Backlighter development at the National Ignition Facility (NIF): Zinc to zirconium

K-shell X-ray emission from laser-irradiated planar Zn, Ge, Br, and Zr foils was measured at the National Ignition Facility for laser irradiances in the range of 0.6–9.5 × 10¹⁵ W/cm². The incident laser power had a pre-pulse to enhance the laser-to-X-ray conversion efficiency (CE) of a 2–5 ns constant-intensity pulse used as the main laser drive. The measured CE into the 8–16 keV energy band ranged from 0.43% to 2%, while the measured CE into the He-like resonance 1s2–1s2p(1P) and intercombination 1s2–1s2p(3P) transitions, as well as from their 1s2(2s,2p)l–1s2p(2s,2p)l satellite transitions for l = 1, 2, 3, corresponding to the Li-, Be-, and B-like resonances, respectively, ranged from 0.3% to 1.5%. Absolute and relative CE measurements are consistent with X-ray energy scaling of (hν)^?3 to (hν)^?5, where hν is the X-ray energy. The temporal evolution of the broadband X-ray power was similar to the main laser drive for ablation plasmas having a critical density surface.