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

Summary form given only, as follows. The Z accelerator at Sandia National Laboratories generates current pulses rising up to ∼20 MA in ∼100 ns. Inside the insulator stack at r∼2 m, power flows inward along four conical, vacuum, magnetically-insulated transmission lines (MITL´s) to the post-hole convolute at r∼10 cm. The convolute adds the current in the four feed MITL´s into the inner MITL, which delivers it to the load. The convolute has magnetic field nulls, which could potentially result in large electron losses. To improve our understanding of the electron flow, we are using the 3-D, electromagnetic, PIC code QUICKSILVER. The simulations accurately model the Z vacuum section out to r∼25 cm. Even with fairly coarse zoning, we still need ∼750,000 cells for a 12-post convolute. The complex cathode structure has ∼20,000 particle-emitting cells. We handle electron motion in the wide-ranging magnetic field strength (0-200 T) with a subcycling model. The field solver uses a fixed time step, but each particle is pushed s times per field solver step, with the integer s dynamically chosen so that the particle makes at least 6 steps per orbit. We have done several series of idealized simulations using a fixed load impedance and 20 ns rise-time pulse. Locations of major electron losses are in qualitative agreement with damage to the Z hardware. We are now doing these simulations on a massively-parallel platform, typically 100 processors on Janus (ASCI Red), making more realistic, time-accurate (400,000 time steps) simulations feasible. A z-pinch load model is under development. In preliminary simulations using only electron emission from the cathode, convolute electron losses are much smaller than experiment. A likely source of the discrepancy is the effect of plasmas created at hot spots on the anode from electron deposition.