Measuring total laser absorption for OMEGA implosions with highly nonuniform scattered light distributions*

The primary method of diagnosing the absorption of laser light in a direct-drive implosion is the measurement of the unabsorbed light scattered from the implosion. Beam energy balance, beam mis-pointing, and polarization dependent 3-D cross-beam energy transfer (CBET) effects due to OMEGA’s polarization smoothing method, all contribute to making the absorption and scattered light distributions very nonuniform over the target. A single scattered light measurement extrapolated over the entire 4π distribution can give over a 10% variation in inferred total absorption depending on the sampling location. The average of several measurements from different locations around the distribution must be used to have confidence in the measurement. On OMEGA implosions, this can be difficult to achieve due to the limited number of calibrated diagnostics, covering less than 0.5% of the distribution at most, and the lack of available diagnostic ports on many implosions. The limitations of the current OMEGA calibrated scattered light diagnostic locations have been examined and shown to be sufficient for accurate measurement, provided they are all deployed. New absolutely calibrated scattered light diagnostics are coming online that will measure the local variations in the distribution, increase the diagnostic cover by a factor of 10, and improve our understanding of the nonuniformity.