Scaling laws of density fluctuations at high-kon Tore Supra

Anomalous 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 k ρ i ∼ 0.1 to k ρ 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 k ρ 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.