  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 $\sim \nu_{*i}^{-0.8}$.

Plasma Physics