The ITER divertor will be more collisional than divertors in existing tokamaks and the neutral modeling tools used for predictive ITER modeling need to be validated for similar divertor conditions in order to increase confidence in the results. The EIRENE neutral code has been applied to the Alcator C-Mod divertor, where collisional effects such as Lyman series photon trapping and neutral viscosity have been observed. The plasma solution used as input to EIRENE agrees well with the available diagnostic data for the divertor and was generated using an empirical Onion-Skin Method interpretive code that requires a large amount of experimental data as input. The calculated divertor pressure is 11 ± 3 mTorr, a factor ∼2 lower than the measured value of 25 ± 3 mTorr. The neutral solution is sensitive to photon trapping, viscosity, elastic collisions between plasma ions and molecules, and the amount of leakage through the outer divertor substructure.
