Pulsed-Power Innovations Needed For Next-Generation, High-Current Drivers

Next-generation, high-current pulsed-power drivers cannot be scaled to currents ~50 to 60 MA using the same design criteria that have been used successfully for the last 25 years. There are fundamental physics issues and engineering constraints that will lead to increases in total inductance. This is driven by water-transmission-line electric-field limits, vacuum-insulator electric-field limits, magnetically insulated transmission lines (MITL) vacuum power flow losses and vacuum electron flow, the post-hole convolute losses, and inner MITL current density limits. Saturn, Z, and ZR were built with similar design criteria. (ZR’s insulator stack operates at a peak electric field of ~150 kV/cm.) A next-generation pulsed-power (NGPP) driver must operate at >2× the present voltage of Z—everywhere—if the current rise time is held to the 100 ns (or less) required for hydrodynamic stability. This higher voltage will have a dramatic impact on load-coupling efficiency. Water breakdown and vacuum-insulator flashover have hard, electric-field constraints. The only way to sustain higher local voltages is to increase those local electrode gaps, which results in higher local inductance. The desire for minimal vacuum electron flow in the multiple, conical MITL’s forces their impedance up with higher voltages. This directly drives the MITL inductances up ( L ~ Zτ ). Similarly, all changes to the vacuum post-hole convolute for operational robustness result in a higher convolute inductance. The inner MITL’s must operate at linear current densities approaching 10 MA/cm. At current densities greater than a few MA/cm the MITL surfaces are fully ionized with dense, multi-eV plasmas in an intense radiation field. In the inner MITL, magnetic insulation must work with dramatically increasing electric fields at a time of declining currents.