Electron density, temperature, and axial charge displacement of an ultrashort pulsed laser plasma filament inferred from electrical conductivity measurements based on strong field approximation

The ionization rate and most probable momentum vector of electrons upon ionization and after optical pulse passage vs. ionization time t₀ is found from a nonadiabatic strong field approximation. The resulting electron temperature T after thermalization, axial current density J, and number density n are found as functions of electric field peak E₀. Conductivity σ as a function of E₀ is then found from its dependence on n and T. Inverting results in E₀ and, therefore, n and T as functions of σ. These are used to determine n and T of a linearly polarized USPL pulse's trailing plasma filament in air at a range of pressures from previously reported measurements of its σ and radial density profile. The measurements are based on the attenuation of a 3.2 GHz TE₁₀ mode externally driven in an S-band waveguide across which the filament passes in the TE direction through holes, and fast framing camera images, respectively. The diagnostic also measures the axial charge displacement Q (current time integral) from the signal when the 3.2 GHz driver is turned off. To help validate the model, Q is independently estimated from the product of its J result, the measured filament's cross-sectional area, and a current decay time estimate.