DEMO divertor limitations during and in between ELMs

Operation of DEMO in comparison to ITER will be significantly more demanding, as various additional limitations of physical and technical nature have to be respected. In particular a set of extremely restrictive boundary conditions on divertor operation during and in between ELMs will have to be respected. It is of high importance to describe these limitations in order to consider them as early as possible in the ongoing development of the DEMO concept design. This paper extrapolates the existing physics basis on power and particle exhaust to DEMO. In phases between ELMs or with mitigated ELMs surface overheating and W sputtering pose challenging boundary conditions. For attached divertor conditions at 90% total radiation fraction a peak power density of about 15 MW m(-2) convected or radiated to the outer divertor is estimated. As this clearly exceeds the tolerable limit, some degree of divertor detachment is regarded as essential for the operation of DEMO. A loss of detachment with a peak power density of more than 30 MW m(-2) cannot be tolerated for more than a second before the divertor would suffer from a destructive event. The combination of the limitations on the peak power flux density and W sputtering rate necessitates divertor temperatures less than 4 eV. For uncontrolled ELMs sizes in the order of 100 MJ are estimated. Results on ELM broadening from JET suggest that in DEMO an energy density limit of 0.5 MJ m(-2) per ELM is exceeded by a factor of about 8 for a large range of relative ELM sizes. This highlights the necessity of a reactor-relevant ELM control technique for DEMO, which is capable of reducing the maximum size of the energy loss per ELM to the divertor by more than an order of magnitude without a strong reduction of confinement.