Understanding L-H transition in tokamak fusion plasmas

This paper reviews the current state of understanding of the L-H transition phenomenon in tokamak plasmas with a focus on two central issues:(a) the mechanism for turbulence quick suppression at the L-H transition;(b) the mechanism for subsequent generation of sheared flow.We briefly review recent advances in the understanding of the fast suppression of edge turbulence across the L-H transition.We uncover a comprehensive physical picture of the L-H transition by piecing together a number of recent experimental observations and insights obtained from 1D and 2D simulation models.Different roles played by diamagnetic mean flow,neoclassical-driven mean flow,turbulence-driven mean flow,and turbulence-driven zonal flows are discussed and clarified.It is found that the L-H transition occurs spontaneously mediated by a shift in the radial wavenumber spectrum of edge turbulence,which provides a critical evidence for the theory of turbulence quench by the flow shear.Remaining questions and some key directions for future investigations are proposed.