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Title: Physical trigger of the L-H transition
Plain: 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: 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.
Settled by: A first-principles simulation that predicts the measured transition dynamics and threshold scalings on at least two devices without tuning.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
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.
Posed since: 1982