Unabsorbed light beamlets for diagnosing coronal density profiles and absorption nonuniformity in direct-drive implosions on OMEGA.

Laser light scattered from a target is the most-direct measurement for diagnosing laser absorption in a direct-drive implosion. The 3ω gated optical imager beamlets diagnostic images unabsorbed light from all 60 OMEGA beams as distinct "beamlet" spots for each beam. The implosion can be diagnosed using the position and intensity of these beamlets. The position of each beamlet in the image is determined by refraction and can be used to fit the coronal plasma density profile of the implosion. The inferred plasma density profiles are comparable to the profiles predicted by the 1D hydrodynamics code LILAC but suggest that the predictions underestimate the density farther out in the corona. The intensity of light in each spot depends on the cumulative effects of absorption and cross-beam energy transfer along the beamlet's path through the corona. The measured variation in intensity and polarization between similar spots indicates that absorption during OMEGA implosions is less uniform than previously known.