MAGNETIC SURFACES AND LOCALIZED PERTURBATIONS IN THE WENDELSTEIN VII-A STELLARATOR
The critical dependence of plasma 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 order 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. In W VII-A the current leads to the helical windings are one symmetry breaking perturbation, and there might also be others. It is possible to avoid field errors of this kind in future stellarators.
Bibliographic Reference: WRITE TO MAX-PLANCK-INSTITUT FUER PLASMAPHYSIK, 8046 GARCHING BEI MUENCHEN (GERMANY), MENTIONING REPORT IPP 2/283, 1986
Record Number: 1989125021300 / Last updated on: 1987-02-01
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