Fusion Science and Technology / Volume 56 / Number 3 / October 2009 / Pages 1417-1431
Technical Papers / Tore Supra Special Issue / dx.doi.org/10.13182/FST09-A9185
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.