Effects of non-vanishing toroidal current densities on stability in the Wendelstein 7-AS stellarator

At the W7-AS stellarator the influence of non-vanishing toroidal current densities on stability is studied over a broad range of externally provided values of the rotational transform (0.301 £ iext £ 0.565). By means of inductive current drive and by varying the heating scenarios, using 1 MW NBI and 0.4 MW ECRH, access is given to a variety of current density radial profiles with significant shear. Disruptive-like events accompanied by fast energy loss are observed as soon as low order major rational resonances appear in the outer region of the plasma in discharges with significant contributions of net toroidal ohmic current in the positive direction (increasing the rotational transform). To some extent the applied loop voltage is able to prevent a total decay of the plasma current. Preceding the partial or total energy loss, a perturbing effect in the equilibrium is observed in the radial profile of the electron temperature, as derived from ECE measurements, and the tearing mode structure is identified by tomographic reconstruction of the emission in the soft X ray wavelength region. A D¢ code is used to calculate numerically the stability of the current density radial profiles against tearing modes, and the predictions are compared with experimental observations. ECR heated plasmas with inductive current drive in the negative direction are observed to be stable at iext = 0.48 and 0.52 in a reversed magnetic shear configuration (as defined in the tokamak sense: dq/dr < 0). No significant tearing mode activity is found in the presence of large bootstrap and Ohkawa current densities in plasmas without ohmic current drive. However, a deterioration of confinement is observed due to other effects on rational surfaces at the edge.