Current transport and loss mechanisms in the Z accelerator

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 10 4 − 10 5 A=cm 2 and current is diverted from the load via transport without magnetic insulation. In regions with electrode current density > 10 6 A=cm 2 , electrode surface plasmas develop velocity-shear instabilities and a Hall-fieldrelated transport which scales with electron density and may, therefore, lead to increased current loss.

Numerical simulations of enhanced ion current losses in the inner magnetically insulated transmission line of the Z accelerator

“…This can be understood from the generalized summation rule given by Eq. (12) and derived in Appendix C; if H þ is present and not MI, the effective loss is a significant fraction of what it would be if only H þ is present and NMI. For instance, for the H þ 2=3, O þ 1=3 case, that fraction is 0.75.…”

“…These dependencies are clearly expressed in Eqs. (10) and (12) if no species is MI (or if there is a single ion species that is NMI). If there is an ion species which, at a given time and position, magnetically insulated the ion current losses, at that time and position, are for all practical purposes negligible.…”

“…Since we only consider steady state (fixed AK gaps) our present work ignores the impact of electrode plasma expansion. Recent simulation studies [11,12] offer an additional current loss mechanism, partially based on identifying turbulently driven plasma transport in the inner MITL. As we discuss in Sec.…”

Examining the molecular origins of anomalously high H2O generation at oxide-passivated metal surfaces for plasma applications

“…This outgassing may limit the efficiency of multi megaampere (MA) pulsed-power accelerators, [10][11][12] such as Sandia National Laboratories' Z Accelerator. The high currents used in these systems heat the electrodes and accelerate the release of surface and bulk impurities [13].…”

“…The plasmas thus formed may transport into the transmission line gaps and shunt some portion of the current away from the 'load' (current delivered to the target). It has been inferred that, even after pumping under vacuum conditions (∼10 −5 torr vapor pressure), up to tens of monolayers (ML) worth of hydrogen-and/or carbon-containing impurities desorb from the surface in vacuum [5,7,11]. This amount of outgassed inventory has not been explained by molecular length-scale studies, and is to some extent a placeholder pending further molecular level studies.…”