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A novel diagnostic, the tunnel probe, is used to investigate the edge plasma of the CASTOR tokamak. Comparison with conventional small, cylindrical Langmuir probes (typical of those usually employed for turbulence measurements in magnetized plasmas) demonstrates the superiority of the tunnel probe. The collectors of the tunnel probe are concave, eliminating in theory all uncertainty of the effective collecting area, and thereby rendering the measurement of parallel ion current density, electron temperature, and floating potential more reliable. Two tunnel probes, mounted back-to-back in a Mach probe arrangement, are used to investigate directional asymmetries of the plasma parameters. The tunnel probes are used as standard Langmuir probes by applying voltage sweeps simultaneously to all their internal conductors. The measured electron temperature is higher on the electron side than on the ion side, and the floating potential is lower. The observed asymmetries, measured at low density and collisionality in CASTOR, could be consistent with a hot tail of non-thermal electrons flowing in the counter-current direction. The ratio of ion saturation currents to the internal conductors of the tunnel probe provides a second independent measurement of electron temperature whose directional asymmetry is less pronounced, in agreement with recent theoretical predictions that tunnel probes should be less sensitive to non-thermal electrons than Langmuir probes in certain conditions.

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Authors: DEJARNAC R, Association EURATOM-IPP.CR, Prague (CZ);STÖCKEL J, Association EURATOM-IPP.CR, Prague (CZ);ADÁMEK J, Association EURATOM-IPP.CR, Prague (CZ);BROTÁNKOVÁ J, Association EURATOM-IPP.CR, Prague (CZ);GUNN J P, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR);IONITA C, Association EURATOM/ÖAW, Institute of Ion Physics and Applied Physics, University of Innsbruck, Innsbruck (AT)
Bibliographic Reference: An article published in: Plasma Physics and Controlled Fusion 49 1791-1808 (2007)
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