EXPERIMENTAL-EVIDENCE FOR ION PRESSURE-GRADIENT DRIVEN TURBULENCE IN TEXT

For high density Ohmic discharges in the TEXT tokamak, a distinct ion mode (i.e. density fluctuations propagating in the ion diamagnetic drift direction) is observed in the microturbulence spectra. The magnitude and spectral characteristics of the mode are identified. A microinstability based transport model is used for the purpose of interpreting anomalous confinement properties. Onset of the ion feature occurs at plasma densities where a clear saturation is evident in the global energy confinement time τE. When the ion feature in the fluctuation spectra is strongest, agreement between predictions from the transport model and experimentally measured values of the global energy confinement time is realized if anomalous ion effects due to the ion pressure gradient driven (ηi) instability are included. By injecting pellets, a high density plasma is created in which the density profile is sharply peaked. Under these conditions the ion feature in the fluctuation spectra is suppressed. A possible connection between this experimentally observed ion mode and the theoretically predicted properties of the ηi instability is explored.