Shock formation and Magneto-Rayleigh-Taylor instability mitigation in double-shell Staged Z-pinch implosions*

Target preheating in a magneto-inertial fusion scheme like the Staged Z-pinch is required to reduce the required convergence ratio for reaching fusion conditions. The current iteration of the Staged Z-pinch uses a single, high-Z gas-puff liner to compress a deuterium (D) target. Prior MHD simulations1 of similar implosions on a 1 MA driver predicted peak and average implosion velocities of 20 and 10 cm/us, respectively, which resulted in shock temperatures far below the ~100 eV required for target preheating\footnote{J. Narkis et al., Phys. Plasmas 23, 122706 (2016)}. Reduction of liner mass is an effective approach to increasing implosion velocity – experimental implosion velocities exceeding 30 cm/$\mu$s have been reported – and therefore shock strength, however this also results in increased magneto-Rayleigh-Taylor (MRT) instability growth. Both using a double liner and an axial magnetic field are effective mitigation mechanisms for MRTI growth. However, a double liner provides better MRTI mitigation and a fortuitous increase in shock strength and implosion velocity over a single liner, as demonstrated in simulated Kr / D and Ne / Kr / D implosions.