Global electromagnetic simulations of the outer core of an ASDEX Upgrade L-mode plasma

Abstract

The outer core of a low confinement discharge performed at the ASDEX Upgrade tokamak is investigated using both global and local nonlinear gyrokinetic simulations. Previous work [Told et al., Phys. Plasmas 20, 122312 (2013)] had shown that local gyrokinetic simulations agree reasonably well with experimental results in terms of transport levels, with minor discrepancies that can be resolved within the uncertainties of the experimental profile. In the present work, the analysis of the same discharge is extended to include global gradient-driven simulations with the GENE code, taking into account the plasma profiles from mid-radius up to close to the separatrix. It is shown that the mean fluxes obtained assuming the local approximation are in general agreement with results from global simulations. Moreover, both types of simulations exhibit large-scale avalanche-like events, both in the ion and electron heat fluxes, with similar basic properties. However, analyzing the statistics of the fluxes reveals that intermittency is more important in the global case. Furthermore, even when averaging over turbulent time scales, radial corrugations in the ion temperature profile are identified in the global simulation. These corrugations are at most of the order of 1%, but correspond to up to 10% variations in the gradient. Thus, while the results on the mean fluxes support the validity of the local approximation, the presence of corrugated structures may pose a challenge for the direct validation of local gyrokinetic simulations against experiments.