Time resolved X-ray diffraction using the FIDDLE diagnostic at NIF

The Flexible Imaging Diffraction Diagnostic for Laser Experiments (FIDDLE) is a new diagnostic tool at the National Ignition Facility (NIF) designed to observe in situ solid-solid phase transitions—and eventually solid-liquid transitions—at high pressures using time-resolved X-ray diffraction. FIDDLE incorporates five Icarus ultrafast X-ray imager sensors that capture 2 ns snapshots, which can be gated to collect X-rays over four frames separated by 2 ns to cover diffraction over tens of ns. The platform leverages the laser power at NIF for both the laser drive and the generation of 10 keV X-rays for approximately 14 ns using a Ge backlighter foil. The primary goal of FIDDLE is to observe diffraction at various stages of compression to investigate the kinetics of phase transitions. Pb serves as an ideal test material due to its well-characterized phase diagram and strong diffraction lines; we focus on the transition from HCP to BCC. Results will be presented from the first experiments using the FIDDLE diagnostic at NIF, where ramp-compressed Pb was subjected to peak pressures to between 70 and 110 GPa. Improvements in background mitigation—achieved through shielding against EMP disruption and unwanted X-ray sources—have enabled the successful observation of time-resolved diffraction in ramp-compressed Pb on all five sensors and at all timesteps.