Divertor Impurity Sources; Effects of Hot Surfaces and Thin Films on Impurity Production
Strong continuum emission has been observed from divertor tiles at visible wavelengths and identified as Planck radiation from surfaces with temperatures of typically ~2600K. Such hot spots (which are not tile edges) are often seen at the inner divertor, and can persist for several seconds. Outer divertor hot spots are much rarer, and were never seen in the MkII GB divertor. Surprisingly, these hot spots do not usually produce significant impurity fluxes. In contrast, ELMs may produce a significant enhancement of impurity fluxes, depending on strike point location and ELM size.