Monochromatic Backlighting Of Direct-Drive Cryogenic Dt Implosions On Omega

Backlighting is a powerful technique to observe the flow of cold and dense material in highenergy- density-plasma experiments. High-performance, direct-drive cryogenic deuterium-tritium (DT) implosions are a challenging backlighting configuration because of the low opacity of the DT shell, the high shell velocity, the small size of the stagnating shell, and the very bright selfemission of the hot core. A crystal imaging system with a Si He-alpha backlighter at 1.865 keV driven by similar to 20-ps short pulses from OMEGA EP was developed to radiograph the OMEGA cryogenic implosions. The high throughput of the crystal imaging system makes it possible to record highquality images with good photon statistics and a spatial resolution of similar to 15 mu m at 10% to 90% modulation. This imager has been used to study the evolution of preimposed mass-density perturbations in the ablator, to quantify the perturbations caused by the stalk that is used to mount the target, and to study the mix caused by laser imprint or small-scale debris on the target surface. Because of the very low opacity of DT relative to carbon, even 0.1% of mix of carbon into the DT ice can be reliably inferred from the images. With the current implosion designs, mix is only observed for an adiabat below alpha = 4. Published by AIP Publishing.