PIC simulations of power flow in the URSA minor linear transformer driver for radiographic applications

URSA Minor is a 21 cavity linear transformer driver (LTD) at Sandia National Laboratories to evaluate LTD technology for driving radiographic diodes. Operating at ~2.0 MV, there is substantial electron flow in the central magnetically insulated transmission line (MITL). An important concern is electron loss to the vacuum insulators, which are recessed only ~2 cm from the MITL. Electron flow has been analyzed with both 2-D, r-z particle-in-cell simulations of the full 21 cavity system, and full 2π r-φ-z 3-D simulations of the last seven cavities to address azimuthal asymmetry. The simulation results show that electron loss to the insulators is negligible, even with large perturbations in the cavity trigger timing. However, they predict peak load currents ~10-15% higher than experiment. This is probably because a comparable fraction of the switches are triggering very late - well after peak power.