Title: Spatially Resolved Electron Number Density Measurements Via Coherent Microwave Scattering

Abstract: This paper reports on the use of coherent microwave scattering (CMS) for spatially resolved electron number density measurements of elongated plasma structures induced at mid-IR femtosecond filamentation. The presented studies comprise one-dimensional mapping of laser filaments induced via 3.9 um, 127.3 fs laser pulses at output energies up to 15 mJ. The axial electron number density and laser intensity were measured to be invariant along the entire filament length for the tested laser pulse energies 5-15 mJ (2x$10^{15}$ cm$^{-3}$ and 30-40 TW/cm$^2$, respectively) which supports that intensity clamping conditions were achieved in the experiments. The proposed approach enables unprecedented, currently unavailable capabilities to conduct direct, absolute, and longitudinally resolved measurements of electron number density in laser filaments and to precisely characterize conditions associated with self-focusing and intensity clamping.