Thomson and Collisional Regimes of In-Phase Coherent Microwave Scattering Off Small Plasma Objects

The total number of electrons in a small plasma object can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). In this paper, we develop a theoretical foundation for the CMS diagnostic approach through fluid-plasma and short dipole formulations – focusing on the Thomson and collisional scattering regimes for elongated microplasmas. We additionally present an experimental validation of this basis via 11 GHz CMS off laser-induced, variable pressure (1-760 Torr) air-based plasmas. The evaluation comprises radiation pattern measurements, intensified charge-coupled device plasma-volume imaging, and the determination of total scattering cross-sections, electron number densities, total electron numbers, relative phases between electron displacements and the driving field, and collisional frequencies (with particular novelty). Preliminary studies suggest an ideality of CMS in the Thomson scattering regime – which shares a space with regions of low optical nonlinearities and, additionally, does not require a detailed knowledge of plasma / gas properties for an accurate characterization of scattering. We further present the possibility of using relative phase measurements (with respect to the Thomson phase) in the mixed Thomson-collisional regime for measurements of the electron-gas collisional frequency.