This dissertation investigates the possibility of using compact and heaterless calcium aluminate electride hollow cathodes in different electric propulsion systems for space applications. As conventional hollow cathodes generally require a heater to reach the high operating temperatures necessary to thermally emit electrons, research on low temperature heaterless hollow cathodes as electron sources has been increasing. Efforts at Technische Universität Dresden have resulted in an operational hollow cathode design that can be reliably used for low current plasma discharges. Hollow cathodes are crucial components in electric propulsion systems to ionize the propellant and neutralize the extracted ion beam. The successful development of an operational hollow cathode opens the possibility of using the design in different low-power electric propulsion systems.
As the electron emission properties of C12A7:e- are still not well understood, a volume-averaged hollow cathode model has been developed as part of this thesis to obtain an improved insight into the plasma processes governing the cathode discharge. The model consists of two computational domains in which the plasma properties are volume-averaged. A lumped-node thermal model coupled with the plasma model provides the cathode temperature distribution for different operating points. The model moreover provides the discharge voltage which can be directly compared to experimental data. The thermal model was compared to thermal measurements to derive adequate values for free model parameters. The discharge voltage fits well for a 1 A discharge but diverges from measurement data at higher currents. The model is a starting point for further modeling efforts and needs to be verified using extensive plasma diagnostics.
The first electric propulsion system developed as part of this thesis is an electrothermal device that takes advantage of high particle temperatures in a hollow cathode discharge. A performance model and preliminary test series were used to derive design parameters for a prototype that was used for an extensive parameter study. The thruster reliably generates thrust over a current range between 1 A – 3 A. The thrust achieved with this device is in the high micronewton to low millinewton range. The specific impulse is on the order of 100 s, which is low for electric propulsion systems, and the high discharge voltages of approximately 50 V limit the achievable efficiency to <1%.
The second thruster concept is a DC discharge gridded ion thruster using a C12A7:e- hollow cathode as the discharge cathode and the neutral gas inlet. An analytical discharge model combined with a particle-in-cell simulation for ion extraction by electrostatically biased grids was used to design a modular testing prototype. The concept requires a low discharge current on the order of 200 mA. Operating the cathodes in a milliamp discharge current range proved to be difficult and was accompanied by high discharge voltages. Extracting an ion beam from the testing prototype was not successful.
The third propulsion system is a magnetoplasmadynamic thruster (MPDT) that takes advantage of a strong magnetic field generated by permanent magnets and an orthogonal current in a plasma discharge using a C12A7:e- hollow cathode. Conventional MPDTs require high current discharges to generate a sufficiently strong self-induced magnetic field. The developed concept is a design alternative to expand the operational envelope to lower powers. A major advantage is the comparatively easy scalability of the device. One prototype for the low amp current range was developed and successfully operated. The generated thrust is in the low millinewton range with a specific impulse up to 1,200 s. The test series highlighted thermal problems with the design. Consequently, a sub-amp version of the concept was developed. The thruster was successfully operated but required high mass flow rates, lowering the specific impulse and efficiency.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:79754 |
| Date | 27 June 2022 |
| Creators | Gondol, Norman |
| Contributors | Tajmar, Martin, Gessini, Paolo, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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