Lithium erosion experiments and modelling under quiescent plasma conditions in DIII-D

Lithium-sputtering erosion and transport has been measured in the outer divertor of the DIII-D tokamak. The Divertor Materials Evaluation System (DiMES) mechanism places a 2.5 cm lithium spot as a plasma-facing surface in the divertor. Plasma diagnostics and atomic lithium visible spectroscopy are used to measure the lithium erosion yield near the outer strikepoint (OSP) with swept-plasma parameters of electron temperature 5-25 eV and electron density (0.03-1.80) × 10¹⁹ m^-3. Solid-phase lithium physical sputtering is measured to be less than 10% (Li/D⁺). The yield increases with incident energy. Physical sputtering models confirm measurements of sputtered energy and spatial distributions showing skewed angular distributions when the sample is exposed near the OSP and isotropic angular distributions when the sample is exposed to the private flux region. REDEP/WBC modelling of near-surface impurity transport agrees well with experimental measurements showing sputtered lithium neutral atoms effectively ionized about a centimetre away from the Li-DiMES probe surface. A reduction in lithium physical sputtering by a factor of 4-5 is measured when the lithium surface forms an oxide, consistent with the physical sputtering behaviour of most metal-oxides. Although of less significance than lithium atom transport, there is a modelling/data discrepancy regarding lithium ion transport with, e.g. the data showing more asymmetric ion transport than predicted.