Spectrally accurate global-local gyrokinetic simulations of turbulence in tokamak plasmas*

We develop a novel approach to gyrokinetics where multiple flux-tube simulations are coupled together in a way that consistently incorporates global profile variation while allowing the use of Fourier basis functions, thus retaining spectral accuracy. By doing so, the need for Dirichlet boundary conditions typically employed in global simulation, where fluctuations are zeroed at the radial boundaries, is obviated. This results in a smooth convergence to the local periodic limit as ρ∗ → 0. In addition, our scale-separated approach allows the use of transport-averaged sources and sinks, offering a more physically motivated alternative to the standard sources based on Krook-type operators. Having implemented this approach in the flux-tube code stella, we study the role of transport barriers and avalanche formation in the transition region between the quiescent core and the turbulent pedestal, as well as the efficacy of intrinsic momentum generation by radial profile variation. Finally, we investigate the role of zonal flows in mitigating the effects of turbulence spreading, and determine whether the Dimits shift can persist in a global setting.