Characterization of Femtosecond Laser Filament Conductivity with Varying Pressure

Our experiments examine the effect of changing gas pressure on the conductivity of an ultra short pulse laser filament plasma and how the conductivity varies longitudinally in the focusing region. By comparing digital images of the plasma fluorescence to its conductivity, we are able to characterize the plasma decay. To determine filament conductivity, we measure microwave scattering along the laser propagation direction. We use a WR284 rectangular waveguide with a 1.5 cm hole that allows the beam through. A 3.2 GHz microwave signal is emitted in the waveguide, and signals are received through a waveguide-to-coax antenna connected to a HP8470B Schottky diode. In addition, a series of glass vacuum tubes is used with a 1.2 cm diameter section encompassing the focusing region. This glass tube is passed through the waveguide so measurements can be obtained along the plasma while air pressure is varied from 760 to 0.10 Torr. Filament conductivity is determined by comparing the signal attenuation during filamentation to COMSOL simulations. Our research provides insight into the extent a visual inspection of a filament plasma is representative of its true extent.