Scaling laws of density fluctuations at high-kon Tore Supra

Abstract

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 kρ i ∼ 0.1 to kρ i >> 1. On 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 non linear dynamics in this range. The resulting very low level of fluctuations at high k does not support a strong effect of turbulence driven by 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 time scales with a/c s ; the turbulence level also scales with ρ i , according to the mixing length rule.

Experimental turbulence studies for gyro-kinetic code validation using advanced microwave diagnostics

“…The fact that these spectra obtained by Doppler backscattering are consistent with earlier results obtained by laser scattering [21], where non-linear saturation effects are excluded to play a role, might indicate that O-mode reflectometry suffers less from non-linear saturation. But then it remains to be understood, why the shape of the O-mode spectra from Tore Supra is different from the one from the GENE simulation in Fig.…”

Spectral properties of electrostatic drift wave turbulence in the laboratory and the ionosphere

“…Spectral results from laboratory experiments. The data from Tore Supra identify a production and a coupling subrange as indicated (Hennequin et al, 2004). The results are supported by different discharges giving nearly similar power spectra.…”

Scale-selective turbulence reduction in H-mode plasmas in the TJ-II stellarator

“…First wavenumber spectra were measured in the mid-70's with electromagnetic wave (infrared and microwave) scattering techniques [18,19], followed up by measurements on a variety of machines [20][21][22][23]. Doppler reflectometry and far infrared scattering have been employed on the Tore Supra tokamak to identify turbulence scales and investigate the collisionality dependence of density turbulence spectra [24][25][26][27]. However, as of today, the scale-dependence of the turbulence reduction in H-mode plasmas has not been addressed thoroughly, although preliminary work has been presented [28][29][30].…”