In magnetically confined hot plasma turbulent transport is the dominating channel for radial density, energy, and momentum transport across the confining magnetic field. However, this transport may be self-regulated by spontaneously generated sheared flows ? often referred to as zonal flows ? capable of reducing the radial turbulent transport. The transport barriers mediated by these flows appear locally at specific radial positions, but most notably near the plasma edge - the so-called Edge Transport Barriers (ETB?s), which are instrumental in the rapid transition to an enhanced confinement state (the H-mode), with suppressed turbulent transport. The transition from low (L-mode) to the high (H-mode) confinement states in magnetically confined plasma is an outstanding issue in magnetic fusion research and is still not understood from first principles. Although the H-mode is routinely achieved in a multitude of magnetic confinement devices, since the first observation more than 30 years ago, the transition still lacks full theoretical explanation and predictive modeling. This is a high priority topic since ITER will rely on operation in H-mode to achieve the goal of ignition.

The self-consistent generation of large scale flows - zonal flows - by the rectification of small scale turbulent fluctuations is not only of relevance in magnetically confined plasmas, but is a generic feature of quasi-2D turbulent flows and also of high importance in, e.g., geophysical flows. The morphology of zonal flows and the basic mechanism for their generation will be discussed.

Particularly, we will present recent investigations of the generation of transport barriers in plasma turbulence and the role of these in the L-H transition by results from both low-dimensional models of the predator-prey type [1] and from first principle simulation applying a four-field, drift-fluid model [2].

[1] Dam, M., Br?ns, M., Rasmussen, J. Juul, Naulin, V., and Xu, G.S., Bifurcation analysis and dimension reduction of a predator-prey model for the L-H transition. Physics of Plasmas (2013) 20, 102302.
[2] Rasmussen, J. Juul, Nielsen, A.H., Madsen, J., Naulin, V, and Xu, G.S. Numerical modeling of the transition from low to high confinement in magnetically confined plasma. Plasma Phys. Control. Fusion (2016) 58, 014031.