International collaboration on development of a stellarator confinement database has progressed. More than 3000 data points from nine major stellarator experiments have been compiled. Robust dependences of the energy confinement time on the density and the heating power have been confirmed. Dependences on other operational parameters, i.e. the major and minor radii, magnetic field and the rotational transform , have been evaluated using inter-machine analyses. In order to express the energy confinement in a unified scaling law, systematic differences in each subgroup are quantified. An a posteriori approach using a confinement enhancement factor on ISS95 as a renormalizing configuration-dependent parameter yields a new scaling expression ISS04; . Gyro–Bohm characteristic similar to ISS95 has been confirmed for the extended database with a wider range of plasma parameters and magnetic configurations than in the study of ISS95. It has also been discovered that there is a systematic offset of energy confinement between magnetic configurations, and its measure correlates with the effective helical ripple of the external stellarator field. Full documentation of the International Stellarator Confinement Database is available at http://iscdb.nifs.ac.jp/ and http://www.ipp.mpg.de/ISS.

Regression analyses have been carried out for the international stellarator database which includes 859 discharges from the medium-sized helical devices ATF, CHS, Heiliotron E, W7-A and W7-AS. Recent results from enhanced confinen1ent regime such as H mode and reheat mode are excluded from the database. Optimum fit of all devices is given by the following expression (International Stellarator Scaling 95, ISS95), No dependence of 'tE on the isotropic mass is indicated in the data set. No distinct difference between ECH and NBI can be diagnosed. Because of the different density ranges in the two heating methods, a possible difference might, however, be hidden in the density scaling properties. The density dependence of 'tE also turns to be more complicated than a simple power law. Figure 1 shows a comparison of all data together with ITER L mode database with the ISS95 expression. Although it is crucial to use the appropriate definition of a and -t in the comparison of stellarators and tokamaks, the ISS95 scaling describes tokamak data in L mode very well. In other words, also, the stellarator and the tokamak L mode are of comparable confmement quality.In Fig.1, the data of heliotron/torsatron devices and shearless stellarator have opposite offsets with respect to the ISS95 scaling. It should be noted that data stored in the database are primarily obtained in each standard operation.Operational modes with better confinen1ent are obtained by means of intense wall conditioning and tailoring the magnetic geometry in each device. The ISS95 scaling should be recognized as an L-mode-like scaling. The ISS95 scaling is based on the selection of the iotadependent scaling for heliotron/torsatron confinen1ent. It was tested whether the choice of the radial position at which the -t value is taken influences the results. Regressions using -t at p = 1/3 or 1 do not, however, qualitatively change the results. If the iota-independent scaling is selected, the offsets reduces to a level similar to that when the LHD-scaling expression is used. The next generation experiments LHD and W7-X will allow to distinguished more clearly between the two scaling expression.The predicted operational regime in LHD is also illustrated in Fig. 1, which suggests that the operational regime of LHD will be close to those of the present large tokamaks in L mode.

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.

In this paper we summarize the present status of experimental tokamak confinement studies. Under quiescent conditions, ion heat and impurity transport can be close to the neoclassical leveL Generally, however, radial transport is enhanced by instabilities. There is evidence that anomalous ion heat and momentum transport may be caused by turbulence driven by the ion temperature gradient The same level of understanding is not reached in electron heat and particle transport. Electron heat transport is characterized by a highly nonlinear relation between heat flux and temperature gradient. Particle transport is strongly governed by off-diagonal contributions. Where possible, the tokamak results are compared with those from stellaraton, in particular with those from Wl-AS. Such a comparison is meaningful because stellaratom and tokamaks share many transport aspects. In both cases transport is generally anomalous, degrades with hedting power and increases toward the edge. Bifurcations such as the tokamak H-mode transition are also observed in the W7-AS stellarator. Transport in stellarators, although anomalous, seems to be less confused by additional complexities such as large off-diagonal contributions, profile resilience, disparities between steady-state and perturbatively determined transport coefficients and an isotopic mass dependence in energy and particle transport Differences in magnetic field and minor radius scaling seem to be introduced by operational restrictions: rotational transform c i s constant in stellarator field or size scans whereas q n generally varies with current, field, or size in tokamaks.

Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared