Coherent microwave scattering for diagnostics of small-size plasma objects

Measurements of electron number density in small-size plasmas are very challenging as many traditional diagnostic approaches cannot be used. The coherent microwave scattering (CMS) offers a convenient diagnostic solution for such small plasmas. It is based on measurement of the total number of electrons present in a small plasma object using the constructive coherent elastic scattering of microwaves. Scattered radiation can be detected and attributed to the absolute electron count in the plasma after appropriate system calibration with dielectric scatterers.

Fundamentals of the coherent microwave scattering with an emphasis on Thomson, collisional, and Rayleigh scattering in short, thin unmagnetized plasma media will be considered. Ideality of the CMS technique in the Thomson “free-electron” regime will be reviewed where a detailed knowledge of collisional properties (which are often difficult to accurately characterize) is unnecessary to extract electron number measurement from the scattered signal. Substantially higher sensitivity of the CMS in comparison with the incoherent counterpart in the optical frequency domain (laser Thomson scattering) will be discussed. Finally, several examples of the CMS application for diagnostics of small-size plasmas will be considered including nanosecond repetitively pulsed discharges in air and laser-induced ionization of various gases in UV, NIR, and mid-IR spectral ranges.