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

Measurements of local electron density , electron temperature and ion temperature 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 standard deviations of , and under these conditions for a value of the normalized toroidal flux of 0.98 are, respectively, , keV, and 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.