Studying Rayleigh-Taylor mitigation mechanisms in gas puff Z-pinches with the FLASH code*

Gas puff Z-pinches have been studied for decades, with different variations on the materials and dimensions of the setup, to increase stability and performance for fusion applications. Here, we show how the new capabilities of the FLASH code have made possible the simulation of such setups in two dimensions, capturing the development of the Rayleigh Taylor instability (RTI). The new code capability was first verified against previously published HYDRA calculations. Taking advantage of the adaptive mesh refinement of FLASH implementation we then studied the effect of mesh resolution on RTI dynamics, and reproduced Argon on Deuterium CESZAR experiments. This allowed us to validate FLASH with experimental data while also shedding light on the underlying RTI mitigation mechanism the experiments pursued. Having built confidence in FLASH's ability to simulate Z-pinch experiments, we used the code to design, for the first time, gas puff Z-pinches experiments to be conducted in the fall of CY24 on ZEBRA, based on previous experiments conducted by F. Conti et al.. The experiments will study how the liner radius affects the pinch stability, inspired by recent results obtained on the Double Eagle facility, which suggest that strong shock heating could mitigate RTI for extended liners. We show the numerical trend for the RTI growth rate as a function of liner radius, while dissecting the mitigation process during the implosion. We describe this mitigation as a thermal snowplow mechanism, in which shock heating and electron heat conduction create a high-pressure zone between the target and the liner, which tempers the RTI growth at the interface. These considerations led us to study the effects of radiation on the liner compression and instability growth, both theoretically and numerically, showing promising features that could inform the design of future gas puff Z-pinches experiments.