This dissertation presents research aimed at extending the efficient operation of 1600 s specific impulse Hall thruster technology to the 2000 to 3000 s range. Motivated by previous industry efforts and mission studies, the aim of this research was to develop and characterize xenon Hall thrusters capable of both high-specific impulse and high-efficiency operation. During the development phase, the laboratory-model NASA 173M Hall thrusters were designed and their performance and plasma characteristics were evaluated. Experiments with the NASA-173M version 1 (v1) validated the plasma lens magnetic field design. Experiments with the NASA 173M version 2 (v2) showed there was a minimum current density and optimum magnetic field topography at which efficiency monotonically increased with voltage. Comparison of the thrusters showed that efficiency can be optimized for specific impulse by varying the plasma lens. During the characterization phase, additional plasma properties of the NASA 173Mv2 were measured and a performance model was derived. Results from the model and experimental data showed how efficient operation at high-specific impulse was enabled through regulation of the electron current with the magnetic field. The electron Hall parameter was approximately constant with voltage, which confirmed efficient operation can be realized only over a limited range of Hall parameters.

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Development and Characterization of High-Efficiency, High-Specific Impulse Xenon Hall Thrusters

The use of Solar Electric Propulsion (SEP) can provide significant benefits for the human exploration of near-Earth asteroids. These benefits include substantial cost savings - represented by a significant reduction in the mass required to be lifted to low Earth orbit - and increased mission flexibility. To achieve these benefits, system power levels of 100's of kW are necessary along with the capability to store and process tens of thousands of kilograms of xenon propellant. The paper presents a conceptual design of a 300-kW SEP vehicle, with the capability to store nearly 40,000 kg of xenon, to support human missions to near-Earth asteroids.

300-kW Solar Electric Propulsion System Configuration for Human Exploration of Near-Earth Asteroids

A laboratory-model Hall thruster with a magnetic circuit designed for high-specific impulse (2000‐3000 s) was evaluated to determine how current density and magnetic field affect thruster operation. Results have shown for the first time that a minimum current density and optimum magnetic field shape exist at which efficiency will monotonically increase with specific impulse. At the nominal mass flow rate of 10 mg/s and between discharge voltages of 300 and 1000 V, total specific impulse and total efficiency ranged from 1600 to 3400 s and 51 to 61%, respectively. Comparison with a similar thruster showed how efficiency can be optimized for specific impulse by varying the shape of the magnetic field. Plume divergence decreased from a maximum of 48 deg at 400 V to a minimum of 35 deg at 1000 V, but increased between 300 and 400 V as the likely result of a large increase in discharge current oscillations. The breathing-mode frequency continuously increased with voltage, from 14.5 kHz at 300 V to 22 kHz at 1000 V, in contrast to other Hall thrusters where a sharp decrease of the breathing-mode frequency was found to coincide with increasing electron current and decreasing efficiency. These findings suggest that efficient, high-specific impulse operation was enabled through the regulation of the electron current with the applied magnetic field.

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High-specific impulse Hall thrusters, part 1: Influence of current density and magnetic field

Tests results and performance comparisons of coated and un-coated skutterudite based segmented unicouples

Performance and plasma measurements of a high-specific impulse (2000‐3000 s) Hall thruster were analyzed using a phenomenological performance model that accounted for a partially ionized plasma containing multiply charged ions. Anode efficiency over discharge voltages of 300‐900 V ranged from 57 to 69%, which corresponded to 89‐97% voltage utilization, 86‐90% mass utilization, 77‐81% current utilization, and 97‐99% charge utilization. Although the net decrease of efficiency due to multiply charged ions was at most 3%, the effects of multiply charged ions on the discharge current could not be neglected because the increase of the discharge current with voltage was primarily due to the increasing fraction of multiply charged ions. This and the fact that the maximum deviation of the electron current from its average value was only +5/−14% illustrated how efficient operation at high-specific impulse was enabled through the regulation of the electron current with the applied magnetic field. The electron Hall parameter, defined by acceleration zone plasma properties, was nearly constant with voltage, decreasing from an average of 210 at 300 V to an average of 160 between 400 to 900 V.

/pdf/high-specific-impulse-hall-thrusters-part-2-efficiency-51w759yag9.pdf

High-Specific Impulse Hall Thrusters, Part 2: Efficiency Analysis

Sample mission application of capacitor powered Hall thrusters

COMPASS Final Report: Advanced Lithium Ion Venus Explorer (ALIVE)

Human Telerobotic Exploration of Venus: A Flexible Path Design Study

NASA has long been conducting studies which apply different in-space propulsion technology assumptions to the mission of sending humans to Mars. Two of the technologies under study that are considered to be the most near-term with respect to technology readiness level (TRL) are traditional chemical propulsion systems and high-power Solar Electric Propulsion (SEP) systems. The benefit of relatively low trip times inherent in using impulsive chemical propulsion systems to perform the full round-trip delta V is hampered by the large propellant mass required to perform these burns for human Mars missions. SEP systems offer the benefit of much lower propellant requirements to perform the same round-trip missions, at the cost of longer trip times. Traditionally, impulsive chemical systems are better suited than SEP when used in a gravity well, and SEP systems are more efficient than traditional impulsive systems when used in interplanetary space. A mission to Mars includes both of these scenarios, and thus several NASA architecture studies performed over the last few years have looked to combine the use of both SEP and chemical propulsion systems where they are the most beneficial to human Mars missions. This combined propulsion system concept has been referred to as a SEP/Chem hybrid Mars Transfer Vehicle and is currently shown as the concept Deep Space Transport (DST) in the March 2017 NASA presentation to the National Aerospace Council (NAC).

/pdf/nasa-grc-compass-team-conceptual-point-design-and-trades-of-ojj3gzpp53.pdf

NASA GRC Compass Team Conceptual Point Design and Trades of a Hybrid Solar Electric Propulsion (SEP)/Chemical Propulsion Human Mars Deep Space Transport (DST) Vehicle

Steven R. Oleson

Papers

Sample mission application of capacitor powered Hall thrusters

COMPASS Final Report: Advanced Lithium Ion Venus Explorer (ALIVE)

Human Telerobotic Exploration of Venus: A Flexible Path Design Study

NASA GRC Compass Team Conceptual Point Design and Trades of a Hybrid Solar Electric Propulsion (SEP)/Chemical Propulsion Human Mars Deep Space Transport (DST) Vehicle

HERRO Mars: Design Study for a Highly-Robotic Human Mission to Mars