Magnetic shear effects on confinement and electron heat transport in Tore Supra discharges with electron heating

Various steady state non-inductive plasmas, with strong electron heating and significant modification of the current density profile, have been routinely obtained in Tore Supra discharges with lower hybrid current drive (LHCD) and/or fast wave electron heating (FWEH) experiments. The dependence of the electron heat diffusivity χ e on the electron temperature gradient ∇T e , the magnetic shear s and the safety factor q is demonstrated. The increase of χ e with ∇T e indicates the existence of a critical temperature gradient. Moreover, the current density profile affects the global confinement and the local transport. The electron heat flux q e is found to be roughly proportional to q 2 . The effect of magnetic shear on χ e is studied in the improved confinement discharges obtained by modifying the current profile. When the magnetic shear increases in the confinement zone and/or vanishes in the plasma centre, χ e decreases. The effect of the current profile is also observed in the saturated ohmic regime. The results for χ e do not agree with the local transport models of Taroni et al. or Rebut-Lallia-Watkins (RLW) for all ranges of electron heating.