Experimental study of ergodic edge plasmas with marfes in Tore Supra

High-density discharges with and without the ergodic divertor (ED) are compared. With increasing density the radiation maximum swings from the outside divertor modules to the inner wall. A strongly radiating cold plasma layer (an ergodic divertor marfe, ED-marfe) appears on the high-field side. Differing from the marfe in the limiter configuration, this ED-marfe has a poloidal extension of more than 120 degrees and can be run in steady-state conditions. It shows a poloidal structure with temperature modulations which reflect the magnetic structure of the divertor configuration. The modifications in the plasma edge electron temperature and density profiles are used for the identification of three different divertor regions with characteristic transport properties. At the plasma edge, within the stochastic region, a flat electron temperature profile is measured by Thomson scattering with temperatures as low as 20 eV. In this region, a positive radial electric field is deduced from rotation measurements. The importance of charge exchange (CX) processes due to a high neutral density in the radiating edge layer is shown. The neutral density within the ED-marfe and the ergodic region is estimated via CX-sensitive intensity ratios of carbon impurity ions. Densities of about 1018 m-3 in the marfe and 1016-1017 m-3 in the ergodic region are estimated.