Theory for the power threshold of the L-H transition in tokamaks

Abstract

Flux-driven two-fluid simulations in a diverted tokamak exhibit a confinement transition when a critical heating threshold is exceeded, as in experimental observations of the L-H transition. The simulations show that electromagnetic drift-wave turbulence spontaneously generates a sheared $E\times B$ flow responsible for transport suppression in the edge. The toroidal-field asymmetry of the transition, here reproduced in simulations for the first time, arises from time-reversal symmetry breaking by finite collisionality, as demonstrated by a quasilinear calculation of the turbulent momentum flux. Scaling laws are analytically derived for the L-H power threshold in the high- and low-density branches, for the density minimum, and for the minimum power, all matching or surpassing the accuracy of existing empirical scalings.

arXiv comment6 pages, 4 figures