The L-H transition in tokamaks: power threshold, density minimum and toroidal-field asymmetry
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| Main Authors: | , , , , , |
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| Format: | Preprint |
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2026
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| _version_ | 1866914530343256064 |
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| author | De Lucca, Brenno Ricci, Paolo Labit, Benoit Mancini, Davide Stenger, Louis Tecchiolli, Zeno |
| author_facet | De Lucca, Brenno Ricci, Paolo Labit, Benoit Mancini, Davide Stenger, Louis Tecchiolli, Zeno |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2605_00624 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | The L-H transition in tokamaks: power threshold, density minimum and toroidal-field asymmetry De Lucca, Brenno Ricci, Paolo Labit, Benoit Mancini, Davide Stenger, Louis Tecchiolli, Zeno Plasma Physics 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. |
| title | The L-H transition in tokamaks: power threshold, density minimum and toroidal-field asymmetry |
| topic | Plasma Physics |
| url | https://arxiv.org/abs/2605.00624 |