Ergodization of the magnetic field at the plasma edge by the dynamic ergodic divertor of TEXTOR 941

The structure of the resonant helical magnetic field of the dynamic ergodic divertor (DED) which is planned for TEXTOR is presented. The ergodic features are investigated by field line tracing calculations by the Gourdon code. The island widths and the related Chirikov parameters are determined by Poincaré plots. The deduced values are smaller than the ones predicted by the standard theory and a correction of the standard theory is presented. The onset of stochastization by the nonlinear interaction of neighboring island chains as well as remnant islands and barriers for field line diffusion at the maximum perturbation level, for which the Chirikov parameter is slightly above 1, are shown. The effect of hot spots sweeping over the limiter surface due to the rotation of the perturbation field is demonstrated. Local effects of the helical field and the divergence of a bundle of field lines are shortly worked out. Poincaré plots for lower mode operation are shown, which can penetrate deeper into the plasma to act at the q=2 surface. The accuracy of the numerical tool is demonstrated to be sufficient for these problems.