Ar Gas Puff Z-Pinches with Applied Bz Field on Cobra at Cornell University

We have conducted Ar gas puff z-pinch studies on the COBRA generator with an external Bz field. Two ~20 cm diameter coils are used to generate up to 6 kG Bz field on the pinch axis. A triple-nozzle injects two concentric annular gas puffs and a center jet into the load region. The z-pinch implosion starts from the outer puff at~3 cm in radius, slows down by the inner puff at the radius of ~ 1.5 cm, and then imploded on the center jet (R~0.5 cm). The de-acceleration of implosion by the inner puff reduces the grows of the magneto-Rayleigh-Taylor (MRT) instability. Stable implosions and ~200 J Ar K-shell yields are achieved with the outer, inner, and jet nozzle plenum pressure at 0.8, 2.4, and ~10 psia, respectively. In the absence of the inner gas puff, the implosion starts from 3 cm in radius accelerates to the center jet. The MRT instability grows rapidly and the implosion is not stable as observed. Subtitling the inner puff with an applied the external Bz field, the compression of the Bz field trapped between the outer and jet plasma slows down the implosion, and the MRT instability is suppressed. We have observed not only the stabilization of the implosion, but also the heating of the center jet plasma by the compression of the trapped magnetic field, and the helix -like structures in the imploding plasma.