Key Role of the Current Density Profile on Core Confinement and Transport in Tore Supra Plasmas: Electron Heat and Particle Transport

From both simulation and theoretical perspectives, the current density profile of magnetized plasma is expected to play an important role in turbulence. Optimization of both the safety factor q and the magnetic shear s can reduce turbulence, and therefore heat transport. Experimentally, external sources of heating and/or noninductive current drive have been used in Tore Supra to modify the current profile. In these experiments, electron heat diffusivity and turbulence level were found to be reduced when increasing s or reversing the q profile (i.e., negative s). As a consequence, confinement was improved. Core electron heat transport has been investigated. A critical threshold temperature gradient, above which turbulence strongly increases, has been experimentally determined. A parametric dependence study of this threshold pointed out the role of the ratio s/q, as expected by turbulence theory and simulations, thus explaining improved confinement regimes. Finally, thanks to the unique Tore Supra experimental conditions, the role of the q profile on turbulent particle transport was investigated. We have demonstrated that the electron density profile peaking is strongly governed by the q profile in low collisionality plasmas with dominant trapped electron modes.