X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

The conversion efficiency of 351 nm laser light to soft x rays ( 0.1 – 5 keV ) was measured for Au, U, and high Z mixture “cocktails” used as hohlraum wall materials in indirect drive fusion experiments. For the spherical targets in a direct drive geometry, flattop laser pulses and laser smoothing with phase plates are employed to achieve constant and uniform laser intensities of 10 14 and 10 15 W ∕ cm 2 over the target surface that are relevant for the future ignition experiments at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)]. The absolute time and spectrally resolved radiation flux is measured with a multichannel soft x-ray power diagnostic. The conversion efficiency is then calculated by dividing the measured x-ray power by the incident laser power from which the measured laser backscattering losses are subtracted. After ∼ 0.5 ns , the time resolved x-ray conversion efficiency reaches a slowly increasing plateau of 95% at 10 14 W ∕ cm 2 laser intensity and of 80% at 10 15 W ∕ cm 2 . The M -band flux ( 2 – 5 keV ) is negligible at 10 14 W ∕ cm 2 reaching ∼ 1 % of the total x-ray flux for all target materials. In contrast, the M -band flux is significant and depends on the target material at 10 15 W ∕ cm 2 laser intensity, reaching values between 10% of the total flux for U and 27% for Au. LASNEX simulations [G. B. Zimmerman and W. L. Kruer, Comm. Plasma Phys. Contr. Fusion 2, 51 (1975)] show good agreement in conversion efficiency and radiated spectra with data when using XSN atomic physics model and a flux limiter of 0.15, but they underestimate the generated M -band flux.