Flux-driven simulations of turbulence collapse

Using three-dimensional nonlinear simulations of tokamak turbulence, we show that an edge transport barrier (ETB) forms naturally once input power exceeds a threshold value. Profiles, turbulence-driven flows, and neoclassical coefficients are evolved self-consistently. A slow power ramp-up simulation shows that ETB transition is triggered by the turbulence-driven flows via an intermediate phase which involves coherent oscillation of turbulence intensity and E x B flow shear. A novel observation of the evolution is that the turbulence collapses and the ETB transition begins when R-T > 1 at t = t(R) (R-T: normalized Reynolds power), while the conventional transition criterion (omega(ExB) > gamma(lin) where omega(ExB) denotes mean flow shear) is satisfied only after t = t(C) (>t(R)), when the mean flow shear grows due to positive feedback. (C) 2015 AIP Publishing LLC.