Radiation distribution and power balance in the ASDEX Upgrade LYRA divertor

Radiation distribution and heat load on the target plates of ASDEX Upgrade are investigated over a wide experimental parameter range with discharges up to 20 MW heating power and ne=(0.4–0.8)nGW. Integrating the radiation density over the whole plasma yields a total radiated power of about 80% of the input power, and radiation from the divertor and X-point region is about 40% of the input power. Whereas the radiated power in the main plasma is comparable for the former open divertor I (Div I) and the new closed LYRA divertor (Div II), the radiation fraction in the divertor has nearly doubled for the LYRA divertor. Simultaneous to the increased divertor radiation in the LYRA divertor, the maximum heat flux density to the target plates, measured by thermographic cameras, is reduced by a factor of about two compared to that in Div I for comparable attached conditions. Furthermore, especially in the inner divertor fan, the radiation contributes to a large part of the measured heat flux.