Influence of the temporal contrast on the characteristics of laser-produced ultrafast Kα Hard X-ray source

Abstract

Characteristics of the intense ultrafast quazi-monochromatic X-ray source at 17.4 keV generated by a multi-TW femtosecond Ti:Sa-laser beam ($λ$=800 nm) tightly focused on a thick molybdenum target are investigated within a wide range of laser pulse peak intensities ($10^{16}$ - $10^{19}$ W/cm$^2$) and temporal contrast ($10^6$ - $10^{10}$). Spatial and absolute spectrally-resolved energetic parameters of produced pulsed X ray emission are measured. Fluxes of Mo K$α$ line radiation reaching $2\times 10^9$ photons/sr per laser shot and energy conversion efficiency as high as $3.2\times 10^{-5} sr^{-1}$ are obtained at high laser contrast of $>10^9$. The interplay of resonance absorption, vacuum heating and ponderomotive (JxB) heating mechanisms of the laser-plasma interaction is clearly demonstrated by different K$α$ conversion efficiency dependencies at different laser pulse intensity and contrast parameters. The results are confirmed by examining dependencies on the laser incidence angle. In the relativistic intensity regime, the K$α$ efficiency shows similar values within the whole examined range of the laser temporal contrast, which varies over 4 orders of magnitude resulting in the electron density scale length L of the pre-formed plasma changing from L<<$λ$ to L>$λ$. It confirms the dominant role of the JxB mechanism accompanied by the effect of plasma interface steepening under laser radiation pressure in the relativistic interaction regime.