Search for the ion temperature gradient mode in a tokamak plasma and comparison with theoretical predictions

Abstract

In recent experiments in DIII-D, comparison of specific turbulence characteristics with linear and nonlinear modeling has identified common features associated with the ion temperature gradient (ITG) mode. A low-frequency turbulence feature is observed in high-density saturated Ohmic confinement discharges, which is absent in low-density linear Ohmic confinement discharges. The feature is in a range of wavelength (k⊥ρs≈0.2–0.5) and the frequency expected for the ITG mode and onset of the feature is coincident with onset of confinement saturation. The density profile is significantly broader in the high-density discharge, a known destabilizing effect on the ITG mode. Gyrokinetic stability calculations of the growth rate of the most unstable drift ballooning mode show the ITG mode to be more unstable in the high-density discharges.