Doppler-Free Spectroscopy Measurements of Isotope Shifts and Hyperfine Components of Near-IR Xenon Lines XENON AS A PROPELLANT GAS FOR ELECTRIC THRUSTERS

Abstract

Abstract. Xenon is currently used as propellant gas in electric thrusters, in which ejection of corresponding ions produces the desired thrust. As such a gas contains 9 stable isotopes, a non-intrusive determination of the velocity distribution function of atoms and ions in the thruster plasma plume, by means of absorption or fluorescence techniques, requires a precise knowledge of the line structure. We used Doppler-free Lamb-dip spectroscopy to determine isotopic shifts and hyperfine components of odd isotopes of several spectral lines of Xe atom and Xe + ion in the 825 -835 nm range XENON AS A PROPELLANT GAS FOR ELECTRIC THRUSTERS Xenon is currently used as propellant gas in electric thrusters like gridded ion engines and Hall effect thrusters A Hall Effect Thruster (HET) can be seen as a hollow annular ceramic channel confining a magnetized low pressure DC discharge generated between an external hollow cathode and an anode. Electrons emitted from the cathode are trapped around Larmor orbits and experience an azimuthal drift. The resulting low electron mobility induces a strong electric field that is concentrated in the vicinity of thruster exhaust. The propellant gas, which is fed through the anode, is efficiently ionized by trapped electrons in the downstream region. Created ions are then instantly accelerated along the thruster axis by the local electric field Profound understanding of physical phenomena governing the plasma dynamics inside a HET necessitates to accurately measure physical parameters such as the gas temperature, the magnetic field magnitude during thrusters' operation and especially the ion velocity distribution function. In order not to modify thruster plasma properties, these three quantities must be determined by means of laser-aided diagnostics providing information about Doppler broadening, Doppler shift and Zeeman splitting of xenon atomic and ionic lines. However, due to the existence of numerous Xe isotopes, some with non-zero nuclear spin, such measurements require a precise knowledge of the isotopic and hyperfine structure (HFS) of considered optical transitions. DOPPLER-FREE SPECTROSCOPY The hyperfine structure of a spectral line can be resolved when overcoming the limitation set by Doppler broadening. In this contribution, a non-linear absorption technique based on selective saturation of individual atomic transitions, the so-called Doppler free, or saturation, spectroscopy method, was applied with the use of a tunable single-mode diode lase