Modeling the RITS-6 transmission line

Sandia National Laboratories’ Radiographic Integrated Test Stand (RITS-6) is a six-cell inductive voltage adder accelerator designed to produce currents of 186 kA at 7.8 MV in 70 ns in its low-impedance configuration. The six inductive-adder cells are connected in series to a coaxial magnetically insulated transmission line. Each cell has a single point feed to an azimuthal transmission line which distributes the pulse around the cell bore. To understand the extent to which power is distributed symmetrically around the coaxial transmission line and its effect on electron power flow downstream, particle-in-cell simulations were used to model the entire RITS-6 transmission line in 3D from pulse forming circuit to the diode load. Simulation results show electron flow current to be asymmetric by 16% at the exit to the sixth cell, but 3% or less at diagnostic positions near the load. Magnetic insulation in the trans-mission line does not appear to be impacted by the asymmetry, though flow impedance is not uniform axially.