2-D PIC Simulations of Electron Flow in the Magnetically Insulated Transmission Lines of Z and ZR

The ZR project [1] to refurbish the Z accelerator at Sandia National Laboratories [2] is scheduled for completion in FY06. An important factor for the success of ZR is limiting current losses in the vacuum section. On Z today, late-time losses of 5 - 10% are observed, which are known to occur in the post-hole convolute region. The most direct way to mitigate these losses is to limit the electron flow into the convolute from the magnetically insulated transmission lines (MITLs). The key design consideration is the radial profile of the MITL gap. The MITL gap profile is a compromise between two competing constraints - limiting both the electron flow into the convolute and the MITL inductance. Larger gaps reduce the flow, while smaller gaps reduce the inductance. 2-D particle-in-cell (PIC) simulations suggest that the current Z A-level MITL profile is close to optimum, in the sense of minimizing the flow for its total inductance. For ZR, operating at ~40% higher voltage and ~30% higher current, the decision was made to limit the flow into the convolute to be no larger than on Z today. Time-accurate simulations, with a Z-pinch load, show that profiles based on Z, but with 20% larger gaps meet this condition. It does not appear to be possible to meet the flow limitation without this increase in inductance.