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 issue on the path to achieving fusion in a magnetic confinement device. Over the past 10 years, the ergodic divertor was successfully operated on the Tore Supra tokamak to fulfil this task. In ergodic divertor discharges, a magnetic perturbation, superimposed upon the equilibrium magnetic field, modifies transport properties by creating open field lines at the plasma edge. The characteristics of the resulting edge ergodic layer (hence, the induced effects on transport, MHD stability and other physical properties) depend on the features of both the existing equilibrium and the applied perturbation. Until recently, the description of the ergodic layer was solely based upon results from numerical simulations which had been poorly validated against experimental data. We report here on the results of a series of experiments dedicated to this issue, and particularly to locating the separatrix position (i.e., the boundary between closed and open field lines).