Blob Transport in the Tokamak Scrape-off-Layer

Recent experimental evidence suggests the importance of fast radial plasma transport in the scrape‐off‐layer (SOL) of tokamaks. The outward transport appears to be convective rather than diffusive, extends into the far SOL, and can produce significant recycling from the main‐chamber walls, partially bypassing the divertor. A plausible theoretical mechanism to explain this phenomenon is the radial transport of “blobs” of locally dense plasma created by turbulent processes. A related process is the inward transport of “holes” of reduced density plasma, which provides a mechanism for rapid inward transport of impurities. The blob model is also consistent with the spatial and temporal intermittency and the non‐Gaussian statistics observed in the SOL plasma. This paper reviews the present status of blob theory, including analytic models and simulations, and discusses the preliminary comparisons of the blob model with experimental data. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)