Laser absorption measurements in OMEGA implosions using scattered light calorimeters and imaging diagnostics.

Diagnosing the absorption of laser light in a direct-drive implosion by measuring the unabsorbed light scattered from the implosion has been critical to the implosion performance improvements on OMEGA. These measurements identified cross-beam energy transfer (CBET) as the primary source of absorption degradation on OMEGA as well as the primary candidate for a systematic l=1 mode nonuniformity. Previously, the primary quantitative diagnostics for absolute scattered light measurements are calorimeters (two in-situ calibrated Full Aperture Backscatter Station (FABS) calorimeters and six offline calibrated TIM based calorimeters). More recently, five offline calibrated scattered light uniformity imagers (SLUIs) have been deployed. Accurate laser absorption measurements require the removal of “blow-by” light pollution form the FABS calorimeter measurements and accounting for two-photon absorption in an optical component of the SLUI diagnostic.