High Impedance Windows for Shock Compression Experiments

Laser velocity or displacement interferometry is a key instrument in shock and quasi-isentropic studies of material behavior, and optically transparent windows make the measurement much more useful by maintaining pressure in the sample, preventing free-surface ejecta, and allowing the structure of complex waves to be resolved. Although a number of materials have been studied as potential windows over the years, they have almost always been relatively low impedance. This includes the workhorse window material LiF, which has an ambient density of only 2.64 g/cm³. When studying high-impedance samples such as copper or tantalum, a low-impedance window will perturb wave structures due to reflections from the sample/window interface. Here, we perform initial scoping of three potential window materials in an effort to minimize those perturbations. The materials studied were single crystals of Bi₄Ge₃O₁₂, CeF₃, and LiTaO₃, which have densities of 7.13, 6.16, and 7.46 g/cm³; all were commercially available at thicknesses of at least 1 mm. Windows were mounted onto copper targets and impacted at velocities of 2, 3, and 4 km/s. Optical behavior and EOS behavior of the three materials were evaluated using both VISAR and PDV diagnostics.