Filtered Low Angle Thomson Scattering for measurements of femtosecond generated plasma filament in air

In this work, we elaborate on details of Filtered low-angle Thomson scattering (FLATS) that is applied to measure electron properties of femtosecond (fs) generated plasma filament in air. The principle of FLATS technique is based on the spectral narrowing of the Thomson scattering (TS) collected in the forward direction. In this approach, the narrowed electron TS component is separated from the spectrally overlapping rotational Raman scattering, and other unwanted background scatterings such as Mie/Rayleigh scatterings are suppressed by an atomic filter and a volume Bragg grating filter. Thomson scattering from the fs filament was collected at the collection angle of 5 - 20 deg. Such low angles yield TS spectra having a spectral linewidth of several hundreds of GHz for electron temperature ~ 2 eV, and custom-built high spectral resolution spectroscopy is realized to resolve the narrowed TS lineshape.