Appearance of hot spots due to deposits in the JET MKII-HD outer divertor

Deposited layers in the JET MKII-HD outer divertor have been investigated on the basis of their transient heating. The Planck radiation in the 400-600 nm wavelength range and IR thermography data were analyzed to correlate the appearance of the layers with plasma conditions. Both methods yielded significantly different surface temperatures: typically >2000 K for the visible light spectroscopy and down to 800 K for the thermography. This is explained by the existence of high temperature emission areas as small as 1-2 mm2. Analysis of the reoccurrence of hot spots in the outer divertor throughout the 2006 campaigns indicated that the formation is determined by the combination of the outer strike point location and the plasma stored energy. The observations did not indicate any changes in thermal properties nor cyclic formation and disintegration of the layers, i.e. it was stable and so-called hard layers.