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Abstract

Coherent precursor oscillations have been observed prior to beta-limit disruptions at high currents in highly elongated Tokamak à Configuration Variable (TCV) plasmas. The mode structure of the precursors is analysed using fast magnetic fluctuation and soft X-ray (SXR) emission measurements. TCV is equipped with a complete poloidal array and two complete toroidal arrays which measure the poloidal magnetic field inside the vacuum vessel. The data is treated with a standard Fourier analysis in space and time. Furthermore, a simultaneous singular value decomposition of toroidal and poloidal measurements is used to identify the corresponding toroidal and poloidal mode numbers n and m. The analysis reveals a kink-type m=2/n=1 mode which grows locked and causes the disruption. This mode is often preceded by cascades of rotating m/n=4/3 and 3/2 modes. These modes also occur in non-disruptive high-current discharges where they cause minor disruptions but do not trigger the 2/1 mode. The growth times of typically one millisecond correspond to resistive time scales. Measurements of the SXR emissivity at four toroidal locations as well as the tomographic reconstruction in the poloidal plane show good agreement with the mode structures obtained from the magnetic measurements.

Additional information

Authors: REIMERDES H, Centre de Recherches en Physique des Plasmas, Association EURATOM � Confédération Suisse, Ecole Polytechnique Fédérale de Lausanne (CH);FURNO I, Centre de Recherches en Physique des Plasmas, Association EURATOM � Confédération Suisse, Ecole Polytechnique Fédérale de Lausanne (CH);POCHELON A, Centre de Recherches en Physique des Plasmas, Association EURATOM � Confédération Suisse, Ecole Polytechnique Fédérale de Lausanne (CH)
Bibliographic Reference: Article: 1998 International Congress on Plasma Physics, 25th EPS Conference on Controlled Fusion and Plasma Physics, Prague (CZ), June 29 � July 3, 1998
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