Three-Dimensional Characterization of Spherical Cryogenic Targets Using Ray-Trace Analysis of Multiple Shadowgraph Views

Backlit optical shadowgraphy is the primary diagnostic for hydrogenic ice-layer characterization in cryogenic targets at the Laboratory for Laser Energetics (LLE). Reflection and refraction of light passing through the ice layer produce characteristic rings on the image. The position of the most prominent of the shadowgraph rings, known as the bright ring, can be resolved to ~0.1-pixel rms, corresponding to less than 0.2 m for typical target shadowgraphs. The LLE target characterization stations use two camera angles and target rotation to record target shadowgraphs from many different views (typically 48) and build a three-dimensional (3-D) topology of the ice layer. The standard method of bright-ring analysis using spherically symmetric ray-trace calculations to determine the ice surface is limited to mode numbers up to around [script l]_max = 10 by gaps in the data and the effects of ice-layer asymmetries that invalidate the symmetric ray trace calculations. A 3-D ray-tracing model has been incorporated into the shadowgraph analysis. The result is a self-consistent determination of the hydrogen/vapor surface structure for cryogenic targets up to higher-mode numbers ([script l]_max = 16). This reduces the standard deviation between the measured bright rings and those predicted for the 3-D ice surface (by 45% from 1.5 m to 0.8 m in the example shown).