Experimental investigation of bright spots in broadband, gated x-ray images of ignition-scale implosions on NIF

Bright spots in the intensity profile of broadband (h$\nu$ $>$ 8 keV), gated (dt = 30-80 ps) x-ray images of implosions have been observed and are attributed to hot-spot mix [Hammel et al, Phys. Plasmas 18, 056310 (2011)]. Bright spots form when the CH ablator material doped with Ge mixes with the lower Z (T, H, D, He) hot spot plasma, enhancing the local x-ray emission. The signal level of the bright spots was quantified for THD, DT, symcap and convergent ablator implosions using Fourier analysis. The power spectral density of the x-ray intensity profile has a low wavenumber (k$<$0.09 m-1) portion characteristic of the envelope of the framed image used to examine implosion symmetry, and high wavenumber (0.09 m-1$<$k$<$0.63 m-1) features caused by the bright spots. The power corresponding to the high-wavenumber portion of the spectrum is compared to hot-spot mix mass inferred from Ge K-shell x-ray spectroscopy [Regan et al., aps/dpp'11]. The potential of using this technique to assess hot-spot mix for capsule shells without Ge dopant are discussed.