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

Measurements in DIII-D of edge ion and electron temperatures (Ti and Te) 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 prior to the transition are approximately proportional to the toroidal magnetic field Bt, for the field 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. (author)