Two dimensional modelling approach to transport properties of the TEXTOR-DED laminar zone

An important operational range for TEXTOR-DED is title case of a high ergod ization at the plasma edge. Under these conditions, the edge magnetic field forms a proper ergodic layer and a laminar zone. The laminar zone is estab lished by magnetic field lines which intersect wall elements after only a f ew toroidal turns (open ergodic system). For this, both the heat and partic le transport in the plasma edge are dominated by the laminar zone which sho ws a well defined structure compared with that of the ergodic layer. For th e present analysis the regions containing field lines with connection lengt hs corresponding to one or to two poloidal turns are of particular interest ; the radial width of these regions is of the same order as the width of th e unperturbed SOL of a 'classical' limiter or divertor configuration. Field lines running poloidally twice around the torus introduce a particular new feature, they impose a connection between radially separated areas and thu s enhance the radial temperature and particle transport. A new 2-D modellin g approach code for the laminar zone of an ergodic divertor has been develo ped which is strongly oriented on the topology of the magnetic field lines in the plasma edge. A 2-D finite element method is used in order to model t he perpendicular transport of particles and energy. For obtaining a solutio n of the parallel transport, an analytical model is discussed providing exp ressions for averaged particle and energy sources. By considering the topol ogical properties of the magnetic field and corresponding assumptions in th e code, the transport calculation in the perturbed plasma edge gives a firs t insight into the generic transport properties of the TEXTOR-DED laminar z one. As an important application for later experiments the power fluxes to the wall elements are estimated. They show a strong variation perpendicular to the perturbation coils and a medium variation over the helical divertor footprints.