Variation of edge gradients with heat flux across L-H and H-L transitions in Alcator C-Mod

Local edge electron parameters are measured in the Alcator C-Mod tokamak, during discharges in which input power is continuously ramped up and down, leading to transitions from the low-confinement mode (L-mode) to high-confinement mode (H-mode) mode and back to L-mode. This allows measurement of the accessible portions of the non-monotonic flux-gradient relationship proposed by models of the H-mode as a critical transition. Results are consistent with a dependence of conductivity chi on the temperature gradient, where a very sharp decrease in chi occurs above a critical delT. Other possible flux-gradient relations are also examined, and the data are compared with theoretical formulations. The initial transient of pedestal energy in the first few ms after the L-H transition is also analysed and found to be consistent with a sudden decrease in chi across the pedestal region.