Plasma formation, evolution, and dynamics in 100 TW vacuum-transmission-line post-hole convolutes

Post-hole convolutes are used in high-power transmission-line systems to join several individual transmission lines in parallel. Such convolutes transfer the currents of the lines to a single transmission line, that in turn delivers the combined current to the load. Magnetic insulation of electron flow, established upstream of the convolute, is lost at the convolute due, in part, to the formation of magnetic nulls. This loss of insulation results in current losses. At very high-power operating levels, the formation of electrode plasmas is considered likely. This work examines the evolution and dynamics of cathode plasmas in the transmission-line/ double-post-hole convolute configuration used on the Z machine. The results of these simulations suggest that net electron-flow-current losses due to cathode plasma evolution in the vicinity of the post-hole convolutes can be of order 2 MA out of a total system current of ∼18.5 MA, consistent with measurements.