Aerospace Laboratory for Plasma Experiments
Electric propulsion (EP) is a type of in-space propulsion that makes use of electrical power to accelerate a propellant via electric and/or magnetic means. Compared to conventional chemical thrusters, the use of electrical power enhances efficiency of EP thrusters, using significantly less mass to accelerate the spacecraft to the same velocity. This is because the propellant mass is ejected from an EP thruster at a rate of up to twenty times that of a classical chemical thruster.
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EP has been in use since the 1960's in various forms. Significant scientific missions include:
At ALPE, we are developing small thrusters and supporting technologies for a variety of uses:
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Testing facility effects to determine electron mobility within vacuum facilities
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Development of microwave thruster technologies for small 10 - 50 W power levels
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Understanding cathode oscillations in magnetic fields
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Development of a micro-thrust stand for measuring thrust on both pulsed and steady state thrusters
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Development of high speed plasma diagnostics for understanding
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Understanding fundamental processes within electrospray thrusters
WHT-44
Hall effect thrusters (HETs) can be designed for many applications ranging from satellite orbit correction to deep space missions. HETs will play an exciting roll in the future of space travel, and NASA has specifically tasked HETs for use in future missions in the Technology Roadmap. In fact, HETs will be used on the Lunar Gateway as the primary propulsion technology.
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Within ALPE, the goal for the WHT-44 project was to design and prototype a small HET that is accessible to any university with standard vacuum, electrical, and machining resources. This HET was a low-cost alternative that provided significant educational value to students working on the project. By opening this field of study to more researchers and encouraging more universities to become involved in electric propulsion research, more knowledge will be contributed to the understanding of HETs. This increased understanding will contribute to better HET designs.
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ALPE produced the 44-mm Western Hall Thruster (WHT-44) with a total fabrication cost of just over $2,000, including the Xenon for operation. Testing of the WHT-44, coupled with a 1.5-A Bantam cathode from Plasma Controls LLC, was performed in ALPE’s vacuum facility.
WHT-44 operating on Xenon in the ALPE vacuum chamber
WHT-44 at startup and after manufacturing
Papers and Conference Proceedings
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Baird, M., Kerber, T., McGee-Sinclair, R., and Lemmer, K., “Plume Divergence and Discharge Oscillations of an Accessible Low-Power Hall Effect Thruster,” Applied Sciences special issue Plasmas for Space Propulsion, Vol. 11, 1973 (2021) DOI: https://doi.org/10.3390/app11041973
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WHT-MS
The WHT-MS is a 200-W magnetically shielded HET with an externally mounted cathode. This thruster is a retrofitted version of the Western Hall thruster, a 44 mm (WHT-44) diameter HET, built for academic investigations.
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The purpose of the WHT- MS's design was to improve upon that of the WHT-44, through the incorporation of magnetic shielding. In addition to magnetic shielding, the channel diameter, was increased to 49 mm. The magnetic field topology is created using an outer magnetic shell configuration to achieve magnetic shielding of the channel walls.
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As new limits in power are reached, HET research continues to be significant to the EP community. However, as many facilities are not capable of operating high-power HETs, low power alternatives enable research to be conducted in smaller facilities. The accessibility of low power HETs allow research of new technologies and challenges facing HET development.
WHT-MS operating at 200W on Xenon
Font view of WHT-MS
WEST
The electrospraying process is a highly efficient way to extract and electrostatically accelerate ions from a liquid. Electrospray thrusters are envisioned as both primary propulsion for micro/nanosatellites and high-precision thrust sources to enable low-noise drag compensation and precision pointing in larger scientific missions.
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Electrospray thrusters are a TRL 6/7 technology facing challenges in lifetime and propulsion efficiency. Electrical shorting between the ion emission site and the extraction electrode along with chemical decomposition of the fluid propellant results in only 10's to 100's of hours of operation. High beam divergence angle, and low mass utilization from anomalous neutral propellant losses have suppressed total propulsive efficiency and requires further investigation. Furthermore, accurately representing the space environment in ground testing facilities has been a recently highlighted issue for electrospray thruster developmen.
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ALPE has developed the Western Electrospray Single-emitter Thruster (WEST), a porous-media, passively-fed, ionic liquid ion source as a test platform for investigating fundamental questions in electrospray performance. Current research includes high-resolution optical imaging and optical emission spectroscopy of luminescence at the emitter tip and characterization of the affect extractor electrode geometry and alignment to the emitter has on performance.
(Top) Assembled WEST with split extractor electrode. (Bottom) Backlit image showing profile of emitter tip and extractor next to image of luminescent plume during ion emission. (Left) Two porous borosilicate emitters manufactured at ALPE before and after operation.
Cathodes
Hollow cathodes provide the necessary electrons for HET and gridded ion thruster operation. Cathodes utilize a thermionic material to produce the electrons. Typical cathode inserts are made of Lanthanum Hexaboride (LaB6) or a tungsten matrix impregnated with Barium Oxide (BaO).
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The main failure mechanism of hollow cathodes is the heating element that heats the insert to emission temperatures. The ALPE Heaterless cathode (AHC-3.2) was designed and manufactured at WMU with the intention of evaluating heaterless ignition characteristics. The insert material is a variation of BaO (featherweight cathode tip) developed by Plasma Controls, LLC. The AHC-3.2 features an interchangeable keeper face of threaded graphite.
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Current research focuses on plasma instability mechanisms in the plume of the cathode instigated by a magnetic field on various propellants (xenon, krypton, and argon). Centrally mounted cathodes in HETs are exposed to high-strength magnetic fields and a magnetic mirror. Under these conditions, a variety of waves can be excited, resulting in large-scale plasma oscillations. APLE investigates these waves with a variety of invasive diagnostic techniques.
H6 hollow cathode. Top: without applied feed. Bottom: applied magnetic field from upstream solenoid.
AHC-3.2 transition from spot mode (with three distinct high density regions) to plume mode
EBeam cathode operating with BIT-1
Plasma controls Bantum-class cathode.
Papers and Conference Proceedings
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Mooney, M., Lemmer K., and Klang, G. "Analysis of Alternative Propellants on the Dynamic and Turbulent Behavior of a Cathode Plasma" Gaseous Electronics Conference, October 9-13, 2023, Ann Arbor, MI
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Mooney, M. and Lemmer, K. "Wave characterization of a hollow cathode plume in a HET-like magnetic field" 37th International Electric Propulsion Conference, June 19-23, 2022, Boston, MA
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Mooney, M., Baird, M., Lemmer, K., “Featherweight Heaterless Hollow Cathode Characterization”, 36thInternational Electric Propulsion Conference, September 16-20, 2019, Vienna, Austria
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Microwave Thrusters
Microwave thrusters utilize electromagnetic waves to generate high density plasmas by depositing power through resonance processes such as electron cyclotron resonance (ECR). These thrusters have the unique advantage of being electrodeless providing increased lifetime and the capability of being compatible with all propellants.
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ECR thrusters are one type of microwave based thruster utilizing ECR as the only plasma generation method. These devices are highly scalable to both high and low thrust applications and could be utilized on all satellite platforms. Current limitations of small scale ECR thrusters are that they suffer from relatively poor efficiency
A 25 mm ECR Thruster has been developed at ALPE with the goal of studying and improving upon the efficiencies of small scale ECR thrusters. Focus has been into optimization of thruster geometry, microwave plasma coupling, and magnetic field topology.
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Operation of ECR thruster at 20 W & 0.3 SCCM (left) and 40 W & 5 SCCM
ECR thruster mounted in ALPE vacuum chamber
Thrust Stand
High-Speed Retarding Potential Analyzer (HSRPA)
A retarding potential analyzer (RPA) are an electrostatic plasma diagnostic which utilize a series of biased meshed grids to determine ion energy distribution functions (IEDFs). RPAs typically come in three and four grid configurations, each of which possess a collector behind the biased grids.
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A standard RPA combined with high-speed circuity to produce time-resolved measurements is regarded as a HSRPA. To capture the desired dynamics within an EP thruster plume, time-resolved diagnostics are required. A HSRPA offers an effective method for determining time-resolved IEDFs within the plume of EP thrusters.
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The ALPE RPA is a four-grid configuration, three electrostatically biased mesh grids and a floating grid. Its housing, mesh grids, retention rings, and wires are fabricated from 316 stainless steel, while alumina and MACOR are used as insulators between the grids and wires.
RPA configuration
Example of time-averaged IEDF
Example of a time-resolved IEDF
Orthogonal Acceleration Time-of-Flight Mass Spectrometry (OA-ToF-MS)
Time-of-Flight is a type of mass spectrometry that differentiates particle charge-to-mass ratio by drift time in a known length, field-free region, assuming the particles are of uniform charge and kinetic energy. It is widely used for evaluating ionic liquid electrospray thruster performance by deter-mining the ratio of monomer, dimer, trimer, and droplet populations in a plume.
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Recent optical emission spectroscopy studies of ionic liquid ion source plumes indicate the fragmentation of molecular ions. However, no fragmented species have been reported using linear ToF. Orthogonal acceleration ToF improves performance over linear ToF by separating the drift direction from the ion source direction. The goal of the OA-ToF-MS in development at ALPE is to characterize this fragmentation.
(Top) Cross section view of developed OA-ToF-MS showing the path of ions from the electrospray ion source to the microchannel plate detector. (Bottom) Simulated performance of the OA-ToF-MS using SIMION showing the theoretical mass resolution of the system separating fragmented species, monomers, dimers, and trimers.
Electrical Coupling
Confinement Cage (EC3)
Electric propulsion (EP) testing is conducted in grounded vacuum facilities across the country to obtain valuable experimental data on EP devices. Vacuum chambers and pumps are used to evacuate air from the sealed vessel simulating an in-orbit environment. However, a perfect representation of space cannot be achieved. The limitations of facilities can directly impact the operation of EP thrusters, resulting in what is termed facility effects. To investigate the impact of facility effects, specifically electrical facility effects caused by operation of an EP device in a grounded facility, EC3 was designed and fabricated.
EC3 is a plasma confinement cage consisting of three individual sections capable of measuring termination current and modifying the electrical boundary conditions surrounding a thruster in a vacuum chamber environment. Each section of EC3 is instrumented with a wideband current sensor with a bandwidth of 10 MHz. As current terminates on the individual sections, it will be measured by the respective current sensor. The termination paths of the charged species can be controlled by biasing individual sections of EC3. This control allows for changing the electrical configuration of the testing facility.
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EC3 individual sections
Inside EC3 with mounted cathode