Internal magnetic fluctuations and electron heat transport in the TORE SUPRA Tokamak. Observation by cross polarisation scattering

Magnetic fluctuations (radial size ∼ 5 mm) are measured by a cross polarisation scattering (CPS) diagnostic in TORE SUPRA. These fluctuations are investigated quantitatively in the ohmic and low confinement regimes over a wide range of plasma currents, densities and additional heating powers. Simultaneously, electron heat diffusivities expected from these fluctuations are compared to those obtained by profile analysis. A radial profile of the magnetic fluctuations in the gradient region ( 0.3 < r/a < 0.7) is established from these measurements. The magnetic fluctuation levels are found to increase towards the plasma edge, and this measurements. The magnetic fluctuation levels are found to increase towards the plasma edge, and this feature is compatible with that of electron heat diffusivity. A strong correlation between the measured magnetic turbulence and the local temperature gradient is observed during the additional heating. A local electron heat diffusivity induced by magnetic fluctuations is estimated using the non-collisional quasi-linear formula: χemag = πqRvth (δ Br / B)2. Both the order of magnitude and the parametric dependence of χemag show similarities with electron diffusivities determined by transport analysis. In particular, a threshold is observed for the dependence of fluctuation-induced heat fluxes on the local temperature gradient, which is close to the critical gradient observed for the measured heat fluxes. (author)