Spall experiments in convergent geometry using the Atlas pulsed power facility

Summary form only given, as follows. The first spall experiments conducted on the twenty-four megajoule capacitor bank, Atlas, are described. These experiments are intended to determine whether there are qualitative differences between spall phenomena in convergent and planar geometries. It is well known that spall, which arises as a result of intersecting release waves putting a material into tension, is a function of shock amplitude, shape, and duration. However, quantifying differences is difficult, and often the best one can do is to obtain the "spall strength" of the material. This important parameter is inferred from laser interferometry measurements (VISAR) of the "pull back" velocity of the free surface of a shocked sample. Principally, planar experiments are performed one of three ways: using a gas gun to launch a freeflying sabot into the target, employing a laser driven mini-flyer, or using high explosives to launch the shock wave. In the case of Atlas, the flyer is a cylindrically imploding metal liner, driven by the magnetic pressure produced by the bank current. After impact of the liner with the target the magnetic pressure continues to accelerate the liner/target assembly radially inward. This continued acceleration combined with converging geometry differentiates these spall experiments from those conducted in planar geometry. VISAR will be used to measure the free surface velocity of a shocked aluminum target, and the signature will be used to infer the spall strength and compare this with that obtained from gas gun experiments. Comparisons of the VISAR signals obtained on Atlas experiments with gas gun signals will be used to provide insight into the effects of convergence on spall phenomena. Axial radiography will also be fielded to determine the location of the spalled material layer, and correlate this with the VISAR.