Measurement of cryogenic target position and implosion core offsets on OMEGA

Abstract

Cryogenic target implosions currently performed on the University of Rochester's OMEGA Laser System [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are of deuterium–tritium-filled polystyrene shells held at near the triple point temperature (∼20 K) inside a cooled shroud that must be retracted from around the target just before the target is illuminated by OMEGA. As a consequence, impulses may be imparted to the target stalk, causing the target to depart from its ideal position centered at the aim point of the laser beams. The positions of cryogenic targets at the start of the laser pulse are determined in this work by comparing the positions of images of the cryogenic target implosion to that of a non-cryogenic target implosion that is accurately centered on the aim point of the beams. Images are taken from an array of up to five digitally recorded, x-ray pinhole cameras. Positions of the resultant implosion cores are determined along a single line of sight from x-ray images of these cores taken with an x-ray microscope operating in the time-integrated mode. The offsets of the cryogenic-target cores relative to the non-cryogenic-target cores are found to have a magnitude and direction consistent with the core forming in the direction of the target offset at the start of the laser pulse. The inferred offsets are therefore consistent with each other within errors. Neutron yields are seen to be affected by the target offset although with considerable scatter.