Spectroscopic measurements of graphite electrode erosion on the ZaP-HD sheared-flow-stabilized Z-pinch device

TL;DR

This study applies the S/XB spectroscopy method to the ZaP-HD SFS Z-pinch device to quantify gross carbon erosion from a graphite electrode during high-current plasma exposure. By calibrating absolute C-III 229.7 nm emission, and selecting S/XB coefficients from DHI-derived $n_e$ and $T_e$ profiles (adapted to account for electrode contact), the authors infer erosion fluxes and compare them to sputtering and sublimation theories. The measured peak erosion flux reaches about $9.7\times10^{30}$ atoms m$^{-2}$ s$^{-1}$, which is orders of magnitude larger than sputtering but of the same order as sublimation, implying sublimation as the dominant erosion mechanism under the studied conditions. Expanding the density profile to conserve linear density (to reflect plasma expansion near the electrode) reduces the inferred flux by roughly a factor of four, yet it remains well above sputtering and near sublimation values, underscoring the role of sublimation and its energy distribution in electrode erosion and redeposition behavior.$ $ These results enhance understanding of PMI in high-temperature, high-current Z-pinch devices and inform material-durability considerations for future fusion energy systems where solid-electrode contact is involved.

Abstract

The ionizations per photon, or S/XB, method uses spectroscopic measurements of radiating impurity ions to determine the influx from a solid surface. It is highly useful as a non-perturbing, in-situ measure of the gross erosion flux of plasma-facing components (PFCs). In sheared-flow-stabilized (SFS) Z-pinch devices, the electrode supplies the plasma current and directly contacts the core Z-pinch plasma. Electrode erosion due to the large particle and heat fluxes affects electrode durability, which is an important factor in existing and future devices. An improved understanding of these plasma-electrode interactions is required, in particular as energy density increases. Experiments on the ZaP-HD device investigate erosion of the graphite electrode by applying the S/XB method for C-III emission at 229.7 nm. The S/XB coefficients are determined from electron density and temperature profiles obtained from Digital Holographic Interferometry (DHI) measurements. An approach for expanding these profiles to represent plasma contacting the electrode is described. In both cases, the measured erosion fluxes are on the order of 10$^{30}$-10$^{31}$ atoms m$^{-2}$s$^{-1}$. These values are significantly larger than the expected erosion flux due to physical sputtering of H$^+$ ions on carbon, but are comparable to theoretical sublimation fluxes. This suggests that the source of carbon erosion flux is primarily from sublimation as opposed to sputtering. The dominance of sublimation over sputtering processes implies a difference in energy of the eroded neutrals which may provide insight on redeposition and net erosion behavior.

Figures (13)

• Figure 1: S/XB coefficients obtained from the ADAS database for the 229.7 nm emission of the C-III ion, showing variation for the range of typical ZaP-HD plasma parameters. These coefficients are used to convert the measured photon flux of C-III emission into an inferred erosion flux of carbon neutrals.
• Figure 2: Cross-sectional machine drawing of the ZaP-HD SFS Z-pinch device showing the Acceleration Region where the plasma is generated and accelerated and the Assembly Region where the Z-pinch plasma is formed and compressed. The Z-pinch plasma (shown schematically in magenta) directly contacts the electrode nose cone. Four rectangular fused silica windows in the Assembly Region provide optical access to the Z-pinch plasma and the electrode for erosion measurements.
• Figure 3: Recent modifications to ZaP-HD placed the electrode further downstream, simplifying optical access with a direct line of sight to the telescope used for S/XB spectroscopy.
• Figure 4: Impact parameters of the 20 spectroscopy chords on the electrode nose cone profile. Plasma flows downstream in the positive $z$ direction. The central axis of ZaP-HD is along $x = 0$. The measurement chords span a total of 23.6 mm across the nose cone. All but the four outer chords on either side of the array have a line of sight which terminates on the electrode surface.
• Figure 5: Absolute calibration setup for S/XB measurements. The output of the light source passes through a fused silica window and is focused by a telecentric telescope onto a 20-chord fiber bundle. The distance from the telescope lens to the light source replicates the distance on the experiment to the Z-pinch plasma. The absolute intensity response of the system is recorded on the ICCD for the known output of the light source.