Direct-drive-implosion experiments with enhanced fluence balance on OMEGA

Direct-drive experiments on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are now regularly performed with enhanced single-beam uniformity (1-THz-bandwidth smoothing by spectral dispersion, including distributed phase plates and polarization rotators). The on-target-beam-energy balance is determined by a set of beam pickoffs that are cross-calibrated to full beam calorimeters and by independent measurements for each beam of the losses incurred in transporting the beams to the target chamber. Variations of the illumination due to beam-to-beam, on-target energy differences and beam-to-beam shape differences can then introduce variations of the illumination and hence drive symmetry that will affect implosions. A new technique has been implemented that determines variations of the beam peak fluences at target chamber center on a full-power target shot by simultaneously measuring the x-ray flux produced by all 60 beams separated on a 4-mm-diam, Au-coated spherical target. The inferred beam-to-beam, peak-fluence variations are minimized by changing the gain of the last laser amplifier (enhanced fluence balance). Results of implosion experiments with enhanced balance show more spherically shaped stagnation cores, consistent with the reduction of low-spherical-harmonic-mode components ( l ⩽6) of the illumination nonuniformity.