3-D PIC simulations of the convolute and inner MITL on the Z accelerator

Summary form only given. The Z accelerator at Sandia National Laboratories delivers current pulses rising up to /spl sim/20 MA in /spl sim/100 ns to a Z-pinch load. The accelerator is cylindrical, with power flowing radially inward through a water pulse-forming section to the insulator stack at r/spl sim/2 m. Inside the stack, power flows along four conical, vacuum, magnetically-insulated transmission lines (MITLs) to the convolute, at r/spl sim/10 cm. The convolute couples the feed MITL currents, in parallel, into the inner MITL that drives the load. The convolute is a critical area of the vacuum power flow, because it necessarily has magnetic field nulls, which could potentially result in large electron losses. We have been using the 3-D, electromagnetic particle-in-cell (PIC) code QUICKSILVER to simulate the convolute and inner MITL region of Z. The simulations are done in cylindrical coordinates and accurately model the Z geometry (to "stair-step" accuracy). The azimuthal extent of the simulation is /spl pi//N/sub p/, where N/sub p/ is the number of posts (typically 12), with mirror boundaries applied on each boundary. The fairly coarse zoning used still requires /spl sim/520,000 cells for a 12-post geometry. The complex cathode structure has /spl sim/11,400 particle-emitting cells.