Effects of radial electric fields on the turbulence and transport in the TEXTOR edge and SOL plasma

Results are given on the changes in edge transport in the presence of positive fields, below and above the H-mode threshold, induced in TEXTOR by means of a biasing electrode. Langmuir probes are used to measure the time-stationary convective and turbulence driven particle transport. Experiments with normal and reversed toroidal field directions provide a first indication of the spatial variation of the particle fluxes. The convective flux locally dominates the transport inside the limiter radius, but appears to be strongly position dependent, both in magnitude and direction. The turbulence driven particle flux is more spatially uniform. With respect to a reference ohmic discharge, the convective flux strongly increases inside the limiter when positive bias is applied with only small changes occurring at the H-mode transition. Radial fluxes as high as 2×10²¹ particles/m²s, inwardly or outwardly directed, can be observed at the limiter radius. The turbulent flux inside the limiter during ohmic discharges is outwards, increases with a +250 V bias voltage, but turns inwards in H-mode.