Experimental Signatures of Critically Balanced Turbulence in MAST

TL;DR

This study uses BES measurements in MAST to demonstrate that ion-scale turbulence exhibits critical balance with multiple timescales being comparable, revealing new insights into turbulence dynamics in tokamaks.

Contribution

The paper provides experimental evidence of critically balanced turbulence in MAST, linking turbulence timescales to local equilibrium and deriving associated scalings.

Findings

Turbulence correlation time is comparable to drift, streaming, and magnetic drift times.

The nonlinear decorrelation time exceeds other times and scales with ion collision rate.

Zonal flows may control turbulence decorrelation, exceeding drift wave amplitudes.

Abstract

Beam Emission Spectroscopy (BES) measurements of ion-scale density fluctuations in the MAST tokamak are used to show that the turbulence correlation time, the drift time associated with ion temperature or density gradients, the particle (ion) streaming time along the magnetic field and the magnetic drift time are consistently comparable, suggesting a "critically balanced" turbulence determined by the local equilibrium. The resulting scalings of the poloidal and radial correlation lengths are derived and tested. The nonlinear time inferred from the density fluctuations is longer than the other times; its ratio to the correlation time scales as $\nu_{*i}^{-0.8\pm0.1}$, where $\nu_{*i}=$ ion collision rate/streaming rate. This is consistent with turbulent decorrelation being controlled by a zonal component, invisible to the BES, with an amplitude exceeding the drift waves' by $\sim \nu_{*i}^{-0.8}$.