Ultrashort pulsed laser atmospheric filament electrical conductivity and radius measurements by guided wave attenuation in an S-band waveguide and visible light imaging

A diagnostic is described and demonstrated to measure the electrical conductivity σ of the plasma filament left behind by an ultrashort pulsed laser's optical pulse after it has self-focused in air via the Kerr effect. The filament is formed by a 30 mJ Ti:sapphire laser pulse with a center wavelength of 800 nm with a linearly polarized 50 mm diameter approx. 50 fs long pulsed beam focused with a 300 cm focal length spherical mirror. It passes through holes in the middle of an S-band waveguide in the TE₁₀ electric field direction. There is a transmitter at one end of the waveguide and a receiver at the other. The receiver signal is recorded with and (to record the effect of self-emission from the filament) without 3.2 GHz TE₁₀ mode excitation from the transmitter. The dependence of the mode's measured attenuation (corrected for self-emission) on σ and filament radius R is determined by 3-D steady state electromagnetic simulations. R is measured by fast optical imaging. Measurements are made along a filament's length at a broad range of atmospheric pressures.