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Abstract

Sheaths are space charge regions at the plasma-wall. They are induced by the differential inertia between ions and electrons, and without external perturbation, they create a floating potential between the neutral plasma and the walls. In Tokamaks, these sheaths are locally enhanced by the RF (radiofrequency) electric field generated by the ICRF (ion cyclotron resonance frequency) antennas used to heat magnetic fusion plasmas at very high temperature. RF sheaths are located at the connection points of magnetic field lines to the wall, or to the bumpers which protect the antenna or to any part of the antenna structure. The asymmetric behaviour of these oscillating sheaths rectifies RF potentials in the plasma in front of antenna, to finally create nonlinearly a DC potential which can be much higher than the floating potential. We study specifically how the space-time distribution of these RF and DC rectified potentials is modified when nearby flux tubes are allowed to exchange perpendicular polarization current. To simulate that, a 2D fluid code has been implemented to compute the 2D RF potential map in a plane perpendicular to magnetic lines, and within the flute approximation the whole 3D potential map is deduced. In simulation, we consider a homogeneous transverse conductivity and use a �test� potential map having, in absence of transverse currents, a Gaussian shape characterized by its width r0 and its amplitude. As a function of these 2 parameters (normalized respectively to a characteristic length for transverse transport and to the local temperature), we can estimate the peaking and the smoothing of the potential structure in the presence of polarization current. So, we are able to determine, for typical plasmas, the amplitude of DC potential peaks, particularly on antenna�s corners, where hot spots appear during a shot. In typical Tore Supra conditions near antenna corners potential structures less than centimetric are involved in the 2D effects.

Additional information

Authors: COLAS L, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR);FAUDOT E, LPMIA, UMR CNRS 7040, Universite Henri Poincare, Nancy (FR);HEURAUX S, LPMIA, UMR CNRS 7040, Universite Henri Poincare, Nancy (FR)
Bibliographic Reference: An oral paper given at: 12th International Congress on Plasma Physics Organised by:CEA Research Centres, France Held at: Acropolis conference center, Nice (FR)
Availability: Available from Association EURATOM-CEA, Département de Recherches sur la Fusion Contrôlée, CEA Cadarache, F-13108 St Paul-Lez-Durance, France Tel: (+33) 4 42 25 70 01; Fax: (+33) 4 42 25 64 21 E-mail: dirdrfc@drfc.cad.cea.fr
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