Observation of a high-density ion mode in tokamak microturbulence

For high-density Ohmic discharges in the TEXT (Texas Experimental) 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 this mode are identified. Onset of the ion feature occurs at plasma densities where a clear saturation is evident in the global energy-confinement time E. A possible connection between this experimentally observed ion mode and the theoretically predicted properties of instabilities driven by ion temperature gradient (i) is explored.