A low density H-mode plasma has been selected for detailed inter-ELM modelling by the SOLPS code package, with the coupled treatment of its plasma (fluid code B2) and neutral (Monte-Carlo code Eirene) parts. Good quality measured midplane density and temperature profiles, covering the pedestal region and stretching far into the SOL, as well as several other parameters and profiles measured in the divertor, have enabled testing the consistency of code solutions with experiment. Once the upstream, midplane profiles, have been fitted, and the global parameters (e.g. input power into the computational grid, radiated power) matched, the code reproduced experimental profiles and control parameters in the divertor with the accuracy within a factor 2. Deviations of modelled parameters from the experiment were found around the strike point position where most of the power was deposited on the target. The deviations are consistent among themselves and all point to one common problem with the modelling: the predicted divertor electron temperature is too low, and the density too high, compared with the experiment. The largest inconsistency between the code and experiment was in the magnitude of the peak H α radiation in the outer divertor, which was larger by a factor of 2 in the code simulations. In addition, the code predicts a somewhat higher sub-divertor neutral flux, but lower carbon impurity content in the edge plasma than in the experiment, as well as lower CIII emission. The discrepancy between H α profiles can to a large degree be attributed to profile effects: the simulated H α emission profiles are narrower than in the experiment, reflecting the tendency for the neutral-plasma mix to congregate excessively around the strike point in the modelling. At the same time, the integrated H α emission matches very well with the experiment.Extensive sensitivity studies of the influence of variations in input parameters and assumptions of the code on the modelled divertor conditions have been conducted. They have not resulted in an identification of any SOLPS input/control parameters capable of removing the main disagreement between the code output and experiment. A possibility for parallel transport effects related to low collisionality to increase the effective plasma temperature near the strike point position, or for increased perpendicular transport by neutrals (due to some missing reactions in Eirene) to widen the target profiles, will be explored in the future.2

The conditions for a magnetically con­ fined plasma to ignite are a plasma tem­ perature above 100 Million degree (10 keV) and a product of density ne and energy confinement time τE in excess of 2 x 1020 m-3 s. (Technical require­ ments such as low plasma contamina­ tion by impurity ions have additionally to be fulfilled.) Under steady state condi­ tions, the energy confinement time de­ termines the energy content of the dis­ charge at given heating power or descri­ bes its decay when the heating power is switched-off. On the large tokamak at Princeton, the PLT, an ion temperature of 7.5 keV was achieved in 1979 with natu­ ral injection (Nl) auxiliary heating 1) and on Alcator C, another tokamak device in the USA, using resistive heating by the plasma current (the same process which heats a normal conductor), the less critical n τE breakeven limit was reached in 1984, albeit with a low tem­ perature of 1.5 keV. Thus the road to an ignited tokamak plasma seemed to be clear. However there turned out to be an un­ expected road block: the good confine­ ment properties of resistively heated plasmas could not be maintained at high plasma temperatures. With increasing auxiliary heating power, the plasma con­ finement was found to degrade severely and ignition conditions were in effect not approached. This degradation in confinement was a worldwide obser­ ved phenomenon which was found to be independent of the heating technique and which seemed to threaten the ulti­ mate goal of the fusion programme. On ASDEX, the AxiSymmetric Diver­ tor tokamak Experiment at Garching, the same problem was encountered: τE decreased from 70 ms, established in a resistively heated discharge with a typi­ cal Ohmic input of 0.4 MW, to 20 ms during a Nl pulse of 3 MW. The particle confinement time was also observed to decrease. However, on ASDEX a solu­ tion of this problem was found by sur­ rounding the plasma with a thermal insulation layer. This solution to the heat leak problem is in itself not unique-every house owner applies it in an effort to increase the room temperatures (im­ prove the energy confinement time) without having to uppgrade the furnace. Fig. 1-Poloidal cross-section of the dou­ ble-null divertor configuration of ASDEX. We should not feel inclined to report on it if the thermal resistivity of the insulation layer surrounding the ASDEX plasma were not to surpass that of a product such as Styrofoam by three orders of magnitude, and if degraded confine­ ment as a limitation of the present expe­ riments were not thereby mitigated. Design Features of ASDEX The toroidal plasma in ASDEX has major and minor radii of 1.65 m and 0.4 m; maximum plasma current is 0.5 MA; stabilization is provided by a toroidal ma­ gnetic field of up to 2.8 T. The Nl system of ASDEX delivers a beam of hydrogen atoms with 40 keV energy and with a maximum power of 4.4 MW. The main research goal of ASDEX is to demon­ strate the efficiency of the magnetic di-vertor concept in providing ultraclean hydrogen discharges by minimizing the interaction of the h...

The elements of transport into and across the scrape-off layer in the poloidal divertor tokamak ASDEX Upgrade are analysed for different operational regimes with emphasis on enhanced confinement regimes with an edge barrier. Utilizing the existing set of edge diagnostics, especially the highresolution multi-pulse edge Thomson scattering system, in combination with long discharge plateaus, radial sweeps and advanced averaging techniques, detailed radial mid-plane profiles of diverted plasmas are obtained. Profiles are smooth across the separatrix, indicating strong radial correlation, and there is no remarkable variation across the second separatrix either. Together with measured input, recycling, pumping and bypass fluxes, a corrected separatrix position is determined and transport characteristics are derived in the different radial zones generally identified in the profile structure. Transport in the steep gradient region inside and across the separatrix shows typical ballooning-type critical electron pressure gradient scaling and, in parallel, even a clear correlation between radial electron density and temperature decay lengths (e.g. η e = d(ln T )/d(ln n) ∼ 2 for type-I ELMy H-modes). These findings indicate the importance of stiff profiles in this region, while diffusion coefficients are secondary parameters, determined essentially by the source distribution. The outer scrape-off layer wing exhibits a more filamentary structure with preferential outward drift especially in high-performance discharges, with formal diffusion coefficients far above the Bohm value in agreement with results on the old ASDEX experiment. A basic mechanism involved there seems to be partial loss of equilibrium and fast curvaturedriven outward acceleration, in principle well known from theory, investigated decades ago in pinch experiments and utilized recently in high-field-side pellet fuelling.