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(EX8/2) MHD Phenomena at ASDEX Upgrade

S. Günter, A. Gude, M. Maraschek, S. D. Pinches, S. Sesnic, R. C. Wolf, Q. Yu, H. Zohm^* and the ASDEX Upgrade Team

MPI für Plasmaphysik, EURATOM-Association, D-85748 Garching, Germany
^* Institut für Plasmaforschung, Universität Stuttgart, D-70569 Stuttgart, Germany

Abstract.  The onset of neoclassical tearing modes leads to the most serious $\beta$ limit at ASDEX Upgrade. The $\beta_{p}^{}$ value for the onset of neoclassical tearing modes is found to be proportional to the ion gyro-radius for collisionless plasmas as proposed by the ion polarisation current model. Larger collisionalities have a stabilizing effect. Sawtooth crashes or fishbones can trigger the mode, and in a few cases it appears spontaneously. Fishbones are shown to be able to cause magnetic reconnection. The fractional energy loss due to a (3,2) mode saturates for large pressures at around 25 %. In discharges with large impurity accumulation unusual MHD phenomena such as cascades of high-n tearing modes and modes driven by positive pressure gradients have been found.

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