Microplasma electron number density measurement by resonant coherent microwave scattering

In this paper, current efforts of measuring sodium plasma decay processes in the mixture of sodium and argon by Radar REMPI, coherent microwave scattering from Resonance Enhanced Multi-Photon Ionization are presented. First the method for quantitative and nonintrusive measurement of electron number density in a microplasma by resonant coherent microwave scattering is described. In principle, coherent microwave scattering reaches its maximum when the microwave frequency is at its resonance frequency. Details are discussed for both Rayleigh and Mie scatterings. A minimal detectable electron number density is estimated based on the resonance condition. In experiments, laser-induced microplasma in the mixture of sodium and argon up to 900K is generated by 2+1 REMPI process of sodium, which generates negligible ionization of argon. Microwave measurement of plasma decay processes in the microplasma is conducted. Prediction from a plasma dynamic model matches the experimental results.