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(EX7/4) Effects of Toroidal Currents upon Magnetic Configurations and Stability in Wendelstein 7-AS

A. Weller¹⁾, M. Anton¹⁾, R. Brakel¹⁾, J. Geiger¹⁾, M. Hirsch¹⁾, R. Jaenicke¹⁾, S. Klose¹⁾, E. Sallander²⁾, A. Werner¹⁾, W7-AS Team¹⁾, IPP-NBI Group^1)
 
1) Max-Planck-Institut für Plasmaphysik, Garching, Germany
²⁾ Alfvén Laboratory, Royal Institute of Technology, Stockholm, Sweden

Abstract.  The proposal of new concepts for current carrying hybrid stellarators has raised the issue if current driven instabilities, in particular major disruptions, may be suppressed or mitigated by the externally provided poloidal magnetic field. In W7-AS the internal toroidal currents such as bootstrap and Okhawa currents are cancelled by opposite currents driven inductively or by electron cyclotron current drive (ECCD). In this way the edge rotational transform is controlled, and net current-free stable plasmas are maintained. On the other hand, the current drive systems provide a flexible tool to investigate current driven instabilities as well as various issues concerning the effect of magnetic shear on confinement and MHD mode behaviour. The stability studies in the presence of significant toroidal currents have been made in the accessible range of the external rotational transform $\mbox{$\iota\llap{-}$}$_ext = 0.30...0.56 involving the low order rational surfaces $\mbox{$\iota\llap{-}$}$ = 1/2, 3/2, 3/4 and 1. In addition the rational surfaces $\mbox{$\iota\llap{-}$}$ = 1/3 and 1/4 could be accessed by reverse current drive. Target plasmas heated by electron cyclotron resonance heating (ECRH), neutral beam injection (NBI) or both were investigated in order to assess to which extent the stability depends on particular current density profiles. Disruption-like events, preceded by tearing mode activity, have been observed in a wide range of the external rotational transform. The mode structures have been analyzed by X-ray tomography, electron cyclotron emission (ECE) diagnostics and magnetic measurements. The experimental data are roughly consistent with stability calculations on the basis of a cylindrical $\Delta{^\prime}$-analysis. In contrast to the tokamak case the plasma equilibrium is maintained even after a thermal collapse enabling a recovery of plasma energy and inductive current. The improved positional stability can result in the formation of very large magnetic islands. Severe disruption-like effects may be controlled by excluding relevant rational surfaces, in particular $\mbox{$\iota\llap{-}$}$ = 1/2, from the outer plasma region.

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