Plasma Diagnostics for Electric Propulsion Thrusters

The plume plasma generated by electric thrusters contains a large number of charged particles, which cause damages such as force-thermal effects, sputtering deposition, and electromagnetic interference to key components of spacecraft, including solar panels, communication systems, and scientific instruments. Plasma diagnostics can obtain information on the density, velocity, and temperature of ions and electrons through methods such as Langmuir probes, Faraday probes, emission probes, Retarding Potential Analyzer (RPA), spectroscopy, and laser-induced fluorescence, thereby reflecting macroscopic parameters such as momentum, thrust, and energy distribution.
Plasma diagnostics can be approached from two aspects: charge and luminescence. Charge-related information allows the acquisition of parameters such as plasma space potential, particle temperature, density, velocity, and charge; while luminescence information provides parameters such as composition, temperature, density, and velocity. Electrostatic and optical diagnostic methods each have their advantages, and it is optimal to use joint diagnostics to obtain more plasma parameter information, thereby clarifying the plasma physical processes.
First, it is recommended to use multiple methods to measure the same parameter under the same operating conditions of a plasma system to obtain consistent results, which can greatly enhance the confidence in diagnostics. For example, plasma electron temperature can be obtained through five methods: Langmuir single probe, Langmuir triple probe, isothermal "Barometric Law", non-isothermal "Poly-tropic Law", and floating space potential inference method. This approach has yielded radial distributions of electron temperature with consistent order-of-magnitude averages but slightly different variation trends.
On the basis of improving the accuracy of individual parameters, efforts should be made to obtain multiple parameters of the same plasma system under the same operating conditions, such as plasma space potential, density, and velocity distribution, which can help explain more physical mechanisms.
Combining contact electrostatic probes with non-contact optical diagnostic methods can broaden the applicable range of plasma diagnostics, acquire more plasma parameters, and provide support for interpreting plasma physical laws.