Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas

The total number of electrons in a classical microplasma can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). Here, we establish a theoretical basis for the CMS diagnostic technique with an emphasis on Thomson and collisional scattering in short, thin unmagnetized plasma media. Experimental validation of the diagnostic is subsequently performed via linearly polarized, variable frequency (10.5-12 GHz) microwave scattering off laser induced 1-760 Torr air-based microplasmas (287.5 nm O-2 resonant photoionization by similar to 5 ns, < 3 mJ pulses) with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipolelike radiation behavior, plasma volume imaging via ICCD photography, and measurements of relative phases, total scattering cross-sections, and total number of electrons N-e in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of CMS in the Thomson free-electron regime-where a detailed knowledge of plasma and collisional properties (which are often difficult to accurately characterize due to the potential influence of inhomogeneities, local temperatures and densities, present species, and so on) is unnecessary to extract N-e from the scattered signal. The Thomson scattering regime of microwaves is further experimentally verified via measurements of the relative phase between the incident electric field and electron displacement.

Automated summary

The paper presents a theoretical basis for the CMS diagnostic technique and validates its application in microplasmas through experiments. Findings suggest that in the Thomson free-electron regime, a detailed knowledge of plasma and collisional properties is unnecessary to extract the total number of electrons from the scattered signal.