A MODEL FOR A SCRAPE-OFF-LAYER LOW-HIGH (L-H) MODE TRANSITION

Increasing the radial mode number has a stabilizing effect on the cond ucting-wall and curvature-driven interchange modes in a tokamak scrape -off layer (SOL), arising from the increased polarization response. Su ch an effect is naturally imposed as the SOL width is decreased, and f or a narrow-enough SOL, the stabilizing effect is stronger than the in crease in the instability drives. By combining a mixing-length estimat e for the thermal diffusivity with energy conservation and heat conduc tion equations and the condition of continuity of the heat flux at the separatrix, it is found that the resultant turbulence-transport syste m admits two solutions, one stable and one unstable, at different SOL widths; the inclusion of additional physics can add a second stable ro ot at lower width. These roots are plausibly identified with SOL behav ior in low (L) and high (H) modes. Particularly when a model is introd uced for finite-beta, finite-k(parallel to) effects on the modes, a po wer threshold for transition to the narrower root is obtained, suggest ing a possible L-H transition mechanism. The non-monotonic dependence of the turbulent heat flux vs SOL width and the possibility of multipl e solutions for the equilibrium SOL width are verified with nonlinear simulations. (C) 1995 American Institute of Physics.