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

Abstract

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