Characterisation of the separatrix position in the ergodic divertor discharges of the Tore Supra tokamak

The control of particle and heat fluxes at the plasma edge is a critical is sue on the path to achieving fusion in a magnetic confinement device. Over the past 10 years, the ergodic divertor was successfully operated on the To re Supra tokamak to fulfil this task. In ergodic divertor discharges, a mag netic perturbation, superimposed upon the equilibrium magnetic field, modif ies transport properties by creating open field lines at the plasma edge. T he characteristics of the resulting edge ergodic layer (hence, the induced effects on transport, MHD stability and other physical properties) depend o n the features of both the existing equilibrium and the applied perturbatio n. Until recently, the description of the ergodic layer was solely based up on results from numerical simulations which had been poorly validated again st experimental data. We report here on the results of a series of experime nts dedicated to this issue, and particularly to locating the separatrix po sition (i.e., the boundary between closed and open field lines). (C) 2001 E lsevier Science B.V. All rights reserved.