Abstract

Heat and particle transport across the nested magnetic surfaces of tokamak plasmas is mainly governed by turbulence. Several underlying instabilities coexist, but the most virulent bear similarity with the Rayleigh-Bénard instability in neutral fluids. In addition, drift waves are also present, the plasma analogue of Rossby waves in rotating atmospheric turbulence. Sheared flows efficiently contribute to their saturation. As will be shown, turbulence self-organization in magnetized plasmas tends to develop localized regions of shear flow layers with fronts propagating in between. This results in the local storage of turbulent energy into corrugations, leading to a staircase pattern of the pressure profile. The segregation of these complementary structures – fronts and layers – is believed to be key to the achieve good confinement properties. One of the issues deals with their possible role in triggering bifurcations towards the macroscopic transport barriers which are observed experimentally. By means of a reduced flux driven nonlinear model that captures “fluctuations – mean” interactions, we explore some of the characteristics of layering and transport. Layering dynamics turns out to depend on the turbulent regime. Flow layer nucleation will be shown to involve phase curvature dynamics and to require the positive feedback loop with the pressure profile. Implications will be discussed.