Laser pulse with power above P_cr~3GW propogating in air will self focuse due to Kerr non linearity and produce lang plasma filaments on it's way. We present a simple non-intrusive experimental method of measuring the conductivity of those laser generated  filaments. The method is based on filament interaction with low intensity microwave radiation in a rectangular waveguide. The suggested diagnostics allow a complete single shot temporal analysis of filament plasma decay with resolution better than 0.3 ns and high spatial resolution along the filament.  The experimental results are compared to numerical simulations, and an initial electron density of 7 × 10¹⁶ [cm^-3] is obtained.

Appl. Phys. Lett. 99, 141503 (2011)