Ultrashort pulsed laser filament-produced plasma electrical conductivity measurement by guided microwave attenuation

The electrical conductivity of USPL filament-produced plasma is measured by guided microwave attenuation at 3 GHz and 40 GHz. A waveguide (WR284 for 3 GHz and WR22 for 40 GHz) is driven and signal received via 50 Ω adapters on opposite ends. A filament cluster is produced by focusing the beam of a Ti:Sapphire 600 nm, 400 fs laser in air using a 50 mm diameter, 3 m focal length mirror. The cluster passes through a hole in the waveguide to attenuate the signal. 3-D transient time-resolved COMSOL Multiphysics™ simulations, where the cluster is represented by a 1 mm diameter cylinder of time-varying conductivity, is used to design the diagnostic and interpret data. Conductivity is inferred from S-band measurements to be approximately 600 mho m⁻¹ from a 26 mJ shot. This is based on attenuation matching that of a simulation of the same conductance, after taking into account the estimated actual filament diameter (100 μm) and number (13). Available Q-band results will be presented.