Physical trigger of the L-H transition
In plain words
When enough heating power is applied, a tokamak's edge turbulence switches off within a fraction of a millisecond and a steep insulating edge forms. More than forty years after its discovery, what causes the switch is not agreed.
Precise statement
In diverted tokamaks, identify the mechanism that suppresses edge turbulence at the L-H transition within about $10^{-4}$ s: turbulence-driven zonal flows, the ion-diamagnetic radial electric field well, X-point ion orbit loss, or their combination. An answer is a flux-driven first-principles model (gyrokinetic or two-fluid, realistic diverted geometry) that reproduces the measured threshold power and its dependences on density, toroidal field direction, isotope and divertor geometry.
What would settle it
A first-principles simulation that predicts the measured transition dynamics and threshold scalings on at least two devices without tuning.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Flux-driven 3D two-fluid simulations reported in 2026 reproduce threshold, density-minimum and field-direction trends (arXiv:2605.00624), not yet validated across devices.
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