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

TL;DR

This paper uses advanced simulations to explore the physical mechanisms behind the L-H transition in tokamaks, revealing how turbulence and asymmetry influence power thresholds and confinement.

Contribution

It introduces three-dimensional flux-driven two-fluid simulations that demonstrate the role of turbulence and symmetry breaking in the L-H transition.

Findings

Electromagnetic drift-wave turbulence generates sheared flows responsible for transport suppression.

Toroidal-field asymmetry results from time-reversal symmetry breaking by collisionality.

Derived scaling laws for the L-H power threshold match or improve upon empirical models.

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.