Quentin E. Walker

Arcjet Plasma Neutralization of Hall Thrusters, Part 1: Hybrid Thruster Mission Analysis

Clustering multiple thrusters has emerged as a favored option for extending Hall effect thruster propulsion to very high powers (100‐150 kW). However, there are inherent difficulties in the simultaneous neutralization of several Hall effect thrusters. Chief among these is the issue of unequal current sharing among multiple cathodes. For this reason, it might prove advantageous to use a single, high current neutralizer. Conventional Hall effect thruster neutralizers, hollow cathodes, typically consume 10% of the propellant flow and produce little or no thrust. An arcjet is an electrothermal electric thruster with moderate efficiency and specific impulse ranges. It is also a high plasma density device that is capable of supporting and amplifying electron current through volume ionization and is capable of neutralizing single or clusters of Hall effect thrusters. By using an arcjet as a neutralizer, Hall effect thrusters will also produce useful thrust from their neutralizers. Because of the arcjet’s lower specific impulse, the hybrid arcjet-Hall system will have a lower specific impulse than that of a pure Hall effect system. However by choosing suitable propellants, the hybrid Hall cluster will retain the high thrust efficiencies typical of pure Hall effect systems with the added benefit of lower total wet mass for select missions caused by a higher system thrust density. This work examines the application of an arcjet-neutralized cluster of Hall effect thrusters for a low Earth orbit to geosynchronous orbit transfer. The analysis shows that hybrid Hall effect clusters neutralized by a single medium power arcjet are advantageous for some orbit raising missions. Helium arcjet neutralizers coupled with conventional xenon Hall effect anodes appear to be the superior configuration because of their relatively high efficiencies and low tankage fractions when compared to alternative propellant mixes.

High Density Plasma Neutralization of a Hall Thruster

In the attempt to fill the performance niche between the arcjet and Hall thruster, we propose the use of a helium arcjet to neutralize Hall thrusters. Since the arcjet is a high plasma density device, one arcjet can potentially neutralize a cluster of Hall thrusters. In this preliminary study, we used a surrogate planar anode in the place of a Hall thruster anode to determine the effects of drawing electron current from a low power arcjet plume on the operation and performance of the arcjet. In all tests we are able to draw currents to the surrogate anode that were greater than the arc current. It is found that biasing the surrogate anode does lead to a perturbation in the arcjet discharge voltage, in some cases resulting in a voltage decrease of up to 40%. The helium arcjet exhibits arc voltage instabilities in the same spectral range of the instabilities that are intrinsic in Hall thrusters. We find however, that these arcjet instabilities decreased in strength and in bandwidth when a bias is applied to the anode. The effect of biasing on the overall arcjet performance (e.g., thrust, specific impulse) is still under investigation. However, preliminary studies made using an impact pressure probe confirms that there is little compromise in the arcjet thrust during the current draw. The use of laser-induced fluorescence to measure the velocity of the arcjet plume flowfield during an applied bias was hindered by the finding that the lower energy state of the helium transition used for the LIF measurements was effectively depopulated by the biasing a result supported by optical emission measurements of the plume.

Arcjet neutralization of Hall thrusters, part 2: Experimental demonstration

The use of an arcjet to provide electron current for a Hall thrusters is examined, as such a hybrid concept can fill a performance niche amongst available space propulsion options. We report on experiments that determine how much electron current can be drawn to a surrogate anode from the plume of low-power arcjets operating on hydrogen and helium and demonstrate the first successful operation of a low-power Hall thruster-arcjet neutralizer package. In the surrogate anode studies, we find that the drawing of current from the arcjet plume has only a weak effect on overall arcjet performance (thrust), with a slight decrease in arc voltage with increased extracted current. A single arcjet Hall thruster hybrid package was assembled for concept demonstration. When operating on helium with a nominal mass flow rate of 4.5 mg/s and at very low power levels (∼70‐120 W), the arcjet was able to effectively neutralize the ∼200‐900-W xenon Hall thruster causing little measurable departure from the hollow-cathode neutralized Hall thruster I-V characteristics up to 250 V. At higher helium mass flow rates, the Hall discharge current was slightly perturbed from its expected values, most likely because of the ingestion of helium in the chamber background.

Doppler-free absorption measurements of Stark broadening in a flowing hydrogen plasma

Abstract This paper presents measurements of electron number density in a low-density hydrogen arcjet plasma flow. A spatially translated Langmuir probe is used to obtain these data, and comparisons are made to measurements of electron density using Doppler-free absorption spectroscopy (DFAS) of the Stark-broadened spectral line of the atomic hydrogen Balmer-alpha (H α ) transition. A lineshape model is developed for the DFAS spectra which includes crossover resonances, and collisional coupling between the electronic states within the n =2 atomic hydrogen level. The electron number densities extracted from the Langmuir probe traces are found to be significantly less than those determined from the DFAS measurements. This difference is attributed to the averaging of the collected current over the extended probe surface.

Characterization of a low-power helium arcjet

A preliminary study of the structure of a low-power helium arcjet plume is presented. Here, we report on the measurements of electron number density, electron temperature, translational temperature, and helium velocities at the arcjet exit plane. These measurements are supported by a limited performance study, obtained using a thrust stand. Thrust efficiencies are found to range from 60 to 70 percent with a specific impulse in the range of 450-490 seconds. Electron densities and electron temperatures measured using a Langmuir probe were found to be comparable to those measured in similar hydrogen-fueled thrusters. The probe measurements served as input to an analytical model developed to investigate the feasibility of performing laser-induced fluorescence (LIF) measurements. A preliminary LIF study was undertaken and proved to be relatively successful. The measured velocities ranged from 9 to 15 km/s and the temperatures ranged from 300 to 900 K. The peak velocities were found to be unusually high considering the specific impulse measured and also in comparison to those seen in hydrogen arcjet flows. This difference is attributed to the higher operating efficiency.

LIF measurements of a low-power helium arcjet

Summary form only given. We are interested in measuring the plasma properties of the helium arcjet flowfield. A number of different diagnostic have been developed to investigate the arcjet plasma plume. Among these diagnostics, laser-induced fluorescence (LIF) has proven to be very useful due to it spatial resolution and species specific nature. This paper presents the results of LIF measurements of the velocity and temperature of the arcjet flowfield. From previous studies, the arcjet flowfield is known to have electron number densities in the range of 10/sup -12/ cm/sup 3/ and electron temperatures of 0.2-0.5 eV. In the preliminary study of the helium arcjet, velocities between 9 and 15 km/s were measured 7 cm downstream of the exit plane with temperatures between 300 and 900 K. In this study, the 587.5 nm transition will be used. A dye ring laser using Rhodamine 6G pumped by a 5 W argon-ion laser will be used to probe this transition. By comparing the measured line center position with that of a stationary reference source, the Doppler shift was measured and converted into an axial velocity. The FWHM of the lineshape will be used to determine the translational temperature.

Measurements of electron density at the exit of an arcjet thruster

This paper presents measurements of electron number density and electron temperature at the exit of a low power 1-kW class radiatively-cooled arcjet thruster operating on hydrogen as a propellant. A spatially translated Langmuir probe is used to obtain these data, and comparisons are made to measurements of electron density using Doppler-free absorption spectroscopy (DFAS) of the Stark-broadened spectral line of the atomic hydrogen Balmer-alpha (Ha) transition. A lineshape model is developed for the DFAS spectra which include cross-over resonances, and collisional coupling between the electronic states within the n = 2 atomic hydrogen level. The electron number densities extracted from the Langmuir probe traces are found to be significantly less than those determined from the DFAS measurements. This difference is attributed to the averaging of the collected current over the extended probe surface.