ZR-convolute analysis and modeling: Plasma evolution and dynamics leading to current losses

Post-hole convolutes are used in high-power transmission line systems and join several individual transmission lines in parallel, transferring the combined currents to a single transmission line attached to a load. Magnetic insulation of electron flow, established upstream of the convolute region, is lost at the convolute due, in part, to the formation of magnetic nulls, resulting in current losses. At very high-power operating levels, the formation of electrode plasmas is considered likely which can lead to additional losses. A recent computational analysis of the Sandia Z accelerator suggested that modest plasma desorption rates in the convolute region could explain measured current losses. The recently completed Sandia ZR accelerator has utilized new convolute designs to accommodate changes to the parallel-plate transmission lines on ZR. Detailed particle-in-cell simulations that are fully electromagnetic and relativistic, and include plasma desorption from electrode surfaces in the post-hole convolutes, are carried out to assess the measured current losses on ZR. We find that the plasma desorption rate used to model the Z convolute also applies to three different ZR convolute designs that have been fielded. Based on these findings, the simulation model is being used to develop newer convolute designs with the goal of reducing the current losses, particularly for higher-impedance loads.