Measurements of the plasma potential fluctuations and the fluctuations-induced flux in the CASTOR tokamak

P. Balan,1 J. Adámek, I. ?uran, M. Hron, C. Ioni??,1 E. Martines,2 R. Schrittwieser,1 J. Stöckel, M. Tichý, G. Van Oost3
Institute of Plasma Physics, Association EURATOM/IPP.CR, Prague, Czech Republic
1University of Innsbruck, Austria
2Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova, Italy
3Ghent University, Belgium

Plasma potential, electron temperature, their fluctuations and the fluctuations-induced flux have been measured in the CASTOR Tokamak, using a double emissive probe system. The CASTOR tokamak has a major radius of 0.40 m, a minor radius of 85 mm, a typical shot length of 30 ms and a toroidal magnetic field of 1 T. The role of the electron temperature fluctuations in approximating the plasma potential fluctuations by the floating potential fluctuations was also investigated. Two loops of tungsten wire of 0.2 mm diameter each, at a poloidal distance of 5.5 mm from each other, were installed on a radially movable shaft. In addition two cylindrical probes were mounted on the same shaft, of which one was biased in ion saturation, and the second one was swept for acquiring the local electron temperature from the I-V characteristic. Significant differences of the statistical properties of the plasma potential from the expected behaviour were found in the Scrape-Off Layer (SOL). In addition it was found that the correlation coefficient of the plasma potential and the electron temperature can reach a maximum of 0.8 in the SOL. The poloidal electric field and the fluctuations-induced flux were also determined and certain differences in the SOL and in the core plasma were found.