Neutral Gas Breakdown Modeling for the Zap Energy Devices

Zap Energy is investigating the sheared-flow-stabilized Z pinch as the core for a fusion generator. Recently, Zap scientists have begun operating FuZE-Q, the newest sheared-flow- stabilized Z-pinch device, achieving ~700 kA pinches using ~10kV capacitor charge, leading to greater than 109 neutrons per pulse, greater than 1 keV plasma temperature, and greater than 10²³ m^-3 plasma density.

FuZE-Q uses an injection of neutral gas into a coaxial accelerator region roughly 0.1m in diameter and 1m long. The details of how the injected neutral gas spreads, ionizes, and continues to feed the Z pinch are all critical to the optimization of FuZE-Q performance. We use simulation to help understand the details of this process. To model the neutral gas filling the accelerator, we use a Navier-Stokes solver in the WARPXM simulation tool.

As a benchmark, we compare the neutral gas distribution from the WARPXM model in 2D and 3D with an axial 1D theoretical model of the gas distribution. Then from the neutral gas pressure and applied voltage, we estimate using avalanche and streamer breakdown theory where the gas is most likely to break down, and we compare this with measured breakdown.

Finally, we use particle-in-cell models to confirm the avalanche / streamer theory.