ELM-resolved divertor erosion in the JET ITER-like wall

Tungsten erosion in H-mode plasmas is quantified in the outer divertor of the JET ITER-Like Wall environment with optical emission spectroscopy on the 400.9 nm atomic neutral tungsten line. A novel cross-calibration procedure is developed to link slow, high spectral resolution spectroscopy and fast photomultiplier tube measurements in order to obtain ELM-resolved photon fluxes. Inter-ELM W erosion is exclusively impurity sputtering by beryllium because of the high sputter threshold for deuterons. Low beryllium concentrations resulted in low inter-ELM sputter yields of around 10−4 with respect to the total flux.

Intra-ELM W sources, which dominate the total W tungsten source, vary independently from the inter-ELM source. The amount of W erosion could only be partly explained by beryllium sputtering, indicating that during ELMs sputtering by fuel species is important. The total W outer divertor source is found to linearly increase with the power crossing the separatrix, whilst excessive divertor fueling can break this trend.

The influence of the W source rate on the tungsten content of the core plasma is investigated using soft x-ray emission to determine the tungsten content. At low source rates the content is determined by the source, but at higher source rates, other phenomena determine the total tungsten content. Indications of impurity flushing by ELMs is seen at ELM frequencies above approximately 40 Hz. The inner/outer divertor asymmetry of the W source during ELMs is investigated, and the outer divertor W source is larger by a factor of $1.8\pm 0.7$ .