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.

In the edge plasma of the ASDEX tokamak, electrostatic fluctuations were observed with Langmuir probes and in Halpha light with high poloidal and temporal resolution. These fluctuations contribute a significant fraction to the 'anomalous' radial particle transport in the scrape-off layer (SOL). The basic properties and the dependence of the fluctuation parameters on the discharge conditions are documented. A model for an instability mechanism specific to the SOL is introduced and the experimentally observed fluctuation parameters are compared with the predictions of the linearized version of this model. For plasma temperatures above approximately 10 eV in the SOL the observed parameter dependences of the fluctuations are well reproduced by the model. By mixing length arguments the radial transport and the resulting density and pressure gradients in the SOL are estimated from the model. Their dependence on plasma temperature and density qualitatively agrees with the behaviour observed in ohmic discharges on ASDEX

The theoretical and experimental development of stellarators has removed some of the specific deficiencies of this configuration, viz., the limitations in β, the high neoclassical transport, and the low collisionless confinement of α particles. These optimized stellarators can best be realized with a modular coil system. The W7-AS experiment [Plasma Phys. Controlled Fusion 31, 1579 (1989)] has successfully demonstrated two aspects of advanced stellarators, the improved equilibrium and the modular coil concept. Stellarator optimization will much more viably be demonstrated by W7-X [Plasma Physics and Controlled Fusion Research, Proceedings of the 12th International Conference, Nice, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 369], the successor experiment presently under design. Optimized stellarators seem to offer an independent reactor option. In addition, they supplement, in a unique form, the toroidal confinement fusion program, e.g., energy transport is anomalous in stellarators too, but possibly more easily understandable in the frame of existing theoretical concepts than in tokamaks.