Detailed investigation on x-ray emission from laser driven high-Z foils in a wide intensity range : role of conversion layer and reemission zone

Detailed radiation hydrodynamic simulations are carried out to investigate x-ray emission process in four high-Z planar targets namely, tungsten (W), gold (Au), lead (Pb) and uranium (U) irradiated by 1 ns, 351 nm flat top laser pulses. A thorough zoning analysis is performed for all laser driven high-Z foils over a wide intensity range of $10^{12}-10^{15} W/cm^{2}$ with appropriately chosen photon energy range and recombination parameter. The resulting variation of conversion efficiency over the full intensity range exhibits an optimum for all materials which is explained by considering the characteristic emission contributions from two different regions of laser irradiated plasma, namely, conversion layer and remission zone. A new generalized single scaling relation based upon smooth broken power law is proposed for conversion efficiency variation along with the separate determination ($\eta_{S}$, $\eta_{M}$) in soft and hard/M-band x-ray regions. It has been observed that $\eta_{S}$ for Pb and W always lies in between that for Au and U for intensities smaller than $\sim 3\times 10^{13} W/cm^{2}$. On further increase in intensity, $\eta_{S}$ is observed to be maximum for Au and U whereas it is minimum for W. Significant contribution to M-band conversion efficiencies is observed in all elements for intensities higher than $\sim 2\times 10^{13} W/cm^{2}$ with maximum and minimum values attained by W and U, respectively. The results are explained by considering the contributions from the emission coefficients of all materials in both conversion layer and reemission zone up to corresponding photon cut-off energies at different laser intensities.
Comment: Submitted to Physics of Plasmas