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Current loss in the vacuum section of the refurbished Z accelerator

There are two contributing factors to the current loss in the vacuum section of the Z accelerator at Sandia National Laboratories. Early in time, the loss can be accounted for solely by vacuum electron flow effects. Electrons emitted in the four magnetically insulated transmission lines (MITLs) flow into the convolute and are lost to anode surfaces, particularly in magnetic null regions. These losses can be modeled with standard electromagnetic, particle-in-cell (PIC) methods. Results of detailed 3-D PIC simulations are presented, showing good agreement with experiment early in the pulse, over a range of Z shots. However, late in the pulse, the simulations underestimate the observed loss in experiment, sometimes substantially. Electrode plasma effects in the convolute must be responsible for the additional loss. New PIC models to create an electrode plasma layer from first principles, using electron and ion particles, have been developed. It is shown that very small cell sizes (≪ 10 µm) are required to accurately resolve the expansion of electrode plasmas with T ∼ 1 eV, into an anode-cathode gap with megavolts applied across it. Such small cell sizes are prohibitively small for 3-D convolute simulations.