Observations of intrinsic shock front anisotropy in diamond

We have fielded a high resolution two-dimensional imaging VISAR at the OMEGA laser facility. Over an 800 $\mu $m field this instrument captures small spatial variations in the velocity across a shock front. The detection limit is \textit{$\delta $V} $\sim $10 m/s giving relative sensitivity \textit{$\delta $V/V} $\sim $ few x 10$^{-4}$, where \textit{V $\sim $ }20 km/s is the shock velocity. The instrument can resolve spatial mode wavelengths as small as 2.5 $\mu $m. We have observed shock front non-uniformities on multi-Mbar shock fronts after passing through polycrystalline diamond. The shock fronts show significant structure at amplitudes below the threshold for shock melting, with a high degree of non-uniformity on spatial scales of a few microns or less. Above the threshold for shock-melting (after the shock front entered the coexistence region, partial melt, near 6 Mbar) the level of non-uniformity diminished significantly. We are continuing experiments on diamond and other materials to elucidate details of the shock front structure above and below the melt transition.