Laser-induced fluorescence spectroscopy for Kr thrusters

Abstract Electric propulsion requires exhaustive ground testing for the characterization of the devices (thrusters). Among the many plasma parameters accurately determined during the tests, ion velocity distribution is of particular importance. Non-intrusive tools like Laser-Induced Fluorescence (LIF) diagnostics enable the detection of ion velocity through Doppler shift measurements. This requires precise knowledge of the ions’ resonant wavelength when they are at rest, followed by analysis to account for mechanisms that alter the spectral lines. Electric propulsion is transitioning towards the use of Krypton as a propellant, given the substantial rise in the cost of the more advantageous and customary Xenon gas. This transition implies suitable adaptation of LIF diagnostic tools. This paper proposes a combined LIF analysis on Kr I and Kr II to enhance the information gathered on the plasma by providing complementary data. When LIF analysis is applied in strong magnetically-confined plasma, the spectral profiles recorded are altered significantly. Performing LIF measurements in both Kr I and Kr II can provide a more accurate interpretation of data by detecting the effect of the magnetic field, and providing additional plasma parameters such as temperature and most probable velocity. The presented apparatus enables a more accurate determination of the wavelength of the investigated Kr II transition, resulting in an improved in accuracy ion velocity determination.