Neutral particle fueling at the midplane of DIII-D

Abstract Data from an array of eight tangential Dα viewchords, located at the outer midplane of DIII-D, are inverted to obtain atomic neutral density profiles across the separatrix and are compared to results of B2.5 and DEGAS simulations. In H-mode and (low density) L-mode plasma conditions, charge-exchange neutrals reflected and desorbed from the outer midplane region of the vacuum vessel wall can account for the midplane neutral density that is deduced from the Dα data, making it unnecessary to invoke main chamber ion recycling. The midplane measurements and DEGAS calculations are used to validate a slab-model of neutrals in the edge. In this model, cold neutrals from the wall fuel the edge and are attenuated by charge-exchange and ionization, and the results are in agreement with measured neutral profiles.