Diagnostics by RADAR REMPI: Microwave scattering from laser-induced small-volume plasmas

This work presents predictions and measurements of microwave scattering properties of laser sparks In air. This work is part of an ongoing study of Resonant Enhanced Multiphoton Ionization (REMPI) for the generation of a species selective ionization region and the use of microwaves (Radar) to detect the presence of that ionization. From a single laser induced breakdown, the model assumes and measurements confirm that the breakdown regime can be viewed as a point dipole scatterer of the microwave radiation and thus the detected microwave signal is directly related to the time evolving number of electrons. The delay between the laser pulse and the rise of the microwave scattering signal is a direct measure of the avalanche ionization process. If the volume is exposed to two sequential pulses, the experiments show that the signal amplitude increases significantly if the pulses are separated by less that a few tens of nanoseconds. Tens of microsecond delay can be used to extend the lifetime of the plasma. An examination of two simultaneous laser induced breakdowns separated in space shows that the microwave scattering from multiple breakdowns is coherent.