Observations of the magneto-Rayleigh-Taylor instability and shock dynamics in gas-puff Z-pinch experiments

We describe a series of experiments performed to study the shock structure generated during the implosion of a gas-puff Z-pinch. The Z-pinch is produced by a double-annular gas-puff with a center jet driven by Cornell University's COBRA generator operating with a 1MA, 200 ns current pulse. Using 532 nm laser interferometry and 100 MHz multi-frame cameras, a shock structure is observed to form early in the implosion. The shock appears to be created by a current layer at the outer radius of the imploding plasma which acts as a piston moving inward at several hundred km s(-1). The dynamics of the shock and its radial position ahead of the piston agree well with a simple uniform density model outlined in the study by Potter [Nucl. Fusion 18(6), 813 (1978)]. The outer surface of the current layer is observed to be Magneto-Rayleigh-Taylor unstable. The growth rate of this instability is found to depend on the radial density profile of the material within the layer of high-density fluid between the shock and the piston, as predicted by recent theoretical work [see, e.g., D. Livescu, Phys. Fluids 16(1), 118 (2004)]. Growth rates measured in krypton implosions, where the post-shock material is found to decay quasi-exponentially away from the piston, were more than ten times smaller than those recorded in otherwise identical implosions in argon plasmas, where the material between the shock and the piston was observed to maintain a uniform density. Published by AIP Publishing.