Experimental Investigation of Plasma-Electrode Interactions on the ZaP-HD Flow Z-Pinch Device

Abstract

The electrodes of sheared-flow-stabilized (SFS) Z-pinch devices directly face the core plasma and supply the pinch current. The high temperature, high density plasma environment produces intense particle and heat fluxes, leading to concerns of electrode erosion that limit durability and contaminate the plasma. An improved understanding of these plasma-electrode interactions is required, especially at the high temperatures and current densities required for fusion applications. An experimental investigation is conducted on the ZaP-HD SFS Z-pinch device, which produces plasma temperatures up to 1 keV, densities of 10^23 m^-3, and drives pinch currents up to 150 kA. In-situ measurements of the gross carbon erosion flux from the graphite electrode are obtained with S/XB spectroscopy. The measured fluxes exceed the theoretical values from physical sputtering, but are comparable with the expected sublimation flux. An analysis of the ionization mean free paths of neutrals produced by both erosion processes indicates that ionization of sublimated carbon occurs within the electrode sheath, while sputtered neutrals are ionized outside of the sheath. This suggests significant redeposition of sublimated carbon, leading to a process of carbon recycling. The sputtered carbon is therefore primarily responsible for the net erosion. Initial measurements of the electrode surface temperature with a two-color pyrometer are also presented. Ex-situ analysis of electrode material is enabled by the design of a removable coupon. Three different plasma exposure cases were tested that involved varying the pinch current and the particle fluence to the electrode. Net mass loss measurements imply net erosion fluxes far smaller than indicated by spectroscopic measurements of total erosion, which supports the theory of high redeposition rates. Erosion rates range from 0.01 to 0.1 mg/C, which are comparable to existing arc discharge devices. Measurements of the microscopic surface morphology and roughness indicate substantial material rearrangement and general smoothing except at high plasma exposure conditions. The granular matrix of graphite is mostly replaced by larger consolidated structures that reduce the number of visible voids. Crack formation is apparent, possibly due to thermal cycling, which suggests the importance of surface heating and possible phase change of graphite. Definitive features of sputtering such as pitting and cratering are absent, and further study is needed to attribute the observed morphology to other physical processes. Overall, these results indicate some alignment with erosion and recycling processes in high-current arc discharges, which have successfully operated with solid electrodes in extreme environments. Further investigation into these similarities may yield useful understanding that can be applied to the management of erosion on SFS Z-pinch electrodes.