TURBULENT TRANSPORT IN MIXED STATES OF CONVECTIVE CELLS AND SHEARED FLOWS

Low-order mode coupling equations are used to describe recent computer simulations of resistive-g turbulent convection that show bifurcation s for the onset of steady and pulsating sheared mass flows. The three convective transport states are identified with the tokamak confinemen t regimes called low-mode (L-mode), high-mode (H-mode), and edge-local ized modes (ELMs). The first bifurcation (L-->H) and the second bifurc ation (H-->ELMs) conditions are derived analytically and compared with direct solutions of the 6-ODE mode coupling equations. First an exact expression is given for the energy transfer rate from the fluctuation s to the sheared mass how through the triplet velocity correlation fun ction. Then the time scale expansion required to derive the Markovian closure formula is given. Markovian closure formulas form the basis fo r the thermodynamic-like L-H models used in several recently proposed models. (C) 1996 American Institute of Physics.