Measurement of Plasma Decay Processes in Mixture of Sodium and Argon by Radar REMPI

This paper presents the current experimental and computational efforts of measuring sodium plasma decay processes in mixture of sodium and argon by using Radar REMPI, coherent microwave Rayleigh scattering of Resonance Enhanced Multi-Photon Ionization. A single laser beam resonantly ionizes the sodium atoms by means of 2+1 REMPI process which can only generate ionization of the sodium atoms and have negligible ionization of the argon. Coherent microwave scattering in-situ measures the total electron number in the laser-induced plasma. Since the sodium ions decays by recombination with electrons, microwave scattering directly measures the plasma decay processes of the sodium ions. A theoretical plasma dynamic model, including REMPI of the sodium and Electron Avalanche Ionization (EAI) of sodium and argon in the gas mixture has been developed. It confirms that the EAI of argon is several-order-magnitude lower than the REMPI of sodium. The theoretical prediction made for the plasma decay process of sodium plasma in the mixture matches experimental measurement.