Dependence of H-mode power threshold on global and local edge parameters

Measurements of local electron density ne, electron temperature Te and ion temperature Ti have been made at the very edge of the plasma just prior to the transition into H-mode for four different single parameter scans in the DIII-D tokamak. The means and standards deviations of ne, Te and Ti under these conditions for a value of the normalized toroidal flux of 0.98 are, respectively, (1.5 ± 0.7) x 1019 m-3, 0.051 ± 0.016 keV, and 0.14 ± 0.03 keV. The threshold condition for the transition is more sensitive to temperature than to density. The data indicate that the dependence is not as simple as a requirement for a fixed value of the ion collisionality. (author)