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Understanding of the scrape-off layer (SOL) physics in tokamaks requiresdiagnostics with sufficient temporal and spatial resolution. This contributiondescribes results of experiments performed in the SOL of the CASTOR tokamak(R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding thewhole poloidal cross section. The individual probes measure either the ionsaturation current of the floating potential with the spatial resolution up to3 mm. Experiments are performed in a particular magnetic configuration,characterized by a long parallel connection length in the SOL, L_par ~q2piR. Wereport on measurements in discharges, where the edge electric field is modifiedby inserting a biased electrode into the edge plasma. In particular, a complexpicture is observed, if the biased electrode is located inside the SOL. Thepoloidal distribution of the floating potential appears to be stronglynon-uniform at biasing. The peaks of potential are observed at particularpoloidal angles. This is interpreted as formation of a biased flux tube, whichemanates from the electrode along the magnetic field lines and snakes q timesaround the torus. The resulting electric field in the SOL is 2-dimensional,having the radial as well as the poloidal component. It is demonstrated thatthe poloidal electric field E_pol convects the edge plasma radially due to theE_pol x B_T drift either inward or outward depending on its sign. Theconvective particle flux is by two orders of magnitude larger than thefluctuation-induced one and consequently dominates.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,  Nice (France

Formation of convective cells during scrape-off layer biasing in the CASTOR tokamak