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

Abstract

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, image-to-image timing (ps) and absolute timing (ps) are presented. Angular averaging of the images provides an average radius measurement of and an error in velocity of. This technique was applied on the Omega Laser Facility [Boehly et al., Opt. Commun. 133, 495 (1997)] and the National Ignition Facility [Campbell and Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)].