Physics of island divertor plasmas in stellarators

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 governing 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 neutrals, 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 analysis of the island divertor physics, with most key issues, however, being related to experimental results. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.