Density profile peaking in low collisionality H-modes: comparison of Alcator C-Mod data to ASDEX Upgrade/JET scalings

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 n(e)//n(G), 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 coll isionalities, C-Mod attained peaking factors n(e) (0) / (n(e)) approaching 1.5 with T-i = T-e, 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.