Kinetic simulations of power flow in the Z accelerator*

The challenge for the Terawatt-class accelerators driving $Z$-pinch experiments, such as Sandia National Laboratories' $Z$ machine, is to efficiently couple power from multiple storage banks into a single multi-mega amp (MA) transmission line. The physical processes that degrade efficiency are identified in the first-ever, multi-dimensional simulations of the $Z$ machine. Kinetic models follow the range of physics occurring during a pulse, from vacuum pulse propagation to charged-particle emission and insulated flow to electrode plasma expansion. Simulations demonstrate that current is diverted from the load through a combination of standard and anomalous transport. Standard transport occurs in the adder region where the electrode current density is a few $10^4-10^5$~A/cm$^2$ and current is diverted from the load via uninsulated charged-particle flows. In regions with $>10^6$~A/cm$^2$, electrode surface plasmas develop velocity-shear instabilities and a Hall-field-related transport which scales with electron density. These results provide the physics basis for designing future pulsed-power systems.