Recent experiments on gas-puffZ pinches using various ion mixtures have demonstrated that the plasma shell separates into two distinct annuli which implode concentrically. This phenomenon is quantitatively explained with the use of a semihydrodynamic model in which the electrons are treated as a collisional fluid and the ions are considered cold and collisionless. Stability in this model is investigated and an expression for the growth rate of a Rayleigh-Taylor type instability is presented.

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... s growth rate is found to be independent of the degree of plasma compressibility and somewhat reduced in the presence of an ion mixture. Comparison of these results with the University of California, Irvine, Z-pinch experiment is discussed.