Turbulent SOL transport in stellarators and tokamaks

Spatially highly resolved Langmuir probe measurements of ion saturation current and floating potential fluctuations in the scrape-off layer (SOL) and edge of several tokamaks and stellarators have revealed the spatial structure of these fluctuations. The knowledge of the phase angle and coherency between the fluctuations of these quantities is important for a comparison with models and for the calculation of the induced radial transport. The fluctuations occur as individual events, causing burst-like E x B flows which are very localised in poloidal position and in time and interchange plasma of different density and temperature radially. The resulting particle transport can account for the confinement time inferred from particle balance considerations and from the observed gradients in the SOL. The role of fluctuations of the electron temperature and our understanding of the instability mechanism acting in the plasma edge is summarised. A brief account is given on the changes of the fluctuations and the resulting transport when discharge parameters are varied. (C) 1999 Elsevier Science B.V. All rights reserved.