Shear flow generation in magnetised plasmas

Mean shear flows are essential to control confinement in magnetised plasmas for fusion. In absence of external torque, and atomic physics processes, two types of processes are known to drive or damp flows: collisional drag and turbulent flow generation. While the theory of flow generation is well documented in axisymmetric magnetic configurations like tokamaks, it is much less understood when 3D effects are accounted for. In tokamaks, 3D effects may come from magnetic ripple due to the finite number of coils, or magnetic perturbations produced with external coils. In this talk, the physics at play, collisional or turbulent, will first be clarified in a unified framework. Analytical theory is compared to a set of comprehensive simulations based on drift-kinetic or gyrokinetic approaches able to treat both collisions and turbulence on an equal footing, thus allowing an assessment of competing processes. This methodology has recently been applied to understand the improvement of confinement with plasma current. When 3D effects are accounted for, it appears that collisional drag prevails over turbulence beyond a threshold in ripple amplitude that is expected to be reached in ITER. The talk will end with some considerations on the edge-core interplay regarding flow generation.