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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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