L to H mode transition: parametric dependencies of the temperature threshold

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.