Combining optical emission spectroscopy and multivariate data analysis methods for an empirical assessment of plasma parameters of ion thrusters

Abstract

Rising demand of electric propulsion spacecrafts has triggered research for better, more reliable thrusters and alternative propellants. To increase the speed of development, testing and space qualification of these new thrusters and propellants, it is necessary to employ a wide range of diagnostics. In this work, a new diagnostic method has been developed for characterizing the plasma of radio-frequency ion thrusters. However, the method should, in principle, also be applicable to other plasma based electric propulsion devices. The method makes use of plasma parameter measurements with established Langmuir probe diagnostics on a reference setup similar to a real thruster. At the same time, optical emission spectroscopy of the plasma near the Langmuir probe is performed. Both of these measurements are combined in order to reveal the correlation between a plasma’s optical emission spectrum and its plasma parameters. An integral part of the evaluation process is the compression of the information contained in the spectrum, which is done using principal component analysis. Contrary to the normal procedure of getting plasma parameters from a spectrum, this method has no need for theoretical plasma models but is purely empirical. Therefore, it does not rely on databases with cross sections and transition probabilities. In particular, these circumstances make it a useful tool for analyzing complex plasmas made from gas mixtures or exotic propellants like iodine. In the future, these diagnostic options may be employed to assist in thruster characterizations to speed up tests and qualification efforts.