Effects of high density peaking and high collisionality on the stabilization of the electrostatic turbulence in the Frascati Tokamak Upgrade

Abstract

Experiments carried out on the Frascati Tokamak Upgrade (FTU) [B. Angelini et al., Nucl. Fusion 43, 1632 (2003)] with the injection of deuterium pellets in ohmic, high density plasmas show that a remarkable enhancement of the energy confinement time is achieved when the pellet penetrates deeply in the plasma core. Here a study of pellet effects on the electrostatic turbulence is carried out with the gyrokinetic codes Kinezero [C. Bourdelle et al., Nucl. Fusion 42, 892 (2002)] and GS2 [M. Kotschenreuther, G. Rewoldt, and M. W. Tang, Comput. Phys. Commun. 88, 128 (1995)]. The analysis shows that the collisionality of FTU plasmas is high enough to detrap the electrons. The low wavelength unstable modes remaining are then pure ion temperature gradient modes that are stabilized by more peaked density profiles. In the postpellet phase the density peaking stabilizes also strongly the high wavelength electron temperature gradient modes.