Scaling laws of density fluctuations at high-k on Tore Supra

International audienceAnomalous transport in tokamaks is generally attributed to turbulent fluctuations. Since a large variety of modes are potentially unstable, a wide range of short-scale fluctuations should be measured, with wavenumbers from kρ i ∼ 0.1 to kρ i 1. In the Tore Supra tokamak, a light scattering experiment has made possible fluctuation measurements in the medium-and high-k domains where a transition in the k-spectrum is observed: the fluctuation level decreases much faster than usual observations, typically with a power law S(k) ≡ k −6. A scan of the ion Larmor radius shows that the transition wavenumber scales with ρ i around kρ i ∼ 1.5. This transition indicates that a characteristic length scale should be involved to describe the fluctuation nonlinear dynamics in this range. The resulting very low level of fluctuations at high-k does not support a strong effect of turbulence driven by the electron temperature gradient. For this gyroradius scan, the characteristics of turbulence also exhibit a good matching with predictions from gyro-Bohm scaling: the typical scale length of turbulence scales with the ion Larmor radius, the typical timescales with a/c s ; the turbulence level also scales with ρ i , according to the mixing length rule