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More than forty years after its discovery, what causes the switch is not agreed.","posed_since":"1982","precise":"In diverted tokamaks, identify the mechanism that suppresses edge turbulence at the L-H transition within about 1e-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.","problem_ref":null,"references":"","settled_by":"A first-principles simulation that predicts the measured transition dynamics and threshold scalings on at least two devices without tuning.","status_note":"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.","title":"Physical trigger of the L-H 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