Interaction of large scale flow structures with gyrokinetic turbulence
Shear flows have a profound influence on turbulence-driven transport in tokamaks. The introduction of arbitrary initial flow profiles into the code ORB5 [Jolliet et al., Comput. Phys. Commun. 177, 409 (2007)] allows the convenient study of how flows on all length scales both influence transport levels and self-consistently evolve. A formulation is presented which preserves the canonical structure of the background particle distribution when either toroidal or poloidal flows are introduced. Turbulence suppression is possible above a certain shearing rate magnitude for homogeneous shear flows, and little evolution of the shearing rate is seen. However, when a flow with a zone boundary, where the shearing rate reverses at mid-radius, is introduced, the shear flow evolves substantially during the simulation. ExB shear flows with a zone boundary of a positive sign decay to a saturation amplitude, consistent with the well known saturation of turbulently generated zonal flows. Unlike the E B flow, the parallel flows relax diffusively.