Advanced plasma diagnostics via in-phase coherent microwave scattering

Thomson in-phase coherent microwave scattering (CMS) offers advanced diagnostic capabilities suitable for a wide range of low-temperature plasma applications. The Thomson CMS technique exhibits substantially higher sensitivity compared to its incoherent counterpart in the optical frequency domain (laser Thomson scattering), owing to its in-phase coherent nature.

This work presents three novel applications of the Thomson CMS technique. First, Thomson CMS measurements of photoionization rates in various gases are reported. A 24-inch cubic stainless steel vacuum chamber, with walls covered by microwave absorber panels, was constructed to provide an anechoic environment ideal for CMS measurements. Measurements of photoionization rates in krypton at 204.1 nm and 212.5 nm over a range of intensities were conducted. These processes are utilized in two-photon laser-induced fluorescence, and knowledge of the corresponding photoionization rates is essential for the unambiguous interpretation of the fluorescence data. Second, a novel focused-CMS technique was developed for diagnostics of conventional glow discharge. This technique enables spatially and temporally resolved measurements of electron number density and can be employed for diagnostics across a broad range of low-temperature plasmas, including glow discharges, pulsed plasmas, and electric propulsion systems. Operating at 60 GHz, the system utilizes a microwave-focusing Teflon lens to achieve spatial and temporal resolutions of approximately 1 cm and 1 ns, respectively, in plasma volumes with electron densities ranging from 10⁸ to 10¹² cm^-3. Third, absolutely calibrated two-color radar-REMPI (Resonance-Enhanced Multiphoton Ionization) with Thomson CMS detection for diagnostics of selective species in gaseous mixtures was demonstrated. The technique employs two non-resonant beams, with REMPI conditions established solely within their intersection region. This configuration enables the creation of fully ionized plasma within the overlap region, allowing for precise evaluation of the number density of the selected species.

This work was supported by the U.S. National Science Foundation (Grant No. 2409559).