Critical edge parameters for h-mode transition in DIII-D

Measurements in DIII-D of edge ion and electron temperatures (T-i and T-e) just prior to the transition to H-mode are presented. A fitting model based on a hyperbolic tangent function is used in the analysis. The edge temperatures are observed to increase during the L-phase with the application of auxiliary heating. The temperature rise is small if the H-mode power threshold is close to the ohmic power level in the absence of auxiliary heating and is large if the H-mode threshold is well above the ohmic power level. The edge temperatures just prier to the transition are approximately proportional to the toroidal magnetic field B-t, for the held either in the reversed or forward direction. However, for the reversed magnetic field, the temperatures are at least a factor of two higher than for the forward direction.