Strong asymmetries in the spectrum and magnitude of low frequency (ω ≪ ωci) density fluctuations in the Texas Experimental Tokamak (TEXT) are described. Microturbulence is investigated under a variety of plasma parameters and a comparison with mixing length theory is made. Broadband fluctuations (Δω/ω ≥ 1, Δk/k⊥ ≥ 1.5, propagating in the electron diamagnetic drift direction) are observed throughout the plasma cross-section, but they peak at the limiter and possess a strong up-down asymmetry which inverts with reversal of plasma current direction. The spatial distribution of these fluctuations can be dramatically altered by the introduction of a point limiter. Inside the magnetic axis, a large-magnitude quasi-coherent fluctuation (Δω/ω ≤ 0.2, Δk/k⊥ ≃ 0.7, propagating in the electron diamagnetic drift direction) is seen to dominate the ubiquitous broadband microturbulence. This type of fluctuation peaks on the midplane. For high density discharges, a distinct ion mode (density fluctuations propagating in the ion diamagnetic drift direction) is observed in the microturbulence spectra. Onset of the ion feature occurs at plasma densities where a clear saturation is evident in the global energy confinement time.
Energetics of turbulent transport processes in Tokamaks