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International audience; The L to H mode transition occurs at a critical power which depends on ă various parameters, such as the magnetic field, the density, etc. ă Experimental evidence on various tokamaks (JET, ASDEX-Upgrade, DIII-D, ă Alcator C-Mod) points towards the existence of a critical temperature ă characterizing the transition. This criterion for the L-H transition is ă local and is therefore easier to be compared to theoretical approaches. ă In order to shed light on the mechanisms of the transition, simple ă theoretical ideas are used to derive a temperature threshold (T-th). ă They are based on the stabilization of the underlying turbulence by a ă mean radial electric field shear. The nature of the turbulence varies as ă the collisionality decreases, from resistive ballooning modes to ion ă temperature gradient and trapped electron modes. The obtained parametric ă dependencies of the derived T-th are tested versus magnetic field, ă density, effective charge. Various robust experimental observations are ă reproduced, in particular T-th increases with magnetic field B and ă increases with density below the density roll-over observed on the power ă threshold.

International audience; Spontaneous transport barrier generation at the edge of a magnetically confined plasma is reproduced in flux-driven three-dimensional fluid simulations of electrostatic turbulence. Here, the role on the radial electric field of collisional friction between trapped and passing particles is shown to be the key ingredient. Especially, accounting for the self-consistent and precise dependence of the friction term on the actual plasma temperature allows for the triggering of a transport barrier, provided the input power exceeds some threshold. In addition, the barrier is found to experience quasi-periodic relaxation events, reminiscent of Edge Localised Modes. These results put forward a possible key player, namely neoclassical physics via radial force balance, for the Low- to High-confinement regime transition observed in most of controlled fusion devices.