Measurements of electron velocity distribution function in microwave cathode plume by incoherent laser Thomson scattering

This paper reports the development of a method of using incoherent laser Thomson scattering (LTS) to directly measure the electron velocity distribution function (EVDF) within a plasma plume. In this method, a 532 nm wavelength laser is injected into a plasma and Thomson scattering is detected with a CCD camera and a triple-monochromator. First, the measurement settings (e.g., laser power, number of integrations, and data analysis) were validated through a comparison between a microwave cathode and a hollow cathode. The results indicate that lower laser power is required as the photo-ionization from the metastable state particles increases the Thomson scattering. Second, the EVDFs of the microwave cathode at the plume were investigated for various anode currents and propellant flow rates. The results suggest that the non-Maxwellian EVDF was clearly observed at the low propellant flow rate, and the propellant flow rate is a more important parameter for the non-Maxwellian EVDFs than the anode current.