3-D PIC simulations of electron flow in the vacuum power flow section of the Z accelerator

The Z accelerator at Sandia National Laboratories delivers current pulses rising up to /spl sim/20 MA in /spl sim/100 ns to the load. Inside the insulator stack at r/spl sim/1.8 m, power flows along four radial, vacuum, magnetically-insulated transmission lines (MITLs) to the post-hole convolute at r/spl sim/10 cm. The convolute adds the current in the four feed MITLs into the inner MITL, which delivers it to the load. The convolute is a critical area for the vacuum power flow, because it has magnetic field nulls, which could potentially result in large electron losses. The 3-D, electromagnetic particle-in-cell (PIC) code QUICKSILVER is being used to simulate the electron flow and losses on Z. The simulation geometry accurately models the convolute and inner MITL up to the load, and the feed MITLs out to r/spl sim/22 cm. The simulations are driven with a time-accurate forward-going voltage waveform adapted from shot data. The Z-pinch load is modeled with a new boundary condition terminating a coaxial transmission line with a time-dependent inductor L(t). Current losses in the simulations agree very well with experiment early in the pulse. Late in time, the simulation losses are noticeably smaller. Potential sources for the discrepancy are considered. Several regions of the anode are very rapidly heated above temperatures at which desorption of neutral impurities is expected.