Plasma parameter measurement on a RIT-10 using empirical correlations between non-invasive optical emission spectroscopy and Langmuir diagnostics

Characterising and understanding the plasma properties of a rf-coupled electric propulsion device is crucial during testing, qualification and development. Therefore, the optimization of existing diagnostic systems as well as the development of new ones is an important area of electric propulsion research. Here, we present an approach to non-invasively determine the plasma parameters of an operating radio-frequency ion-thruster. For this purpose, a correlation between non-invasive optical emission (OE) spectroscopy and intrusive Langmuir probe diagnostics measurements is established for a reference system. Both types of measurements are performed simultaneously for a wide range of operation points yielding a large reference data set. Based on a principal component analysis (PCA), a correlation between plasma parameters and corresponding OE spectra at different operational points is established. This correlation can then be applied to OE spectra of the plasma of an operating thruster to obtain non-invasively the corresponding plasma parameters, i.e., without having to employ intrusive Langmuir probes. This approach for evaluating optical spectroscopic data in terms of plasma parameters has no need for a theoretical microscopic modeling of the plasma. This makes this approach very versatile and easily transferable to cases where other propellants are used, since no knowledge of excitation cross sections or transition matrix elements and other microscopic parameters of the species of the plasma is required. Such an approach enables continuous monitoring of a thruster’s behavior during the qualification process.