Improvements in backlighter efficiency and target shielding for time-resolved x-ray diffraction measurements at the National Ignition Facility (NIF)

At the National Ignition Facility, the time-resolved x-ray diffraction campaign records four snapshot diffraction patterns—with a temporal resolution of 2 ns and separated by 2 ns—during a single compression experiment. Data are collected with the Flexible Imaging Diffraction Diagnostic for Laser Experiments (FIDDLE) using five Icarus hybrid complementary metal–oxide–semiconductor (hCMOS) sensors. This multi-frame capability enables measurements of phase-transition kinetics, including solid–solid and eventually solid–liquid transformations, and supports efficient mapping of phase space with four data points per experiment. Achieving these measurements requires a high signal-to-noise ratio with a backlighter (BL) x-ray source, but the background is complex, and generally increases over the course of a shot. This paper details improvements in continuous x-ray generation with a Ge BL foil for >15 ns. In addition, we investigate a background contribution originating in the BL plasma, which generates hot electrons that impact the target and produce secondary x-rays. In the prior target configuration, secondary x-rays generated on the target sidewall had a direct line of sight to the Icarus sensors, increasing the background on the sensor as time progressed. We redesigned the target to reduce this line of sight, thereby more effectively “shadowing” the sensors. The modified design improved signal-to-noise ratio dramatically, thereby enabling us to improve from observing diffraction on only two sensors at 1–2 time steps to observing diffraction on all five Icarus sensors at all time steps.