On parametric variation of Cu Kα x-ray emission in intense ultra-short laser pulse solid interaction and correlation with fast electron transport processes

Abstract Experimental measurements of K-shell x-ray emission from the front of Cu target irradiated by intense ultra-short duration Ti:Sapphire laser pulse have been performed using x-ray crystal (HAPG/HOPG) spectrograph, for different thicknesses of target, laser angle of incidence and polarization. A wide range of laser intensity (non-relativistic to relativistic) was covered through variation of laser pulse energy, pulse duration and target position with respect to best focus. Maximum x-ray emission flux was observed for 7 µm thick Cu foil for p-polarized 25fs duration laser pulse, for angle of incidence of 550 and maximum laser intensity of ~4x1019 W/cm2 used in the experiment. For both thinner and thicker targets x-ray flux reduced. Interestingly, x-ray flux variation with laser intensity in the relativistic regime (~4x1018- 4x1019 W/cm2), for shortest 25fs duration laser pulse, showed two slightly different scalings i.e. slower/faster variation towards lower/higher end of the intensity regime, and measurement with thicker target of 50µm also showed two scalings but with opposite trend i.e. faster/ slower variation towards lower/higher end. X-ray emission flux decreased with decrease in laser intensity (up to ~1x1017 W/cm2) for longer laser pulse duration (up to 9.8ps), but increased slightly for larger laser spot size ( intensity ~5x1017 W/cm2) on the target and reduced subsequently. Maximum Kα x-ray flux of ~1.4×1010 photons/sr. was estimated for 7 μm target and laser to Kα conversion efficiency is found to be ~1.1x10-5. Geant4 simulations performed to study Kα x-ray flux variation with target thickness suggest that refluxing of electrons lead to ~2.9x enhancement in x-ray flux for the 7 µm foil compared to the 50 µm foil. Further, laser-to-electron conversion efficiency was also estimated to be ~6.5%, which was found to be similar with simulations performed with PrismSPECT spectroscopic code. X-ray flux variations with laser and target parameters are discussed in terms of applicable fast electron generation mechanisms and associated processes of refluxing and reacceleration, along with x-ray reabsorption.