Turbulent SOL transport in stellarators and tokamaks

Spatially highly resolved Langmuir probe measurements of ion saturation cur rent and floating potential fluctuations in the scrape-off layer (SOL) and edge of several tokamaks and stellarators have revealed the spatial structu re of these fluctuations. The knowledge of the phase angle and coherency be tween the fluctuations of these quantities is important for a comparison wi th models and for the calculation of the induced radial transport. The fluc tuations occur as individual events, causing burst-like E x B flows which a re very localised in poloidal position and in time and interchange plasma o f different density and temperature radially. The resulting particle transp ort can account for the confinement time inferred from particle balance con siderations and from the observed gradients in the SOL. The role of fluctua tions 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 discha rge parameters are varied. (C) 1999 Elsevier Science B.V. All rights reserv ed.