Thomson and Collisional Regimes of In-Phase Coherent Microwave Scattering Off Gaseous Microplasmas

TL;DR

This paper develops and experimentally validates a microwave scattering diagnostic technique for measuring electron numbers in microplasmas, emphasizing Thomson and collisional regimes, and demonstrates its effectiveness in various plasma conditions.

Contribution

It provides a theoretical foundation and experimental validation for a non-intrusive microwave scattering method to determine electron counts in microplasmas, especially in the Thomson regime.

Findings

Diagnostic is effective in the Thomson regime for electron counting.

Experimental validation includes plasma imaging and phase measurements.

The method is robust despite plasma inhomogeneities and collisional effects.

Abstract

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 microwave scattering off laser induced air-based microplasmas with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via intensified charge-coupled device (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 the diagnostic 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.