Contrib. Plasma Phys. 46, No. 7-9, 504 – 514 (2006) / DOI 10.1002/ctpp.200610037 Physics of Island Divertor Plasmas in Stellarators Y. Feng ∗1 , F. Sardei 1 , P. Grigull 1 , K. McCormick 1 , J. Kisslinger 1 , and D. Reiter 2 1 Max-Planck-Institut f¨ ur Plasmaphysik, Teilinstitut Greifswald, Euratom Association, Greifswald/Garching, Germany 2 Institut f¨ ur Plasmaphysik, Forschungszentrum J¨ ulich Gmbh, Euratom Association, Trilateral Euregio Cluster, D-52425 J¨ ulich, Germany Received 19 October 2005, accepted 9 February 2006 Published online 22 August 2006 Key words Stellarator, island, divertor. PACS 52.55.Hc, 52.55.Rk c  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Using the W7-AS island divertor as a guideline, the paper presents a physics-analysis of the plasma, neutral and impurity transport in the island divertor, aimed at showing the basic similarities and differences in SOL- transport between tokamaks and a typical helical device. Discussion is mainly based on simple models and estimations, while 3D modeling calculations serve only for control of self-consistency and for determining basic functional dependencies not accessible otherwise. The paper focuses on the explanation of the govern- ing physics and mechanisms for the SOL or SOL-related phenomena in W7-AS, such as the absence of a high-recycling regime, rollover of the total recycling flux and of the core-fueling rate from the recycling neu- trals, island neutral screening, partial detachment, geometry-related detachment stability and Marfes. A special emphasis is given on a stellarator-specific, impurity retention effect of the edge islands recently obtained by EMC3/EIRENE simulations for high-density, high-confinement plasmas. The paper presents a theoretical anal- ysis of the island divertor physics, with most key issues, however, being related to experimental results. c  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction While in tokamaks divertors have been extensively investigated since several decades and the poloidal-field diver- tor as a successful concept has been accepted and installed in most of the existing axisymmetric devices, divertor programs for stellarators were started only recently [1,2,3]. Both theory and experiment are at the very beginning. Because of the large differences in magnetic-field topology among individual stellarators, the question as whether there exists, like the poloidal-field divertor for tokamaks, a general divertor concept for helical devices remains still open. Nevertheless, purely from the geometry point of view, there exits at least one common feature shared by all stellarators, i.e. the existence of natural magnetic islands. The magnetic islands have a divertor potential and their feasibility and suitability for a divertor solution of plasma exhaust in low-shear stellarators has been demonstrated in W7-AS [1]. Following the same principle, the large helical device LHD is exploring the 1/1 island for improving the exhaust conditions [3]. The island divertor program started in W7-AS will continue in W7-X where the enlarged islands are even more suitable for further investigating the island divertor concept [4]. Generally, the island divertor follows the same principle as that of the tokamak poloidal-field divertor. The magnetic islands either have a natural open structure or are opened by intersecting target plates. The open field lines inside the islands guide the power entering the islands across the inner separatrix towards the targets positioned at the outer separatrix. The islands form an intermediate SOL between the confinement core and the plasma-surface interaction region, preventing the core plasma from a direct exposure to the recycling neutrals and the sputtering impurities. On the other hand, however, there exist large differences in the respective magnetic and divertor geometries between tokamaks and stellarators, including 3D effects, which influence the plasma, neutral and impurity transport in the SOL. Using the W7-AS island divertor as an example, aim of this paper is ∗ Corresponding author: e-mail: yuehe.feng@ipp.mpg.de c  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim