Experimental determination of spatially-dependent sub-surface impurity content in tungsten exposed to helium plasma operation in WEST and a controlled laboratory plasma source*

Tungsten (W) will be used for the divertor in ITER and is a leading candidate in future fusion reactors. The divertor will face an extremely hostile environment leading to severe material degradation of W in the form of surface erosion, migration, redeposition, and impurity transport. This imposes a significant challenge on divertor performance and thus, understanding W behaviour under fusion-relevant environment and developing strong diagnostics tools is essential for the development of advanced plasma facing components for future fusion reactors. In this presentation we investigate the depth-dependent helium (He) and oxygen (O) concentration in single crystal W exposed to approximately 1 second of He plasma in the WEST C4 campaign. He exposures range from ~4x10²¹ to 3.6x10²³ m^-2. Additionally, polycrystalline W was exposed to a controlled, low flux plasma source for 1 to 10 hours to fluences ranging from 5E19 to 5E21 m^-2 at The University of Tennessee at Knoxville(UTK). Comparison between these samples will provide insight to the impact of different plasma conditions, including impurities and plasma flux, on the depth dependent He concentration. He and O have been spatially depth profiled in plasma-exposed W using Laser Ablation Mass Spectrometry (LAMS) in conjunction with Laser Induced Breakdown Spectroscopy (LIBS). The uncertainties of these results will be discussed through detailed introduction of the calibration processes of these two techniques.