Summary of the Rayleigh-Taylor instability studies at the Pegasus facility

Summary form only given, as follows. The goal of the RTMIX series on Pegasus was to study Rayleigh-Taylor instability growth and mixing, at a high/low density interface in a convergent geometry, as a function of material strength and initial perturbation amplitude. In this series of experiments a solid multi-layered core was compressed in a Z-pinch configuration using an approximately 5.5 MA sinusoidal current driven in the outer layer of aluminum. The 800-micron thick aluminum liner drives a 200-micron thick layer of dense metal (Cu or In/Sn) against a lower density solid core initially made of foam. The results of the four experiments completed are summarized. Preliminary results of the first three experiments were reported in the two previous Pulse Power Conferences. For the first experiment the high/low density interface was smooth and no instability growth or mixing was observed within the resolution limits of the diagnostics. In the second and third experiments, axially symmetric sine-wave perturbations were machined on to the inner surface of the dense material. The wavelength of the perturbations was 1 mm. On half of the interface the perturbation amplitude was 12.5 microns, on the other half the amplitude was 50 microns. Initial predictions indicated that with copper the large amplitude perturbations would grow and the small perturbations would remain stable. Since the In/Sn alloy used in the third experiment was predicted to melt, both perturbations should have experienced strong growth. In all cases the actual experiments showed no growth in either case. This enigma was resolved in the fourth experiment by replacing the foam inner core with liquid butane. The radiographs for the fourth experiment clearly show growth of the perturbations and recovery of the expected fluid-on-fluid behavior. This result indicated that the complex nature of the foam material used in the central core suppressed growth of the Rayleigh-Taylor instability.