D. Hildebrandt

First island divertor experiments on the W7-AS stellarator

1. Abstract In the past, under limiter conditions, it has been impossible to produce high-power, highdensity, quasi-stationary neutral beam injection (NBI) discharges in W7-AS. Such discharges tended to evince impurity accumulation, lack of density control and subsequent radiation collapse (Normal Confinement). Presently, W7-AS is operating with a modular, open island divertor similar to that foreseen for W7-X. The divertor enables access to a new NBI heated, high density (ne up to 4·10 20 m -3 ) operating regime (High Density H-mode). It is extant above a threshold density, and is characterized by flat density profiles, high energyand low impurity confinement times and edge-localized radiation. The HDH-mode shows strong similarity to ELM-free H-mode scenarios previously observed in W7-AS, but in contrast to these avoids impurity accumulation. These new features enable full density control and quasi steady-state operation over many confinement times (at present only technically limited by the availability of NBI) also under conditions of partial detachment from the divertor targets. In HDH-mode, even in attached discharges, the divertor target load is considerable reduced. This is mainly due to favourable upstream conditions (higher nes), edge localized radiation and increased power deposition width. The benefits of the HDH-mode do not restrict only to hydrogen plasmas. They also occur ‐ albeit in a modified manner ‐ in deuterium plasmas. Undoubtedly, there are clear isotope effects between hydrogen and deuterium discharges. The results obtained in W7-AS render good prospects for W7-X and support the island divertor concept as a serious candidate for devices with magnetic islands at the edge. 2. Results Fig. 1 summarizes the behaviour of the energy confinement time E =W/Pabs, the normalized radiated power Prad/Pabs, and separatrix density nes obtained from quasi-stationary discharges with Pabs=1.4 MW as a function of the line-averaged density ne. E-values in NC follow the scaling E ISS95 =0.26· a 0.4 ·Bt 0.83 ·a 2.21 ·R 0.65 ·ne 0.51 ·Pabs -0.59 , [2], whereas for the HDH-mode one finds E ~ 2· E ISS95 . P rad /P abs grows smoothly with ne until partial plasma detachment, where a jump in the normalized radiated power occurs. The separatrix density n es increases sharply at the NC HDH-mode transition point, then continues to climb with ne and saturates

Short and long range transport of materials eroded from wall components in fusion devices

Abstract Carbon sources and the sinks have been quantified in TEXTOR and are discussed in terms of short and long range transport. The major source (22 g/h) is the graphite belt limiter, but part (10 g/h) of the carbon is directly re-deposited after short range transport. Long range transport causes flake formation on obstacles and neutralisers, but little and deuterium rich (D/C≈0.7) deposition in remote areas. The rest is leaving via the pumps in gaseous form. This behaviour is different from that in JET where large amounts of deuterium rich deposits were found in the louvers. Tungsten is favoured for the ITER divertors because of its low sputtering yield for hydrogen, but melting and erosion by carbon may be an additional concern. The short range transport of tungsten has been investigated in a well defined experiment and quantitatively re-constructed by means of the ERO-TEXTOR code. Code validation is necessary in order to increase the confidence and the applicability to JET and ITER.

Mixed-material coating formation on tungsten surfaces during plasma exposure in TEXTOR-94

Abstract A polished graphite sample with a tungsten layer on the plasma-facing surface was exposed to the scrape-off plasma of TEXTOR-94 for about 200 s of discharge time. The plasma exposure caused a contamination layer with a thickness up to 300 nm consisting mainly of the deposited plasma impurity carbon and the tungsten substrate material. The processes and conditions involved in the growth of such mixed material layers are examined. In accompanying experiments, the penetration of carbon into tungsten by diffusion and ion implantation at different temperatures has been measured. The influence of synergistic effects on the carbon penetration into tungsten by the simultaneous impact of carbon and hydrogen ions was studied by irradiation of tungsten with CH 3 ions. The discussion also includes processes such as non-uniform deposition, preferential erosion and prompt local redeposition.

Island divertor experiments on the Wendelstein 7-AS stellarator

A promisingnew operational reg ime on the Wendelstein stellarator W7-AS has been discovered, fulfillingthe conditions of optimal core behavior in combination with edge parameters suitable for successful divertor scenarios. This regime, the high density H-mode (HDH), displays no systematically evident mode activity, and is edge localized mode (ELM)-free. It is extant above a power-dependent threshold density and characterized by flat density profiles, high energy- and low impurity-confinement times and edge-localized radiation. Impurity accumulation, normally as

Influence of magnetic field configurations on divertor plasma parameters in the W7-AS stellarator

Abstract The new island divertor in W7-AS enables quasi steady-state operation with NBI at very high density including scenarios with stable detachment from the targets. Experiments with reversed B-field indicate that the interaction zones on the targets are affected in first order by E×B drifts. Stable detachment is restricted to magnetic field configurations with sufficiently large separation between x-points and targets and not too small field line pitch inside the islands. It is always partial in the sense that it does not extend over the full target area. This inhomogeneity is ascribed to an in/out asymmetry of the electron temperature at the upstream separatrix position.

Island divertor studies on W7-AS

Abstract Basic topological features of the island divertor concept for low shear stellarators are discussed with emphasis on the differences to tokamak divertors. Extensive measurements of the edge structures by two-dimensional plasma spectroscopy and by target calorimetry are in excellent agreement with predicted vacuum and equilibrium configurations, which are available up to central β values of ∼ 1%. For this β value the calculated field-line pitch inside the islands is twice that of the corresponding vacuum case. Video observations of the strike points indicate stability of the island structures for central β values up to ∼ 3.7%. The interpretation of the complex island divertor physics of W7-AS has become possible by the development of the three-dimensional plasma transport code EMC3 (Edge Monte Carlo 3D), which has been coupled self-consistently to the EIRENE neutral gas code. Analysis of high density NBI discharges gives strong indications of stable high recycling conditions for n e ≥ 10 20 m −3 . The observations are reproduced by the EMC3/EIRENE code and supported by calculations with the B2/EIRENE code adapted to W7-AS. Improvement of recycling, pumping and target load distribution is expected from the new optimized target plates and baffles to be installed in W7-AS.

Deuterium trapping in divertor tiles of ASDEX-Upgrade

The deuterium inventory of divertor target tiles used in ASDEX-Upgrade for up to 2000 discharges has been analyzed by thermodesorption spectrometry. In addition, surface analysis techniques as auger electron spectrometry, secondary ion mass spectrometry, nuclear reaction analysis, electron microscopy and optical surface profilometry have been applied for investigating erosion and deposition phenomena. The original plasma facing surfaces were graphite (EK98) and plasma sprayed tungsten, respectively. The total deuterium inventories have been found to vary between 4 · 10 21 D-atoms/m 2 and 3 · 10 23 D-atoms/m 2 . The deuterium is contained in a deposit on the surface of the graphite and tungsten tiles consisting mainly of carbon, boron and the hydrogen isotopes. There is strong indication that morphological eAects influence impurity deposition, deuterium trapping and re-erosion of the contamination. ” 1999 Elsevier Science B.V. All rights reserved.

Results from a double-sided langmuir probe in T-10, and an extended model of a probe in a streaming magnetized plasma

Abstract By means of a JANUS-type Langmuir probe operated in the scrape-off layer (SOL) in T-10 the temporal evolution and radial profiles of plasma density, electron temperature, and toroidal asymmetry of saturation currents were measured in dependence on discharge parameters. Of particular importance turned out to be the B T field direction and the radial probe position relative to the active limiter structure (rail and aperture limiter). The interpretation of the results is based on the local SOL structure in terms of connection lengths to the active limiter and special flow effects in T-10, and makes use of a theoretical probe model by Hutchinson. As to the latter model, a refinement concerning its basic principles is presented independently.

Core-edge studies with boundary island configurations on the W7-AS stellarator

Core, edge and scrape-off-layer plasma behaviour is studied principally under conditions of an a = 5/9 boundary island configuration - which is relevant for the upcoming W7-AS divertor campaign - but for now with ten inboard sector limiters. The major focus is on compatibility between good core confinement and attainment of high recycling at the limiter. At low input power Pin0.4 MW, operation at densities necessary to attain effective divertor action in the future invariably leads to a transition to the ELM-free H-mode accompanied by lower edge densities and increased core radiation until radiation collapse ensues. Thereby, enhancement factors in E of nearly two above the international stellarator confinement scaling are transiently achieved. The threshold density ethr, necessary to attain the H-mode increases with heating power, such that at 2 MW NBI heating power the H-mode is completely suppressed and peak densities at the limiter exceeding 1.5 × 1020 m-3 are realized. The efficacy of newly-installed control coils designed to manipulate the island geometry is tested. Their influence on the core plasma is verified. Due to geometrical effects associated with the mutual shadowing of the inboard limiters, statements regarding the influence on island physics must await the divertor configuration.

Divertor operation in stellarators: results from W7-AS and implications for future devices

The research on divertors for stellarators is at the beginning. Extensive studies are being prepared on large helical device (LHD) and W7-X. W7-AS is now being operated with an open island divertor (ID) which serves as a test bed for the W7-X diverter. The divertor enables access to a new NBI-heated, high-density operating regime with improved confinement properties. This regime-the high-density H-mode (HDH)-displays no evident mode activity, is extant above a threshold density and characterized by flat density profiles. high-energy- and low-impurity-confinement times and edge-localized radiation. Impurity accumulation, normally associated with ELM-free H-modes, is avoided. Quasi-steady-state discharges with n e up to 4 x 10 20 m -3 , edge radiation levels up to 90% and plasma partial detachment at the divertor targets can be simultaneously realized. The accessibility to other improved confinement modes in W7-AS (conventional H-mode anti OC-mode) is not restricted by the divertor. The results provide a promising basis for future experiments, in particular on W7-X, and recommend the ID as a serious candidate for solving the plasma exhaust problem in stellarators.