Effect of opacity on the density pedestal structure in DIII-D and Alcator C-Mod

Analysis of experimental data from DIII-D and C-Mod is used to study the role of opacity on the density pedestal structure. Dα measurements at the midplane on DIII-D indicate that we were able to push the ionization front further out by increasing the separatrix and SOL electron densities. We find that in pressure-record breaking EDA H-modes on C-Mod, adding more gas fueling does not result in an increase of the pedestal density, only an increase in the SOL density. In DIII-D, using gas puff modulations, we observe that when the outer strike point detaches, this strongly reduces the amplitude of the electron density response in the SOL and results in a larger amplitude inside the separatrix. Moreover, we observe the creation of an up-down asymmetry in the electron density with increased gas puff/opacity. Close to the X-point, up to ψN ~ 0.98, the electron density is larger than upstream in DIII-D. SOLPS modeling will be performed to get a better evaluation of the poloidal and radial ionization profiles as well as to better understand the origin of the up-down asymmetry of the electron density in DIII-D.