Global gyrofluid simulations of turbulence in tokamak plasmas

We develop a tokamak plasma turbulence simulation code based on a fluid model using the BOUT++ framework. The model corresponds to a global extension of the local gyrofluid model (Beer and Hammett, 1996 [11]), where external sources and neoclassical equilibrium flows are included but some higher-order kinetic effect terms are ignored for the global nonlinear simulation. Hyper-viscous damping terms are introduced to compensate for the stabilizing effect of the missing kinetic terms. With an appropriate hyper-viscosity, we obtain reasonable linear and nonlinear results on ion temperature gradient modes compared to global gyrokinetic simulations. We perform global flux-driven simulations with self-consistently evolving profiles with external heat sources/sinks. A spectral analysis of heat flux in flux-driven turbulence exhibits a broad 1/f (f is the frequency) spectrum observed in gyrokinetic simulations, indicating the prevalence of self-organized criticality-like heat avalanches. We also investigate the effect of an externally imposed vorticity source on turbulence suppression and transport barrier formation. It is shown that internal transport barrier can form even for a monotonic safety factor profile when a sufficiently strong vorticity source is applied.