Gyrofluid simulations of turbulence suppression in reversed-shear experiments on the Tokamak Fusion Test Reactor

TLDR

The gyrofluid model incorporates key kinetic effects for ions and electrons and matches linear kinetic theory. The study investigates the confinement improvement in reversed‑shear experiments on the Tokamak Fusion Test Reactor and discusses two positive‑feedback mechanisms for the sudden core confinement enhancement. Nonlinear gyrofluid simulations with a bounce‑averaged trapped‑electron fluid model are used to examine how the Shafranov shift suppresses the trapped‑electron mode and how radially sheared E×B flows suppress turbulence. Both reversed shear and the Shafranov shift reduce trapped‑electron precession drifts, lowering the dominant instability growth rate, and the study shows that Shafranov shift stabilization alone can trigger the transition.

Abstract

The confinement improvement in reversed-shear experiments on the Tokamak Fusion Test Reactor [Plasma Phys. Controlled Fusion 26, 11 (1984)] is investigated using nonlinear gyrofluid simulations including a bounce-averaged trapped electron fluid model. This model includes important kinetic effects for both ions and electrons, and agrees well with linear kinetic theory. Both reversed shear and the Shafranov shift reverse the precession drifts of a large fraction of the trapped electrons, which significantly reduces the growth rate of the trapped electron mode, found to be the dominant instability in the core. Two positive feedback transition mechanisms for the sudden improvement in core confinement are discussed: (1) Shafranov shift suppression of the trapped electron mode, and (2) turbulence suppression by radially sheared E×B flows. While both effects appear to be playing roles in the transition dynamics in most experiments, we show that Shafranov shift stabilization alone can cause a transition.