MHD phenomena in ASDEX Upgrade and TEXTOR

Abstract

MHD phenomena in ASDEX Upgrade in standard shear discharge regimes as well as in weak and reversed shear discharges are investigated. The onset of neoclassical tearing modes leads to the most serious β limit at ASDEX Upgrade. The βp value for the onset of neoclassical tearing modes is found to be proportional to the poloidal ion gyroradius for collisionless plasmas, as proposed by the ion polarization current model. Larger collisionalities have a stabilizing effect. Sawtooth crashes or fishbones can trigger the mode, and in a few cases it appears spontaneously. The fractional energy loss due to a (3,2) mode saturates for large pressures at around 25%, in agreement with the result of theoretical calculations. Owing to its large and flexible heating power, ASDEX Upgrade is well equipped for investigations of fast particle driven or influenced MHD modes. The observed TAEs have been shown to agree well with theoretical predictions regarding the frequency as well as the drive by fast particles. Fishbones have been shown to not only affect the fast particle distribution but also directly to affect the background plasma. For low and reversed shear discharges, theoretical predictions for the growth of neoclassical tearing modes in regimes with a large fraction of bootstrap current at the rational surface, as well as for neoclassical double tearing modes, are given. In discharges with large impurity accumulation in the plasma centre, unusual MHD phenomena such as cascades of high n tearing modes and modes driven by positive pressure gradients have been found at ASDEX Upgrade. For similar discharges, double tearing modes have been investigated at TEXTOR.