Modeling picosecond-laser-driven neonlike titanium x-ray laser experiments

Abstract

The technique of first using a nanosecond pulse to preform and ionize the plasma and then using a picosecond pulse to heat the plasma enables low-Z neonlike and nickellike ions to lase, driven by small lasers, with only 10 J of energy. Recent experiments at the Compact Multipulse Terawatt laser facility at Lawrence Livermore National Laboratory have demonstrated lasing in neonlike titanium by irradiation of 1-cm-long slab targets of titanium with a 4.8-J, 800-ps prepulse that is followed 1.6 ns later by a 6-J, 1-ps drive pulse. In this study we model the neonlike titanium x-ray laser under those experimental conditions. The LASNEX code is used to calculate the hydrodynamic evolution of the plasma and to provide the temperatures and densities to the XRASER code, which then performs the kinetics calculations to determine the gain. The temporal and spatial evolution of the plasma is studied both with and without radiation transport included for the 3d and the 3s→2p neonlike titanium resonance lines. Large regions with gains greater than 80 cm-1 are predicted for the 3p 1S0→3s 1P1 neonlike titanium laser line at 32.6 nm. The gain is shown to be quasi-steady-state over these time scales with regard to the equilibration of the excited-state populations. The transient nature of the gain is shown to be due to the ionization balance in the plasma. Given the large gain and the large gradients in these plasmas, we calculate x-ray laser propagation, including refraction effects, to understand which regions have the right combination of high gain and low density gradients for an optical contribution to the x-ray laser output. Calculations with different delays between the long and the short pulses and with different durations for the short pulse are presented to provide a better insight into optimization of the laser output. High gain is also predicted and observed for the self-photopumped 3d 1P1→3p 1P1 laser line at 30.1 nm in neonlike titanium, and calculations are presented to help understand this lasing mechanism.