Global gyrokinetic simulations of intrinsic rotation in ASDEX Upgrade Ohmic L-mode plasmas

TL;DR

This paper uses global gyrokinetic simulations to study intrinsic rotation in ASDEX Upgrade plasmas, revealing the importance of density profiles and kinetic electrons in shaping flow gradients consistent with experimental observations.

Contribution

It demonstrates that non-linear, global gyrokinetic simulations can accurately reproduce intrinsic flow profiles and highlights the critical role of density gradients and kinetic electrons in these flows.

Findings

Simulations agree with experimental intrinsic flow gradients.

Kinetic electron models produce hollow flow profiles.

Flow gradient sensitivity is strongly affected by density profile curvature.

Abstract

Non-linear, radially global, turbulence simulations of ASDEX Upgrade (AUG) plasmas are performed and the nonlinear generated intrinsic flow shows agreement with the intrinsic flow gradients measured in the core of Ohmic L-mode plasmas at nominal parameters. Simulations utilising the kinetic electron model show hollow intrinsic flow profiles as seen in a predominant number of experiments performed at similar plasma parameters. In addition, significantly larger flow gradients are seen than in a previous flux-tube analysis (Hornsby et al {\it Nucl. Fusion} (2017)). Adiabatic electron model simulations can show a flow profile with opposing sign in the gradient with respect to a kinetic electron simulation, implying a reversal in the sign of the residual stress due to kinetic electrons. The shaping of the intrinsic flow is strongly determined by the density gradient profile. The sensitivity of the residual stress to variations in density profile curvature is calculated and seen to be significantly stronger than to neoclassical flows (Hornsby et al {\it Nucl. Fusion} (2017)). This variation is strong enough on its own to explain the large variations in the intrinsic flow gradients seen in some AUG experiments. Analysis of the symmetry breaking properties of the turbulence shows that profile shearing is the dominant mechanism in producing a finite parallel wave-number, with turbulence gradient effects contributing a smaller portion of the parallel wave-vector.