Preliminary experiments in the TORE SUPRA tokamak using the ergodic divertor configuration have shown a strong effect on plasma impurities. The result is a decontamination of the central plasma which is due to a decreased carbon content. This is the consequence of a screening effect of the peripheral ergodic layer which is due to an important increase of the recycling impurity flux, as well as of a modification of the impurity source terms.

The stabilization of both the m s=s 2, n = 1 tearing mode and a detached plasma has been obtained with the use of the ergodic divertor in Tore Supra Ohmically heated discharges at the density limit and for low plasma pressure. This has allowed us, for the first time, to define and to check a discharge piloting strategy to prevent density-limit disruptions and to create a stable edge radiating layer which dissipates 100% of the input power.PACS numbers: 52.35.Py, 52.55.Fa, In future thermonuclear reactors such as ITER (International Thermonuclear Experimental Reactor), the plasma-facing components will have to absorb tremendous power fluxes. Any inhomogeneity in the power flux or in the power deposition can cause damage to these components. For instance, in the case of conductiveconvective power losses, the misalignment of first wall components can focus the power on salient surfaces [1]. The simplest way to get rid of this inhomogeneity is to maximize radiative losses at the plasma edge. Such an ideal radiating regime is directly achieved on tokamaks when the density limit is approached, inducing radiative phenomena such as MARFEs (multifaceted asymmetric radiation from the edge) and detached plasmas which radiate up to 100% of the input power [2,3]. This attractive scenario is counterbalanced by major disruptions occurring at the density limit and restricting the stable operating regime of present day tokamaks [4].With long-pulse operation (> 30 s), high auxiliary heating power (<20 MW), and a first wall cooled by fully pressurized water (30 bars, 230°C), Tore Supra already meets two major problems arising on future large tokamaks: the power exhaust and disruptions. For these reasons we are looking for specific operational strategies on Tore Supra to solve these problems. In this paper we report the results of experiments where for the first time the ergodic divertor (ED) [5,6] has been used to stabilize simultaneously the radiative structure of a detached plasma and the w-2, /z = l tearing mode (m and n are respectively the poloidal and toroidal numbers), both encountered in the predisruptive phase of density-limit disruptions. This allows us to avoid disruptions and to create plasmas with a stable edge radiating layer which is poloidally symmetric and dissipates 100% of the input power. These properties enlarge the application field of the ED, the first purpose of the ED being the screening of the plasma from the wall and the reduction of impurity contamination by increasing the transport in the ergodic layer.The ED in Tore Supra allows us to destroy magnetic surfaces in an "ergodic layer" of about 0.1 m radial width at the edge. Each point in this layer is connected to the wall by a field line performing 3 to 12 toroidal turns. The Tore Supra ED is composed of six coils equally spaced toroidally. The poloidal and toroidal extensions of the coils are A0 = 12O° and A0 = 11°, respectively. This set of coils produces a total radial field perturbation SB^D which is the sum of resonant components along field lines....

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