Microwave diagnostics of laser-induced small-volume plasma

This work presents the preliminary results of an ongoing study of microwave scattering from a laser-induced, small-volume plasma in room air. Microwave scattering from the precursor and after-spark evolution reveal the dynamic characteristics of the laser spark formation, growth and decay. In particular, the avalanche ionization phase is detected by microwave scattering in the precursor part, during and shortly after the laser pulse. A simplified model of the microwave scattering from this early stage of laser breakdown is presented and shown to qualitatively fit the experimental results. Subsequent features of the microwave signal provide further insight into the after-spark evolution and are still under study. This work also discusses the possibilities and limitations of measuring the local electron number density by laser breakdown enhanced microwave scattering.