Abstract
Non-invasive assessment of the plasma parameters is a useful tool for a reliable characterization of many electric thrusters for space applications. Due to high costs, limited availability, and growing use of electric propulsion in spaceflight, alternatives to Xe as a propellant are becoming increasingly important. One option is to use the lighter noble gas krypton or xenon/krypton gas mixtures as a propellant. We propose a versatile analytical approach for establishing empirical correlations between plasma parameters and optical emission (OE) spectroscopy utilizing principal component analysis (PCA). Our approach allows us to establish a surjective mapping of individual OE spectra via their PCA scores onto the corresponding plasma parameters. We prove the feasibility of this approach for Xe, Kr, and Xe/Kr mixed plasmas demonstrating that it is applicable for a wide range of propellant candidates. A major advantage is that the approach does not rely on any microscopic modeling of the OE spectra of the plasma. After having established corresponding reference mappings, the approach can be explored for determining non-invasively and spatially resolved plasma parameters of the propellant plasma of various kinds of operating ion thrusters, which operate in the same plasma regime as the reference plasma. Thus, this method may contribute to shorter qualification and testing times of ion thrusters.
Highlights
Nowadays, electric propulsion (EP) systems are an established option as spacecraft engines.[1,2] While they deliver less thrust compared to their chemical counterpart, EP systems excel due to their high fuel efficiency and the vast number of usable propellants yielding a large variability for implementing them in space mission scenarios
We have developed an empirical approach for correlating optical emission spectra of gas plasmas obtained at different operational points with the corresponding plasma parameters
The different operational points were defined by tuning two external parameters, the propellant gas flow, as well as the power coupled into the plasma for each set of combined optical emission and Langmuir measurements
Summary
Electric propulsion (EP) systems are an established option as spacecraft engines.[1,2] While they deliver less thrust compared to their chemical counterpart, EP systems excel due to their high fuel efficiency and the vast number of usable propellants yielding a large variability for implementing them in space mission scenarios. The high spatial resolution achievable in optical spectroscopy may allow establishing spatial maps of the plasma parameters of an inhomogeneous plasma An example, where this may be of interest, is the mapping of the radial distribution of plasma parameters inside the discharge vessel of gridded ion thrusters, such as RITs. In principle, plasma parameters can be extracted from OES using theoretical modeling of the electronic states of the atoms and ions responsible for the optical emission. Plasma parameters can be extracted from OES using theoretical modeling of the electronic states of the atoms and ions responsible for the optical emission Such theoretical plasma models can be found, e.g., for argon[31–44] and xenon,[4,21,22,24–27,41,45–47] but rarely for gas mixtures where even more microscopic processes need to be accounted for in order to obtain a reliable description. This method might be applied to iodine plasmas as well, as iodine is another promising alternative to xenon as a propellant.[65,66] We demonstrate that the challenges met when attempting a microscopic description of such plasmas can be circumvented by our approach, and the plasma parameters can be reliably extracted
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