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

Experimental measurements of K-shell x-ray emission from the front of a Cu target irradiated by an intense ultra-short-duration Ti:sapphire laser pulse have been performed using an x-ray crystal spectrograph (highly annealed pyrolytic graphite/highly oriented pyrolytic graphite) for different target thicknesses, laser angle of incidence (AOI) and polarizations. A wide range of laser intensity (non-relativistic to relativistic) was covered by varying the laser pulse energy, pulse duration and target position with respect to the best focus. The maximum x-ray emission flux was observed for a 7 µm thick Cu foil with a p-polarized 25 fs duration laser pulse with an AOI of 55° and maximum laser intensity of ∼4 × 1019 W cm−2. For both thinner and thicker targets, the x-ray flux was reduced. Interestingly, variation of x-ray flux with laser intensity in the relativistic regime (∼4 × 1018–4 × 1019 W cm−2) for the shortest duration laser pulse (25 fs) showed two slightly different scalings, i.e. slower/faster variation towards the lower/higher end of the intensity regime, and measurements with a thicker target of 50 µm also showed two scalings but with an opposite trend, i.e. faster/ slower variation towards the lower/higher end. The x-ray emission flux decreased with decrease in laser intensity (up to ∼1 × 1017 W cm−2) for a longer laser pulse duration (up to 9.8 ps) but increased slightly for a larger laser spot size (intensity ∼5 × 1017W cm−2) on the target and reduced subsequently. A maximum Kα x-ray flux of ∼1.4 × 1010 photons sr–1 was estimated for the 7 μm target and the laser to Kα conversion efficiency (CE) was found to be ∼1.1 × 10−5. Geant4 simulations performed to study variation of Kα x-ray flux with target thickness suggest that refluxing of electrons led to a ∼2.9× enhancement in x-ray flux for the 7 μm target and the laser to Kα CE was found to be ∼1.1 × 10−5. Further, laser-to-electron CE was also estimated to be ∼6.5%; this was found to be similar to simulations performed with the 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.