Breakup of transport barriers in plasmas with flow described by symplectic maps

Abstract

Transport barriers associated with the last KAM surfaces to break up are studied in the context of turbulent transport produced by a discrete spectrum of waves in two dimensions using a 2D symplectic map. It is based on the guiding center motion of test particles but finite Larmor radius (FLR) effects are included taking an average over the gyroperiod. Poloidal sheared flows are included which are responsible for the creation of the transport barrier. For large wave amplitudes, widespread chaos dominates the phase space interrupted only by the most stable KAM surfaces. The torus breakup depends on the values of the parameters of the map: wave amplitude A, flow velocity C and the Larmor radius ρ. For two types of sheared poloidal flows, having monotonic and non-monotonic shear, phase diagrams for barrier breakup are obtained by following the iterations of two points on either side of the barrier until the trajectories get mixed. The threshold that delimits the barrier breakup has the typical feature of a fractal curve for all phase diagrams.