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(EX/C5-4) Improved Performance of the W7-AS Stellarator with the New Island Divertor

R. Brakel1), P. Grigull1), K. McCormick1), R. Burhenn1), Y. Feng1), F. Sardei1), J. Baldzuhn1), H. Ehmler1), F. Gadelmeier1), L. Giannone1), D. Hildebrandt1), K. Ida2), R. Jaenicke1), J. Kisslinger1), T. Klinger1), J.P. Knauer1), R. König1), G. Kühner1), H.P. Laqua1), D. Naujoks1), H. Niedermeyer1), E. Pasch1), N. Ramasubramanian1), N. Rust1), F. Wagner1), A. Weller1), U. Wenzel1), A. Werner1), W7-AS Team1)
1) Max-Planck-Institut für Plasmaphysik, Garching, EURATOM-Association, Germany
2) National Institute of Fusion Science, Toki, Japan

Abstract.  The island divertor concept has successfully been realized at W7-AS. The divertor gives access to a new NBI-heated high density regime with densities up to 4×1020m- 3 and energy confinement well above customary scalings. This regime appears promising with respect to the requirements of both confinement and exhaust. Many features are reminiscent of a quiescent H-mode, but plasma particle transport behaves differently. Beyond a threshold density energy confinement approximately doubles whereas particle confinement dramatically decreases. The density profile flattens and the inward impurity pinch is reduced. This eases density control, prevents impurity accumulation, and allows for quasi-stationary conditions with radiation profiles peaked at the edge. At the highest densities partial detachment occurs with a radiated power fraction up to 90% at a tolerable expense of plasma energy. The sub-divertor pressure is sufficient for pumping. Major experimental results, such as the lack of a high recycling phase preceding detachment, are also predicted by the EMC3/EIRENE code.

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IAEA 2003