A future fusion reactor is expected to have all-metal plasma facing materials (PFM) to ensure low erosion rates, low tritium retention and stability against high neutron fluences. As a consequence, intrinsic radiation losses in the plasma edge and divertor are low in comparison to devices with carbon PFMs. To avoid localized overheating in the divertor, intrinsic low-Z and medium-Z impurities have to be inserted into the plasma to convert a major part of the power flux into radiation and to facilitate partial divertor detachment. For burning plasma conditions in ITER, which operates not far above the L-H threshold power, a high divertor radiation level will be mandatory to avoid thermal overload of divertor components. Moreover, in a prototype reactor, DEMO, a high main plasma radiation level will be required in addition for dissipation of the much higher alpha heating power. For divertor plasma conditions in present day tokamaks and in ITER, nitrogen appears most suitable regarding its radiative characteristics. If elevated main chamber radiation is desired as well, argon is the best candidate for simulataneous enhancement of core and divertor radiation, provided sufficient divertor compression can be obtained. The parameter P sep /R, the power flux through the separatrix normalized by the major radius, is suggested as a suitable scaling (for a given electron density) for the extrapolation of present day divertor conditions to larger devices. The scaling for main chamber radiation from small to large devices has a higher, more favourable dependence of about P rad,main /R 2 . Krypton provides the smallest fuel dilution for DEMO conditions, but has a more centrally peaked radiation profile compared to argon. For investigation of the different effects of main chamber and divertor radiation and for optimization of their distribution, a double radiative feedback system has been implemented in ASDEX Upgrade. About half the ITER/DEMO values of P sep /R have been achieved so far, and close to DEMO values of P rad,main /R 2 , albeit at lower P sep /R. Further increase of this parameter may be achieved by increase of the neutral pressure or improved divertor geometry.

Abstract. A multidiagnostic approach, utilizing Langmuir probes in the midplane, X-point and divertor walls, along with Lithium beam and infrared measurements is employed to evaluate the evolution of the Scrape-off Layer (SOL) of ASDEX Upgrade across the L-mode density transition leading to the formation of a density shoulder. The flattening of the SOL density profiles is linked to a regime change of filaments, which become faster and larger, and to a similar flattening of the q profile. This transition is related to the beginning of outer divertor detachment and leads to the onset of a velocity shear layer in the SOL. Experimental measurements are in good agreement with several filament models which describe the process as a transition from conduction to convection-dominated SOL perpendicular transport caused by an increase of parallel collisionality. These results could be of great relevance since both ITER and DEMO will feature detached divertors and densities largely over the transition values, and might therefore exhibit convective transport levels different to those observed typically in present-day devices.

The physics base for the ITER Physics Design Guidelines is reviewed in view of application to DEMO and areas are pointed out in which improvement is needed to arrive at a consistent set of DEMO Physics Design Guidelines. Amongst the proposed improvements, the area of power exhaust plays a crucial role since predictive capability of present-day models is low and this area is expected to play a major role in limiting DEMO designs due to the much larger value of Ptot/R in DEMO than in present-day devices and even ITER.

Abstract. In this article a new experimental classification of divertor detachment in ASDEX Upgrade is presented. For this purpose a series of Ohmic and L-mode density ramp discharges at different heating powers, magnetic field directions and plasma species were carried out. For the first time at ASDEX Upgrade the electron density in the divertor volume and the occurrence of volume recombination were measured by means of spectroscopy. It is shown that detachment is not a continuously evolving process but rather undergoes three distinct states while the characteristics of the inner and outer divertor are strongly coupled. Before the complete detachment of the inner and outer divertor, radiative fluctuations occur in the inner divertor close to the Xpoint, observed for the first time via new fast diode bolometers. Finally, the effect of an externally applied magnetic perturbation field on the detachment process is investigated.

Abstract. Detachment of high power discharges is obtained in ASDEX Upgrade by simultaneous feedback control of core radiation and divertor radiation or thermoelectric currents by the injection of radiating impurities. So far 2/3 of the ITER normalized heatflux P sep /R= 15 MW/m has been obtained in ASDEX Upgrade under partially detached conditions with a peak target heatflux well below 10 MW/m 2 . When the detachment is further pronounced towards lower peak heatflux at the target, substantial changes in ELM behaviour, density and radiation distribution occur. The time-averaged peak heat flux at both divertor targets can be reduced below 2 MW/m 2 , which offers an attractive DEMO divertor scenario with potential for simpler and cheaper technical solutions. Generally, pronounced detachment leads to a pedestal and core density rise by about 20-40 %, moderate ( 20 %) confinement degradation and a reduction of ELM size. For AUG conditions, some operational challenges occur, like the density cut-off limit for X-2 ECRH heating, which is used for central tungsten control. Partial detachment of high power discharges in ASDEX Upgrade2

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The rst stable, completely detached H-mode plasma in full tungsten ASDEX Upgrade has been achieved. Complete detachment of both targets is induced by nitrogen seeding into the divertor. Two new phases are added to the detachment classication described in [1]: First, the line integrated density increases by about 15 % with partial detachment of the outer divertor. Second, complete detachment of both targets is correlated to the appearance of intense, strongly localized, stable radiation at the X-point. Radiated power fractions, f rad , increase from about 50 % to 85 % with nitrogen seeding. X-point radiation is accompanied by a loss of pedestal top plasma pressure of about 60 %. However, the core pressure at ρ pol < 0.7 changes only by about 10 %. H 98 = 0.8 − 1.0 is observed during detached operation. With nitrogen seeding the ELM frequency increases from the 100 Hz range to a broadband distribution at 1 − 2 kHz with a large reduction in ELM size.

Abstract. 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 15MW/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 30MW/m 2 can not 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 4eV.For uncontrolled ELMs sizes in the order of 100MJ are estimated. Results on ELM broadening from JET suggest that in DEMO an energy density limit of 0.5MJ/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.