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A key element in future toroidal magnetic fusion machines like ITER is the design of a divertor, which allows for safe particle and power exhaust in parallel with high bulk plasma performance. Correspondingly, the definition and design of an optimized divertor is a major task within the ongoing international ITER research and development effort. In order to provide a profound physics basis for such a divertor optimization, different divertor geometries are being tested on major tokamaks. This paper describes the effects of these divertor modifications on plasma performance in ASDEX Upgrade and in JET. In conclusion, increasing closure improves divertor performance without limiting the core plasma performance in ELMy H-modes. more...

The influence of impurity radiation on the and transitions is investigated for highly radiative divertor discharges in the ASDEX Upgrade tokamak. The transitions between H- and L-mode depend on the net heat flow across the separatrix, , calculated from the heating power and bolometric radiation profiles. For typical radiative boundary conditions in ASDEX Upgrade, the radiation distribution is dominated by line emission in the vacuum-ultraviolet region and peaks near the separatrix. For the case of neon used as seed impurity, about 2/3 of the main chamber radiation is emitted inside, but close to the separatrix. Argon seed results in a higher fraction of core radiation, while the line emission is shifted further outside for nitrogen. The radiation-corrected threshold is not affected by gas puffing and is described by [MW, , T, amu]. The threshold power, which is typically lower by a factor of two without strong deuterium puffing, is increased by heavy gas puffing leading to . In the vicinity of the radiation-induced transition, a general alignment of H and L mode is observed with regard to global energy confinement time and edge density and temperature profiles. The transition itself exhibits a smooth evolution in time. Reduction of target plate power load down to about 10% of the total heating power is easily achieved by edge radiation in the CDH-mode for low conditions. However, this reduction is attributed mainly to radiation from inside the separatrix and is connected to relatively high values of the core . These results emphasize the importance of the development of more closed divertor concepts, allowing for higher divertor radiation levels in connection with lower core . more...

ITER or any other future fusion device will operate in H-mode to take advantage of the improved confinement. Stationary operation in H-mode, however, requires the occurrence of ELMs, and the resulting power load on the divertor target plates is not acceptable from the engineering point of view. Beyond this, the present divertor concepts rely on a cold and dense (possibly detached) divertor plasma. Large type-I ELMs, however, carry too large a power and possibly burn through such cold divertor plasmas. The completely detached H-mode (CDH-mode), recently established in ASDEX Upgrade shows detachment at both divertor target plates and small ELMs (so-called type-III ELMs), that do not burn through the divertor plasma. The extrapolation of this scenario to ITER, however, is an important question. more...

The radiation and transport characteristics of ASDEX Upgrade discharges with a neon-driven radiative mantle are modelled using a one-dimensional radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The code is well suited to describing the measured impurity line radiation, total, soft x-ray and bremsstrahlung radiation in regions of the plasma that are not dominated by two-dimensional effects. The recycling and pumping behaviour of neon as well as the bulk transport of neon for radiative boundary scenarios are discussed. more...

The completely detached high confinement (CDH) regime established recently in high power ASDEX-Upgrade single null divertor discharges is described. The standard CDH mode scenario with feedback controlled external deuterium and neon puffing is analysed in detail with respect to its divertor detachment and confinement properties. Impurity transport and pumping effects and the density operation windows of the CDH mode are outlined. First results with other impurity radiators (N,Ar) are given. Finally the CDH mode operation space is discussed in terms of main chamber and scrape-off layer plus divertor radiation and possible consequences for future fusion experiments are outlined. more...

Simultaneous operation of lower hybrid current drive and pellet injection could be successfully achieved. With peripheral ablation of the pellets by suprathermal electrons, the same net inward flux of particles is found as with deep penetration of pellets into Ohmically heated plasmas. The density profile ne(r) peaks with the same increment of the peaking factor Qn=ne0/(ne) in both cases. The global energy confinement time rises with density, tau E approximately ne, in the combined operation. more...

There exist two classes of discharges on ASDEX which overcome the severe confinement deterioration at high densities or heating power. The first class is characterized by unusually peaked density profiles and the second one by exceptionally broad ones. These profiles and the concomitant confinement enhancement imply a change in both the particle and heat transport. Whereas the changes in particle transport are not fully understood, transport analyses point out that the improved heat transport in the first profile class can be explained by reduced ion contribution losses coming close to the neoclassical ones. The different results for the ion transport with flat and peaked density profiles are qualitatively consistent with that expected from eta i-modes. This conclusion is supported by momentum transport studies. The analyses, however, cannot yet explain the electron heat transport which is anomalously high in all regimes. more...

Neutral beam (NI), ion cyclotron resonance (ICRH) and lower hybrid resonance (LHRH) heating on ASDEX are discussed with regard to their effect on plasma confinement. Comparison of NI and ICRH shows that the L and H-regimes are universal confinement modes of auxiliary-heated tokamak plasmas (i.e. independent of the heating method), and that the edge electron temperature (or a related parameter) dictates which mode prevails. In this connection it is noted that carbonization of the vessel walls impedes transition to the H-mode in the case of NI heating. Studies of energy confinement in the intermediate regime from ohmic to NI heating reveal a gradual transition from ohmic ( approximately ne) to neutral injection L-mode ( approximately I) scaling. At the same time a remarkable invariance of electron temperature profile shape with increasing heating power is observed. Changing the NI power deposition profiles from central to off-axis leaves gross energy confinement times unchanged while central confinement is substantially improved. more...

Interaction of lower hybrid waves at 1.3 GHz with ions and electrons was studied in the density range 0.2-5*1013 cm-3 in the ASDEX tokamak. At high densities, ne>or approximately=4*1013 cm-3, fast ions with mainly perpendicular velocities are produced by the RF power at the plasma periphery. They are not well confined and do not lead to any bulk plasma heating. At lower densities, 2*1013 more...

First ICRF experiments on ASDEX have been performed at 67 MHz, corresponding to 2 Omega CH-heating of a hydrogen plasma at B0=2.2 T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up to 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The RF heating is found to depend strongly on plasma preheating. In combination with neutral beam injection the ICRF heating efficiency is even higher than the one of NI. Confinement degrades with ICRH to values in between NI-L-type and OH confinement. more...