The Dynamic Ergodic Divertor

The concept of the Dynamic Ergodic Divertor (DED) is based on plasma edge ergodisation by a resonant perturbation. Such a divertor concept is closely related to helical or island divertors in stellarators. The base mode of the DED perturbation field can be m/n = 12 /4, 6/2 or 3/1. The 3/1 base mode with its deep penetration of the perturbation field provides the excitation of tearing modes. This topic was presented elsewhere. In this contribution we concentrate on the divertor properties of the DED. We report on the characterisation of the topology, transport properties in ergodic fields, divertor regimes, impurity transport and density limit behaviour. The 12/4 base mode where the perturbation is restricted to the plasma edge is suitable for divertor operation. With increasing perturbation field island chains are built up at the resonance layers. Overlapping islands lead to ergodisation. The plasma is guided in the laminar region via open field lines of short connection length to the divertor target. The magnetic topology is not only controlled by the coil current but especially by the edge safety factor. For appropriate edge safety factor we observe a strong temperature drop in the plasma edge, indicating an expanding laminar region, which is necessary to decouple the divertor plasma from the core plasma. This temperature drop is accompanied by a redistribution of the heat and particle flux on the divertor target which is measured by thermography, visible spectroscopy and Langmuir probes. The modifications of the magnetic topology by the DED are reflected in the distribution of the plasma edge density and temperature measured by atomic beams and can be directly seen for example from carbon emission lines. The magnetic structure is calculated by the ATLAS code and shows good agreement with the experimental findings. The particle and energy transport is modelled with the EMC3-EIRENE code package and is in qualitative agreement with the measured densities and temperatures. (Author)