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 electron temperature $T$ and mean axial electron momentum $⟨p_z⟩$ vs. peak laser intensity $I_0$ are then used to infer the filament's $I_0$, $T$, major species particle densities, current decay rate $\nu$, and current time integral $Q$ after pulse passage. $Q$ is independently measured via the filament's self-emission in the waveguide signal for comparison. The theoretical far field microwave radiation pattern for the later $Q=Q\left(z\right)$ is calculated. It compares favorably to published measurements, provided $\nu$ is reduced to the value where the total radiated power is equal to the center-of-mass electron kinetic energy imparted by the laser's magnetic Lorentz force. Results are presented for a $30$ cm long filament at a broad range of atmospheric pressures.

arXiv comment19 pages, 30 figures/subfigures. Phys. Plasmas, as resubmitted with minor changes