Two and three-dimensional MITL power-flow studies on RITS

The Radiographic Integrated Test Stand (RITS) (Smith, ID, et al., 1999) is an inductive voltage adder accelerator being developed at Sandia National Laboratories for driving high brightness flash radiographic sources. Three separate 70-ns, 1.35-MV pulses are generated in 8-Ohm parallel water dielectric pulse forming lines and are added in series with induction cells to form a single high voltage drive pulse. Each induction cell has a single point feed with a tapered azimuthal transmission line which distributes the incoming pulse around the bore of the cell. The individual cells are joined in series by a vacuum coaxial magnetically insulated transmission line (MITL). The first three induction cells (4 MV total) have been assembled and tested with detailed current and voltage measurements. These measurements provide a first ever analysis of three dimensional behavior in single feed IVA's and offer unique data for comparison to theory and PIC simulations. Comparison of experimental measurements and folly three-dimensional particle in cell (PIC) simulations with the Lsp code are presented. It is found that current asymmetries produced within the induction cells are propagated downstream into the MITL. The azimuthally asymmetric modes perturb the electron flow in the MITL but in general do not cause loss of magnetic insulation and thus current. Simulation results are in good agreement with current measurements made on both the anode and cathode of the MITL for a number of variables including: 1) azimuthal variation, 2) operating impedance and 3) pulse rise-time. Differences exist between the measured and calculated values of the anode to cathode current ratio. An extension of one-dimensional laminar insulation flow theory (Creedon, JM, 1975) to a two-dimensional (r,/spl theta/ plane) laminar flow model is also presented. The theory supports the experiments in part by suggesting that large asymmetries in the cathode current density can be maintained without degrading or changing significantly the total current in the MITL (i.e. no loss of insulation). We conclude that there is wide variability in the operating point of the MITL without significant loss of power transmission.