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(EX/S3-1) Investigation of the Beta-Limit in the W7-AS Stellarator

A. Weller¹⁾, J. Geiger¹⁾, M. Zarnstorff²⁾, E. Sallander¹⁾, S. Klose¹⁾, A. Werner¹⁾, J. Baldzuhn¹⁾, R. Brakel¹⁾, R. Burhenn¹⁾, H. Ehmler¹⁾, F. Gadelmeier¹⁾, L. Giannone¹⁾, D. Hartmann¹⁾, R. Jaenicke¹⁾, J. Knauer¹⁾, H.P. Laqua¹⁾, C. Nührenberg¹⁾, E. Pasch¹⁾, N. Rust¹⁾, E. Speth¹⁾, D.A. Spong³⁾, F. Wagner¹⁾, U. Wenzel¹⁾, W7-AS Team¹⁾, NBI-Group^1)
 
1) Max-Planck-Institut für Plasmaphysik, IPP-Euratom Association, Garching, Greifswald, Berlin, Germany
²⁾ Princeton Plasma Physics Laboratory, Princeton, NJ, USA
³⁾ Oak Ridge National Laboratory, Oak Ridge, TN, USA

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 $\beta$ 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 $\beta$ 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 $\beta$ is limited by the available heating power and not by stability effects. The equilibrium $\beta$-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.

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