Plasma evolution and dynamics in high-power vacuum-transmission-line post-hole convolutes

Abstract

Vacuum-post-hole convolutes are used in pulsed high-power generators to join several magnetically insulated transmission lines (MITL) in parallel. Such convolutes add the output currents of the MITLs, and deliver the combined current to a single MITL that, in turn, delivers the current to a load. Magnetic insulation of electron flow, established upstream of the convolute region, is lost at the convolute due to symmetry breaking and the formation of magnetic nulls, resulting in some current losses. At very high-power operating levels and long pulse durations, the expansion of electrode plasmas into the MITL of such devices is considered likely. This work examines the evolution and dynamics of cathode plasmas in the double-post-hole convolutes used on the $Z$ accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)]. Three-dimensional particle-in-cell (PIC) simulations that model the entire radial extent of the $Z$ accelerator convolute---from the parallel-plate transmission-line power feeds to the $z$-pinch load region---are used to determine electron losses in the convolute. The results of the simulations demonstrate that significant current losses (1.5 MA out of a total system current of 18.5 MA), which are comparable to the losses observed experimentally, could be caused by the expansion of cathode plasmas in the convolute regions.