Abstract
A challenge for the TW-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-MA transmission line. The physical processes that lead to current loss are identified in new large-scale, multidimensional 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 magnetically-insulated current flow to electrode plasma expansion. Simulations demonstrate that current is diverted from the load through a combination of standard transport (uninsulated charged-particle flows) and anomalous transport. Standard transport occurs in regions where the electrode current density is a few $1{0}^{4}\ensuremath{-}{10}^{5}\text{ }\text{ }\mathrm{A}/{\mathrm{cm}}^{2}$ and current is diverted from the load via transport without magnetic insulation. In regions with electrode current density $g{10}^{6}\text{ }\text{ }\mathrm{A}/{\mathrm{cm}}^{2}$, electrode surface plasmas develop velocity-shear instabilities and a Hall-field-related transport which scales with electron density and may, therefore, lead to increased current loss.
Highlights
Experimental studies of dense Z-pinch implosions require magnetic fields of 100–1000 T generated by multi-MA currents within mm-diameter cylindrical geometries [1,2]
The physical processes that lead to current loss derive from rapid heating of the electrode surfaces, which may generate charged particle emission undergoing transport without magnetic insulation and current-shunting plasma formations
Transmission-line-circuit models of Z [9] and the Primary Test Stand (PTS) [10] have increased our understanding of the electrical coupling of the pulse-forming components to the transmission lines and the magnitude of power lost in transit to the load
Summary
Experimental studies of dense Z-pinch implosions require magnetic fields of 100–1000 T generated by multi-MA currents within mm-diameter cylindrical geometries [1,2]. The physical processes that lead to current loss derive from rapid heating of the electrode surfaces, which may generate charged particle emission undergoing transport without magnetic insulation and current-shunting plasma formations. An adaptive time step and particle management [14] enable both the time step and macroparticle count to adjust to the plasma generated in the unified electrode plasma model [13] With these algorithms and sufficient resolution, the simulations are able to span the range of physical processes that occur in a Z shot, including vacuum pulse propagation, charged-particle emission, magnetically insulation current flow, and plasma formation and expansion.
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