Turbulence simulations of transport barriers with toroidal velocity

The effect of a sheared toroidal velocity on a transport barrier is studied. This analysis is done by using three-dimensional global fluid simulations of electrostatic ion temperature gradient driven turbulence in tokamaks. The barrier is produced with a reversed magnetic shear. For a flat density profile, and at low collisionality, co-rotation leads to an outward motion of the barrier, whereas counter rotation leads to an inward displacement. However, the barrier displacement saturates when increasing the torque at fixed heat source. This saturation is attributed to the onset of Kelvin-Helmholtz modes. Also the central temperature is larger without external torque because the width of the transport barrier is wider. The consequence is that better confinement is obtained in absence of external torque. (C) 2002 American Institute of Physics.