Theory and simulations to characterize impedance modes due to HERMES III transmission line modifications

HERMES-III (18 MV peak, 650 kA peak, 30 ns radiation pulse) uses an inductive voltage adder (IVA) to set up stable, magnetically insulated transmission line (MITL) power flow by adding voltages in 20 stages with constant current. However, the MITL operating impedance changes with electron emission and voltage. Thus, the IVA must be designed with a target voltage where efficiency is maximized, since any lower or higher voltage will lead to impedance reflections at each stage. Operating the IVA at the voltage that maximizes efficiency requires designing the output MITL and diode to operate at the impedance that minimizes reflections.



Experiments tested configurations of conical anode and cathode tapers to determine if the IVA could drive the MITL and load. Creating a variable impedance MITL after the IVA is a low-cost way to field new loads. Staggering the cathode taper to begin fully before the anode taper achieved full voltage transmission to the load. The upstream and downstream MITLs operated at different impedances without voltage reflection due to electron current losses in the taper acting as a current divider. This argument also explains efficient coupling between MITLs and high impedance diodes. Theory and Empire simulations highlight the differences between these two modes.