Measurements of the Imploding Plasma Sheath in Triple Nozzle Gas-Puff Z Pinches on 1-Ma Cobra*

Dynamic gas-puff z-pinch implosions are efficient sources of intense x-ray radiation and are of interest for magneto-inertial fusion studies with applied magnetic fields. However, these implosions are susceptible to magneto-Rayleigh-Taylor instabilities (MRTI) which can lead to axially non-uniform implosions, reducing the effective compression ratio and implosion kinetic energy-to-radiation efficiency. Recent observations of triple nozzle gas-puff implosions on the 1 MA, 220 ns COBRA generator at Cornell University 1 , 2 suggest that these implosions are more stable than predicted by linear MRTI theory based on the time-varying acceleration profiles. Furthermore, the reduced instability growth rates, characterized by effective Atwood numbers much less than one, are observed to depend on gas species and initial fill density. To explore the nature of these differences, detailed Thomson scattering and laser interferometry measurements have been obtained for neon, argon, and krypton gas puffs at various fill densities midway through the implosion. The results reveal two distinct radial profiles for varying initial conditions – a shock-like profile with a sharp discontinuity in density and velocity across the sheath and a snowplow-like profile with a gradual increase in density and velocity across the sheath. Snowplow-like implosions exhibit additional non-thermal broadening of the Thomson scattering spectra in a manner consistent with non-directed hydrodynamic turbulence. These observations suggest that under the appropriate conditions the forward shock may become unstable, resulting in turbulent flow within the imploding plasma sheath. These findings, and implications to the overall implosion stability and performance are presented.