Particle-in-cell simulations of the ZR magnetically insulated transmission lines

Summary form only given. An important issue for the refurbishment of the Z accelerator at Sandia National Laboratories is limiting electron losses to the anode in the vacuum section. As on Z, the ZR vacuum section consists of four magnetically insulated transmission lines (MITLs) coupled in parallel, with a double-post-hole convolute, into the final MITL delivering the full current to the load. On Z, there is clear evidence of 5-10% current losses late in the power pulse, in the convolute. 3-D particle-in-cell (PIC) simulations show that almost all electron loss to the anode occurs in the convolute, localized primarily at the magnetic-null regions of the anode. We believe that the observed convolute loss on Z is due to the effects of dense plasmas formed in these regions, e.g. gap closure. These effects cannot be modeled with standard PIC methods, so we cannot directly scale the convolute current loss up to the higher power levels on ZR. However, an important result from the 3-D simulations is that all electrons lost in the convolute are emitted out in the MITLs, and E/spl times/B drift into the convolute. This suggests that if we can limit the flow into the convolute on ZR to be no higher than on Z today, convolute losses will not be excessive. This problem can be efficiently addressed with 2-D, r-z PIC simulations, modeling an entire MITL, from just outside the convolute out to the vacuum insulator. The goal is to find a profile of gap as a function of radius that optimizes the tradeoff between two competing requirements: minimizing the electron flow into the convolute, and minimizing the overall MITL inductance.