A physics-based model for convolute current loss on Z

Sandia's Z machine [1] is a large pulsed-power accelerator used to create extreme conditions of high energy density. Research areas of Z include dynamic properties of materials, radiation and electromagnetic effects on materials and systems, and inertial confinement fusion. In its current embodiment, four magnetically-insulated radial transmission lines (MITLs) are combined at a double post-hole convolute (PHC). Current loss in the convolute can be quite large (20% of peak load current) for certain pulse shapes and physics loads, thus there is need for a simple physics-based model of this current loss in order to accurately predict load-current waveforms using a transmission-line circuit model of Z in the BERTHA [2] code. Sources of charged particles and plasma in the post-hole convolute are believed to consist of electrons from Child-Langmuir space charge limited field emission from the cathode, as well as cathode and anode plasma production from adsorbed contaminants. The generated loss current path is the subject of some debate, but the result of experiments and PIC simulations [3] indicate that cathode plasma forms initially due to emission from the sides of the cathode hole, then fills a magnetic well downstream of the post, effectively reducing the AK gap there. Current subsequently flows across this plasma-filled gap to complete the circuit. We shall describe the physics contained in the simplified models, show some preliminary results, and discuss how we try to incorporate experimental observations and PIC simulations in our model. We will also describe the inherent uncertainty in the electrode conditions, and how that affects the model results. Based on the model and its integration with BERTHA, recommendations for reducing the current loss will be offered.