Observations on Alcator C-Mod confirm and extend previous measurements from ASDEX Upgrade (AUG) and JET, where density peaking in H-mode was seen to increase as collisionality was lowered. In the earlier work, peaking was also strongly correlated with ne/nG, the ratio of the plasma density to the density limit, and it was not possible to eliminate the latter parameter as the principal scaling variable. This led to divergent predictions for the density profile on ITER. The addition of C-Mod data allows the correlation to be broken, strengthening the case for collisionality as the controlling parameter. At the lowest collisionalities, C-Mod attained peaking factors ne(0)/⟨ne⟩ approaching 1.5 with Ti = Te, high edge neutral opacity and no core fuelling. Taken together, data from the three devices strongly suggest that density peaking factor on ITER in ELMy H-modes will be in the range 1.4–1.6.
Transport phenomena in Alcator C-Mod H-modes