Numerical simulations of a post-hole convolute driven by high power magnetically insulated transmission lines: Analysis of current loss in steady-state and time-dependent operating modes

Electron power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied using 3D particle-in-cell simulations. At sufficiently high voltages, electron emission upstream of the convolute results in a portion of the current carried by the transmission lines to flow in an electron sheath along the cathode surfaces. The simulations show that at 50-200 TW, the transition from the individual MITLs to the convolute results in a portion of the MITL sheath current being lost to both anode and cathode structures. The losses are identified as a function of radius and correlated with Poynting vector stream lines which can be followed by individual electrons. For a fixed MITL-convolute geometry, the difference between the current in the system upstream of the convolute and current delivered to the load (defined as the loss current) increases with both operating voltage and load impedance. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases in both steady-state and time-dependent operating modes. The impact of cathode plasma formation on the loss current is also considered for the time-dependent simulation results. Collectively, these simulation results are being used to help formulate design criteria for high-power convolute-MITL systems.