Configurational effects on the confinement in the stellarator Wendelstein W 7AS

The plasma containment of W 7AS exhibits a strong dependence on characteristics of the magnetic configuration: rotational transform, shear and remaining plasma current. During the first experiments by application of ECF 70 GHz in the accessible range for β(0)≤0.5% optimal confinement is found operating close to, but not at rational values of the rotational transform: ι-bar = 1/4, 1/3, 1/2, 2/3. Small shear is beneficial, if such ''resonances'' can be excluded from the entire plasma column. (author). 6 refs, 3 figs