ERGODIZATION OF THE MAGNETIC-FIELD AT THE PLASMA EDGE BY THE DYNAMIC ERGODIC DIVERTOR OF TEXTOR-94

The structure of the resonant helical magnetic field of the dynamic er godic divertor (DED) which is planned for TEXTOR is presented. The erg odic features are investigated by field line tracing calculations by t he Gourdon code. The island widths and the related Chirikov parameters are determined by Poincare plots. The deduced values are smaller than the ones predicted by the standard theory and a correction of the sta ndard theory is presented. The onset of stochastization by the nonline ar interaction of neighboring island chains as well as remnant islands and barriers for field line diffusion at the maximum perturbation lev el, for which the Chirikov parameter is slightly above 1, are shown. T he effect of hot spots sweeping over the limiter surface due to the ro tation of the perturbation field is demonstrated. Local effects of the helical field and the divergence of a bundle of field lines are short ly worked our. Poincare plots for lower mode operation are shown, whic h 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 th ese problems. (C) 1997 Elsevier Science S.A.