Characterization of Low-density Foams for Use in Strength Experiments at NIF

To infer strength in compressed solids by observation of Rayleigh-Taylor growth, we are engaged in an experimental campaign at NIF. To produce a drive that is sufficiently cool to prevent melting and also long enough to observe growth over tens of ns, we are developing a target that mediates the high-intensity NIF laser drive by shocking a multi-component reservoir and driving the target sample by the stagnation of the reservoir's release after crossing a large gap (~1mm vacuum). This design depends on the shock and release of an ultra-low density foam layer (10-30 mg/cc), which in turn requires that these layers be well-characterized for uniformity and reproducibility. We describe efforts to characterize C and SiO2 based foam targets to quantify variations in rho-R. Our fabrication and measurement goal is taken from simulations that indicate that targets must be uniform at the 10\% level over spatial scales from 20 microns to 2 mm. We present data from differential radiography of witness samples by soft x-rays (E$<$1.5keV) and compare our results to other techniques.