Transport and turbulence in TORE SUPRA ohmic discharges

The mechanisms underlying the energy confinement behaviour in ohmic tokamak discharges are not yet understood. It is well known that the confinement time increases with the average density and saturates above a critical value of the density, but several explanations exist for this saturation: the onset of ionic turbulence, an impurity content effect on Drift Trapped Electron Modes, or ηe modes stabilization by the increase of β. The present study is an analysis of a set of ohmic discharges in TORE SUPRA with Ip=1.6 MA, B=4T, R=2.35 m and a=0.78 m, where the average density was increased from 0.9 to 4.2 1019 m-3. For these plasma parameters, the energy confinement time given by magnetic measurements saturates for ≥ 2.5 1019 m-3. It is emphasized here that the onset of ionic turbulence is unlikely in TORE SUPRA. This conclusion relies on transport analysis and turbulence measurements by CO2 laser scattering, whose results are presented in this paper. (author) 6 refs., 3 figs