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The critical dependence of plasm a confinement in low-shear stellarators, such as Wendelstein VII-A, on the external rotational transform can be explained on the basis of magnetic surface destruction. External symmetry-breaking perturbations generate islands on the low order rational magnetic surfaces. The islands are largest at t = 1/2 and t = 1/3. Confinement is optimum in close proximity to these values. In o rd er to study the structure of surfaces under the influence of perturbations, a mapping procedure is used instead of field line integration. It is found that the neighbourhood of low-order rational surfaces is particularly robust against surface destruction. The reason is that in this vicinity only rational surfaces with large m and n exist (t = m/n). On these surfaces the external perturbation only generates small islands.

Magnetic Surfaces and Localized Perturbations in the Wendelstein VII-A Stellarator