Non-invasive assessment of plasma parameters inside an ion thruster combining optical emission spectroscopy and principal component analysis

We present a non-invasive approach for determining plasma parameters such as electron temperature and density inside a radio-frequency ion thruster (RIT) using optical emission spectroscopy (OES) in conjunction with principal component analysis (PCA). Instead of relying on a theoretical microscopic model of the plasma emission to extract plasma parameters from the OES, an empirical correlation is established on the basis of conducting simultaneous OES and Langmuir diagnostics. The measured reference spectra are simplified and a PCA is performed. The PCA results are correlated with the plasma parameters of the Langmuir measurements yielding a one-to-one correspondence. This correlation allows us to derive the plasma parameters by analysis of a non-invasively determined emission spectrum without additional Langmuir measurements. We show how the plasma parameters can be calculated from OES measurements using this correlation. Under the assumption that the electronic system thermalizes on much shorter time scales than the period of the RF signal driving the thruster, we can also use time-resolved spectral data to determine the time evolution of plasma parameters. In future, this method may contribute to shorter test and qualification times of RITs and other ion thrusters.