ref. 11. were insensitive to negative bias potential. The farfield measurements were made using the FMT in the Tests were conducted in two vacuum facilities. The large vacuum facility. The high pumping speed of far-field ion current density and beam/surface impacts this facility was required because the thruster/probe were measured in NASA's large space propulsion test separation could result in substantial impacts from bed, a 4.6-m diameter, 18-m long tank with 22 0.8-m charge-exchange collisions. The charge-exchange diameter oil diffusion pumps and 27 m2 of 20 K mean-free-path ranged from -60-m to ~80-m for gaseous helium cryopumping surface.12 The facility pressures between 1.3x10-4 and 1.1xl0 "4 Pa. pressure with the NSTAR thruster operating at 2.3 kW and 0.5 kW were 1.9 x 10-4 Paand 1.0 x 10-4 Doubly to Singly Charged Ion Density Pa, respectively. These pressures, and all those Ratio reported below, use an ion pressure gauge correction An ExB probe, fabricated at the Jet Propulsion , factor of 2.87 to adjust for the differential sensitivity Laboratory, was used to measure the relative of the gauges to air and xenon. The near-field ion concentration of doubly to singly charged ions in the current density and doubly-to-singly charged ion EMT plume. The probe consisted of a 9.5-cm long , collimator with 1.7-cm high, 0.10-cm wide slits at current ratios were measured in a smaller facility, a 1.5-m diameter, 4.3-m long tank with four 0.8-m either end, a 14-cm long drift region, and a tungsten diameter oil diffusion pumps. The facility pressures coated current collector located at the rear of the in this tank with the NSTAR thruster operating at probe. Two permanent magnets and two bias electrodes oriented perpendicularly to provide the