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Abstract:
Investigations of the performance and stability of high-beta plasmas have been restarted in W7-AS utilizing higher heating powers and the new divertor system. The combination of beneficial effects resulted in a significant increase of the volume averaged β from 2 % up to 3.1 % and in MHD-quiescent, quasi-stationary discharges at low radiation levels. Experimental studies of equilibrium effects and of MHD mode activity have been performed with the X-ray tomography system. In addition results of computational MHD stability studies are presented, which show an increase of the stability with increasing β due to the pressure induced deepening of the magnetic well in combination with increasing magnetic shear, in qualitative agreement with the experimental data. The maximum achieved β is limited by the available heating power and not by stability effects. The equilibrium β-limit is approached in the case of low-iota configurations. Particularly with regard to current carrying stellarators and comparisons with tokamaks the modification of the stability of high-beta plasmas by significant OH-currents has been investigated.
