Characterization of the DT Layer of ICF Targets by Optical Techniques

The characterization of the solid DT layer, in terms of thickness and roughness, in the LMJ geometry (hohlraum) is not trivial. The DT layer measurements will be done using a Maksutov-Cassegrain telescope, 39 cm away from the target. This telescope will be used to acquire shadowgraphy images and spectral-interferometry measurements. Shadowgraphy imaging probes the DT layer geometry at the equator of the target. Spectral-interferometry gives the DT layer thickness on one spot on the shell, in the polar regions of the target. By scanning around the poles, several points can be acquired to probe the roughness and the local shape of the DT layer at the poles. This paper presents the spectra-interferometry technique and explains how the DT layer thickness could be deduced from channelled spectra. First experimental results on a 125 m thick empty shell are also reported.