Measurements of the ablation-front trajectory and low-mode nonuniformity in direct-drive implosions using x-ray self-emission shadowgraphy

Self-emission x-ray shadowgraphy provides a method to measure the ablation-front trajectory and low-mode nonuniformity of a target imploded by directly illuminating a fusion capsule with laser beams. The technique uses time-resolved images of soft x-rays (> 1 keV) emitted from the coronal plasma of the target imaged onto an x-ray framing camera to determine the position of the ablation front. Methods used to accurately measure the ablation-front radius ($${\it\delta}R=\pm 1.15~{\rm\mu}\text{m}$$), image-to-image timing ($${\it\delta}({\rm\Delta}t)=\pm 2.5$$ ps) and absolute timing ($${\it\delta}t=\pm 10$$ ps) are presented. Angular averaging of the images provides an average radius measurement of$${\it\delta}(R_{\text{av}})=\pm 0.15~{\rm\mu}\text{m}$$and an error in velocity of$${\it\delta}V/V=\pm 3\%$$. This technique was applied on the Omega Laser Facility and the National Ignition Facility.