Divertor performance of high density H-mode discharges in ASDEX Upgrade

The high density H-mode is one of the most probable operational modes in next step devices like ITER for ignition as well as for continuous burning. H-mode operational conditions, however, raise difficult physical questions, especially with respect to the expected enormous heat load of the divertor. The H-mode in ASDEX Upgrade divides into three submodes, namely ELM free, type-I and type-III ELMs. The ELM free mode seems to be incompatible with continuous plasma operation owing to the loss of plasma particle inventory control and concomitant strong impurity accumulation. The type-I ELM regime might be disadvantageous owing to the time averaged strong local concentration of the power load of the divertor plates. The type-III ELM scenario may not be accessible under ITER conditions. Energy removal by significant impurity radiation in the plasma boundary will probably be necessary to reduce the divertor heat load and enable easier access to the desired cold divertor plasma regime but may be detrimental to bulk plasma properties. The investigations presented in this contribution aim at the development of a stable and stationary type-I ELMing H-mode regime with cold divertor characteristics. (author) 7 refs., 3 figs