Plasma-electrode interactions on the ZaP-HD sheared-flow-stabilized Z-pinch device

Experiments on the ZaP-HD sheared-flow-stabilized (SFS) Z-pinch device investigate the plasma-material interactions (PMI) occurring at the electrodes. The electrodes are critical plasma-facing components that supply the plasma current while also making direct contact with the high density and high temperature Z pinch. Subject to such extreme conditions, the durability of solid electrodes in future SFS Z-pinch power plants is therefore of significant importance [Shumlak, J. Appl. Phys. (2020)]. Incident particle and heat flux are concentrated at the cathode tip, which coincides with the peak of the radial density and temperature profiles. Substantial erosion of tungsten on the original ZaP-HD cathode motivated replacement with graphite in order to reduce high-Z plasma contamination and improve durability. In this work, carbon erosion at the cathode was measured in-situ via the S/XB spectroscopy method, while the surface temperature was monitored with a two-color pyrometer. In addition, the cathode assembly was redesigned to allow rapid coupon exchanges, enabling ex-situ surface analysis of the electrode material and mass-loss measurements. The coupons were exposed to various plasma conditions and exposure durations on ZaP-HD. Electron microscopy, X-ray spectroscopy, and profilometry were used to monitor changes in the surface morphology, composition, and profile before and after plasma exposure. These qualitative and quantitative measurements provide insight to the relevant PMI processes and their relationship with plasma parameters and pinch performance.