Zero-dimensional model for the critical condition for the L-H transition in the flux-gradient relation

Abstract

In this paper, the physics of the L- to H-mode transition is discussed, focusing on the mechanism induced by the localized mean radial electric field. In addition to the neoclassical effect, an additional source of the electric field is introduced. The influence of turbulence-driven Reynolds stress is taken into account here. The case where the zonal flows are not excited is analysed. Even if the spontaneous zonal flows by turbulence cannot be excited, the bifurcation of the mean radial electric field can occur, if the electric field is strong enough eρpEr/Ti∼-1. Hard transition of the electric field takes place. The enhancement of electric field above this critical value can induce a bifurcation in the gradient-flux relation, i.e. the disappearance of the L-mode branch. The critical condition of the heat flux for the onset of transition is obtained.