Evidence for reduction of the toroidal ITG instability in the transition from saturated to improved Ohmic confinement in the tokamak TEXTOR

In high density Ohmically heated discharges in the tokamak TEXTOR a transition from the saturated Ohmic confinement (SOC) to the improved Ohmic confinement (IOC) was observed triggered by a sudden reduction of the external gas flow. The SOC–IOC transition was investigated regarding the influence of the toroidal ITG instability driven by the ion temperature gradient (ITG). The ion temperature profiles were measured with high radial resolution by means of charge-exchange recombination spectroscopy (CXRS) with a high-energetic diagnostic hydrogen beam recently installed at TEXTOR. On the basis of the measured ion temperature distributions the ηi parameter (ratio of the density and ion temperature decay lengths) and the growth rate of the toroidal ITG instability were calculated. After the SOC–IOC transition ηi drops and lies in a noticeably smaller radial region over the threshold for the toroidal ITG. In consequence of it, the IOC regime is characterized by a clear reduction of the ITG growth rate γITG which was calculated including finite Larmor radius effects. The steepening of the plasma density profile after the decrease of the external gas flow is the main reason for the reduction of the ITG growth rate and the subsequent confinement transition to the IOC regime.