Transport barrier onset and edge turbulence shortfall in fusion plasmas

A central problem in magnetized fusion plasmas is to understand whether the plasma edge sustains a turbulent state and transitions to improved confinement. The authors simulate the detailed organization of turbulence from the primitive flux-driven kinetic equations including plasma-solid interaction and identify basic mechanisms which allow to cure the oft-reported shortfall of turbulence power in the plasma edge and to describe spontaneous onset of a transport barrier. Magnetic confinement fusion offers the promise of sustainable and safe energy production on Earth. Advanced experimental scenarios exploit the fascinating yet uncommon ability of confined plasmas to bifurcate into states of enhanced performance upon application of additional free energy sources. Self-regulation of small-scale turbulent eddies is essential to accessing these improved regimes. However, after several decades, basic principles for these bifurcations are still largely debated and clarifications from first principles lacking. We show here, computed from the primitive kinetic equations, establishment of a state of improved confinement through self-organisation of plasma microturbulence. Our results highlight the critical role of the interface between plasma and material boundaries and demonstrate the importance of propagation of turbulence activity beyond regions of convective drive. These observations strongly suggest a paradigm shift where the magnetised plasma at the onset of enhanced performance self-organises into a globally critical state, 'nonlocally' controlled by fluxes of turbulence activity.