Evaluation of the power and particle exhaust performance of various alternative divertor concepts for DEMO

To mitigate the risk that the ITER conventional divertor solution, based on a single-null (SND) magnetic configuration and tungsten (W) targets, will not extrapolate to a fusion power plant, EUROfusion is assessing alternatives including advanced configurations (AC) and the use of liquid metals (LM) as target material. The paper describes the present status of modelling activity devoted to comparing the stationary plasma exhaust properties of possible AC including LM solutions with the standard SND divertor for the EU version of DEMO, leaving out the ELM problem. Feasibility studies have been performed to demonstrate the ability of various codes to predict the basic exhaust characteristics of alternative divertor configurations, including the snowflake divertor (SFD), X divertor (XD) and Super-X divertor (SXD), for DEMO. The main objective is to establish trends in the dependence of the power and particle exhaust characteristics on the geometrical divertor characteristics, with main focus on: a) Divertor radiation losses; b) Accessibility of detachment, c)Peak heat flux at the target It has been found, that the different alternative configurations do not result in significantly different volume losses with or without added impurities. In case of SFD, this is because the main changes of the divertor magnetic topology with respect to the standard single null configuration occur in a region close to the primary X point. Here, rather far from the target, temperatures are too high and neutrals density too low to drive significant volume losses, either by radiation or by charge exchange. Simulations show that the reduction of the heat load to the target in DEMO reactor by Li ions is rather limited and accompanied by strong core plasma contamination. A Evaluation of the power and particle exhaust 2 limit for Li concentration of about 15% has been found and this results is not sensitive to the Li production mechanism. Operation of the liquid lithium divertor has to be accompanied by seeding of noble gasses (e.g. Ar) to reduce heat load to the target plates and simultaneously to reduce plasma dilution allowing for higher fusion power. In the case of Sn liquid metal divertor, sputtering processes determine conditions in the divertor leading to low plasma temperature and semi-detached conditions in the divertor. In spite of high radiation fraction for Sn target (> 82%), seeding with additional impurities (e.g. Ne, Ar) is necessary to reduce the power to the divertor targets to acceptable levels. PACS numbers: 52.55.Fa, 52.55.Rk, 52.30.Ex