Injector Design with the FLASH Code:An Application to the Characterization of Gas-puff Z-pinch Density Profiles

Numerical modeling of gas injectors is a central component in the design of gas-puff Z-pinch experiments. During recent campaigns on the Double-Eagle platform, a key challenge has been to reconstruct the gas density distribution inside the chamber based on experimental diagnostics and correlations with pinch timing. To support this effort, a fully integrated gas injection capability was developed within the FLASH simulation framework, including validation and verification against previously published computational fluid dynamics (CFD) results and experimental data from CESZAR. Simulating the full 3D geometry of the injector, including moving poppets and multiple nozzle geometries, requires further code development that is still in progress. In the meantime, we have derived a theoretical model for the mass inflow, guided by both simulations and experimental observations. This approach enabled robust 2D axisymmetric simulations that capture the time-dependent evolution of gas entering the chamber. We take advantage of these new capabilities to study the effect of the plenum pressure and atomic number on the gas density profiles in the chamber, with a particular focus on the mixing between two gases (typically fuel and liner materials) that could affect the magneto-Rayleigh-Taylor instability growth during the implosion phase. These developments pave the way for future gas-puff Z-pinch designs and for any experimental campaigns requiring accurate gas injection modeling using the FLASH code.