Edge turbulence during ergodic divertor operation in Tore Supra

We report new measurements of turbulence during ergodic divertor (ED) operation. At low density, some de-correlation of the turbulence is observed with a decrease of the long timescale structures. It is shown that the typical time involved is compatible with a de-correlation mechanism through radial separation of the B field lines by the ED, with an associated parallel length of the order of the distance between two modules of the ED. This observation reinforces the conclusion drawn in [1] and based on computer simulations. The situation changes when the density is increased: the turbulence level is found to increase. At the highest density, the structure of the turbulent signal is modified and the bursty behaviour suppressed by the ED at low density reappears. These observations lead to the conclusion that the turbulence measured at high density is not sensitive to the ED stabilization effect. This indicates that it could be carried by the ions.