In direct drive inertial confinement laser fusion, a pellet containing D-T fuel is imploded by ablation arising from absorption of laser energy at its outer surface. For optimal coupling, the focal spot of the laser would continuously decrease to match the reduction in the pellet's diameter, thereby minimizing wasted energy. A krypton-fluoride laser (λ = 248 nm) that incorporates beam smoothing by induced spatial incoherence has the ability to produce a high quality focal profile whose diameter varies with time, a property known as focal zooming. A two-stage focal zoom has been demonstrated on the Nike laser at the Naval Research Laboratory. In the experiment, a 4.4 ns laser pulse was created in which the on-target focal spot diameter was 1.3 mm (full width at half maximum) for the first 2.4 ns and 0.28 mm for the final 2 ns. These two diameters appear in time-integrated focal plane equivalent images taken at several locations in the amplification chain. Eight of the zoomed output beams were overlapped on a 60 μm thick planar polystyrene target. Time resolved images of self-emission from the rear of the target show the separate shocks launched by the two corresponding laser focal diameters.

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We report on our continued development of the advanced x-ray plasma diagnostics based on spherically curved crystals. The diagnostics include x-ray spectroscopy with one-dimensional (1D) spatial resolution, 2D monochromatic self-imaging, and backlighting, and can be extended to the x-ray collimating and 2D absorption and emission spectroscopy. The system is currently used, but not limited to diagnostics of the targets ablatively accelerated by the NRL Nike KrF laser. A spherically curved quartz crystal (2d=6.687 03 Å, R=200 mm) has been used to produce monochromatic backlit images with the He-like Si resonance line (1865 eV) as the source of radiation. The spatial resolution of the x-ray optical system is 1.7 μm in selected places and 2–3 μm over a larger area. Another quartz crystal (2d=8.5099 Å, R=200 mm) with the H-like Mg resonance line (1473 eV) has been used for backit imaging with higher contrast. Sherically curved mica (2d=9.969 Å in the second order or reflection, R=200 mm) has been used for backlighting of the low density foam cryotargets with the backlighter energy of 1.26 keV. Time resolution is obtained with the help of a four-strip x-ray framing camera. Time resolved, 20×magnified, backlit monochromatic images of CH planar targets driven by the Nike facility have been obtained with spatial resolution of 2.5 μm in selected places and 5 μm over the focal spot of the Nike laser. A second crystal with a separate backlighter has been added to the imaging system. This makes it possible to use all four strips of the framing camera. As a result we have four monochromatic snapshots of developing instabilities. We are currently exploring the enhancement of this technique to the higher and lower x-ray energies. A progress in high energy (4.5 keV) backlighting that has been made in cooperation with the LLNL will be reported.

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