The shadowgraphic technique has been analyzed using a 1-D ray-trace simulation (DOE Monthly Report March 2003) and a 3-D optics code (courtesy of Prof. Thomas Brown, UR/The Institute of Optics). The former gave precise information about the locations and origins of the many different rings that may be observed. By contrast, the optics code was used to understand the importance of outer-surface perturbations on perceived inner-ice-surface perturbations. Figure 2 shows simulated bright ring positions for a target with a perfect outer surface and a perturbed inner ice layer. The ice-layer perturbations are fully measurable in this case (a). For a perturbed outer surface and a perfect ice layer, the ring may or may not be distorted depending on the exact location of the outer-surface perturbations[(b) and (c)]. This demonstrates that outer-surface perturbations have to be kept small compared to the expected inner-ice-surface perturbations. It also confirms observations where small outer-surface perturbations or miniscule “dust” particles may lead to partial or complete disruptions of the ring.
The limits of accuracy of the shadowgraphic characterization technique are illustrated in Fig. 3. A sapphire sphere was first characterized with an atomic force microscope (AFM) and then with our optical shadowgraphy. The average spectra obtained by these two techniques are shown in Fig. 3 and demonstrate that the AFM technique is four times more accurate than the optical technique. However, only the optical technique can be used for cryogenic target characterization and its accuracy satisfies our experimental requirements.
The current effort at LLE to characterize cryogenic D2-ice layers prepares the way to future experiments on the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). In the near term this effort provides characterized cryogenic targets for current implosion experiments on OMEGA and for layering studies necessary to create the requisite D2– or DT-ice layers.