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.Experimental investigations carried out in the ASDEX Upgrade tokamak under various conditions demonstrate that the ion heat flux at the plasma edge plays a key role in the L-H transition physics, while the electron heat flux does not seem to play any role. This is due to the fact that the ion heat flux governs the radial electric field well induced by the main ions which is responsible for the turbulence stabilization causing the L-H transition. The experiments have been carried out in the low density branch of the power threshold where the electron and ion heat channels can be well separated. In plasmas heated by electron heating, the edge ion heat flux has been increased to reach the L-H transition by using separately three actuators: heating power, density and plasma current. In addition, the key role of the edge ion heating has been confirmed in experiments taking advantage of the direct ion heating provided by neutral beam injection. The role of the ion heat flux explains the non-monotonic density dependence of the L-H threshold power. Based on these results, a formula for the density of the threshold minimum has been developed, which also describes well the values found in tokamaks of various size. For ITER it predicts a value which is close to the density presently foreseen to enter the H-mode and indicates that operation at half field and current would benefit from a very significantly lower density minimum and correspondingly low threshold power.

Abstract. An overview of the H-mode threshold power in ASDEX Upgrade which addresses the impact of the tungsten versus graphite wall, the dependences upon plasma current and density, as well as the influence of the plasma ion mass is given. Results on the H-L back transition are also presented. Dedicated L-H transition studies with electron heating at low density, which enable a complete separation of the electron and ion channels, reveal that the ion heat flux is a key parameter in the L-H transition physics mechanism through the main ion pressure gradient which is itself the main contribution to the radial electric field and the induced flow shearing at the edge. The electron channel does not play any role. The 3D magnetic field perturbations used to mitigate the ELMs are found to also influence the L-H transition and to increase the power threshold. This effect is caused by a flattening of the edge pressure gradient in the presence of the 3D fields such that the L-H transitions with and without perturbations occur at the same value of the radial electric field well, but at different heating powers.