Comparison of turbulence and transport characteristics between global flux-driven and gradient-driven gyrofluid simulations in tokamak plasmas

Abstract

We perform global nonlinear simulations of ion temperature gradient (ITG) turbulence in a concentric circular tokamak geometry with the gyrofluid code GF2-BOUT++ and compare the dynamics between flux-driven and gradient-driven global simulations. Both approaches present broad 1/f-type power spectra for the turbulent ion heat flux, indicative of self-organized criticality (SOC)-like transport avalanches. Despite the similar levels of the total ion heat fluxes, the two approaches yield markedly different normalized ITG length (\(R_0/L_{T_i}\)). This discrepancy is attributed to differences in turbulence characteristics and low-frequency \(\varvec{E}\times \varvec{B}\) flow shear. In the flux-driven simulations, mesoscale heat transport is prominent, and the system self-organizes toward a marginally stable state through enhanced mesoscale avalanche-like transport, leading to a lower average temperature gradient than in the gradient-driven case. Strong and persistent \(\varvec{E}\times \varvec{B}\) shear in the gradient-driven simulations sustains the same heat flux at higher \(R_0/L_{T_i}\) than in the flux-driven ones. The combined effects of these mechanisms cause \(R_0/L_{T_i}\) in the flux-driven case to remain near the critical value of approximately 6, which is comparable to full-f gyrokinetic results, and lead to stronger profile stiffness compared to the gradient-driven case.