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arXiv · 2605.00624

The L-H transition in tokamaks: power threshold, density minimum and toroidal-field asymmetry

Brenno De Lucca·Paolo Ricci·Benoit Labit·Davide Mancini·Louis Stenger·Zeno Tecchiolli
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Abstract

The physical mechanism underlying the L--H transition in tokamaks has remained an open problem for over forty years. We present three-dimensional flux-driven two-fluid simulations in a diverted geometry that exhibit a confinement transition at lower power in the favourable toroidal-field configuration. The simulations show that electromagnetic drift-wave turbulence spontaneously generates a sheared $\bm{E}\times \bm{B}$ flow responsible for transport suppression. The toroidal-field-direction asymmetry arises from time-reversal symmetry breaking by finite collisionality, as demonstrated by a quasilinear calculation of the turbulent momentum flux. First-principles scaling laws are derived for the L--H power threshold in both density branches, the density minimum, and the minimum power, all matching or surpassing existing empirical scalings.

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Brenno De Lucca, Paolo Ricci, Benoit Labit, Davide Mancini, Louis Stenger, Zeno Tecchiolli. 2026-05-01. The L-H transition in tokamaks: power threshold, density minimum and toroidal-field asymmetry. https://arxiv.org/abs/2605.00624

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