Modeling of short-pulse-driven nickel-like X-ray lasers and recent experiments

The technique of using a nsec pulse to preform and ionize the plasma followed by a psec pulse to heat the plasma has enabled low-Z nickel-like ions to achieve saturated output when driven by small lasers with less than ten joules of energy. We model experiments done using the COMET laser at LLNL and the P102 laser at Limeil to produce Ni-like Pd and Ag lasers. The COMET experiments use a 2 J, 600 ps prepulse followed 700 psec later by a 6 J, 6 psec drive pulse in a 1.6 cm long line focus. The P102 experiments used a somewhat larger energy and were able to use different combinations of frequency doubled light for both the prepulse and short pulse drive. The LASNEX code is used to calculate the hydrodynamic evolution of the plasma and provide the temperatures and densities to the CRETIN code, which then does the kinetics calculations to determine the gain. The temporal and spatial evolution of the plasmas are studied both with and without radiation transport included to understand the role of the self photopumping process on the gain of the Ni-like 4f --> 4d laser lines as well as the gain of the usual collisionally driven Ni-like 4d --> 4p laser lines. In particular we study why the 4f --> 4d line lases well only when frequency doubled light is used with the prepulse in the P102 experiments. Experimental results are presented for Ni-like Pd including two-dimensional near-field and far-field images.