GYROFLUID SIMULATIONS OF TURBULENCE SUPPRESSION IN REVERSED-SHEAR EXPERIMENTS ON THE TOKAMAK FUSION TEST REACTOR

The confinement improvement in reversed-shear experiments on the Tokam ak Fusion Test Reactor [Plasma Phys. Controlled Fusion 26, 11 (1984)] is investigated using nonlinear gyrofluid simulations including a boun ce-averaged trapped electron fluid model. This model includes importan t kinetic effects for both ions and electrons, and agrees well with li near kinetic theory. Both reversed shear and the Shafranov shift rever se the precession drifts of a large fraction of the trapped electrons, which significantly reduces the growth rate of the trapped electron m ode, found to be the dominant instability in the core. Two positive fe edback transition mechanisms for the sudden improvement in core confin ement are discussed: (1) Shafranov shift suppression of the trapped el ectron mode, and (2) turbulence suppression by radially sheared E x B flows. While both effects appear to be playing roles in the transition dynamics in most experiments, we show that Shafranov shift stabilizat ion alone can cause a transition. (C) 1997 American Institute of Physi cs.