Thermal effects of surface layers on divertor target plates

Measurement of the heat flux to the divertor by infrared thermography is complicated by processes at the surface leading to excess IR radiation. There are at least two possible causes of the excess radiation: (1) an emission process from the plasma just above the target surface, and (2) a surface layer of low thermal conductivity resulting in higher temperatures for a given heat flux. Understanding these extra processes is important for an accurate temperature measurement and assessment of the heat flux. Comparison of IR emission and visible spectra during very bright events have led us to conclude that the excess IR radiation observed at the JET target plates is genuinely due to blackbody radiation.