Ultrashort pulsed laser atmospheric filament properties and microwave radiation inferred from S-band guided wave interaction and self-emission

Abstract

The electrical conductivity $\sigma$ of the plasma filament left behind by an ultrashort pulsed laser (USPL) optical pulse after it is geometrically then self-focused in air via the Kerr effect is measured by attenuation of a 3.2 GHz TE${}_{10}$ mode within an S-band waveguide through which the filament passes, taking into account the characteristic radius $R$ of the filament, as determined by fast camera visible light imaging. Models of the major air constituents' ionization rate $W_i$ vs. local laser intensity $I$, and of temperature $T$ and mean axial electron momentum $⟨p_z⟩$ vs. peak laser intensity $I_0$ are then used to infer a hypothetical steady state filament's $I_0$, $T$, major species particle densities, and assumed axially invariant current time integral $Q$ and current decay rate $\nu$ after pulse passage. $Q$ is independently measured via the filament's self-emission signal in the waveguide for comparison. The theoretical far field microwave radiation pattern due to the actual axial variation in $Q=Q\left(z\right)$ is compared favorably to published measurements. A much lower upper bound on $\nu$ is inferred once such radiation is taken into account. Results are presented along a $30$ cm long filament at a broad range of atmospheric pressures.

19 pages, 30 figures/subfigures. Journal submission pending