DIVIMP modeling of the toroidally symmetrical injection of 13CH4 into the upper SOL of DIII-D

As part of a study of carbon-tritium co-deposition, we carried out an experiment on DIII-D involving a toroidally symmetric injection of 13CH4 at the top of a LSN discharge. A Monte Carlo code, DIVIMP-HC, which includes molecular breakup of hydrocarbons, was used to model the region near the puff. The interpretive analysis indicates a parallel flow in the SOL of M∥ ∼ 0.4 directed toward the inner divertor. The CH4 is ionized in the periphery of the SOL and so the particle confinement time, τc, is not high, only ∼5 ms, and about 4X lower than if the CH4 were ionized at the separatrix. For such a wall injection location, however, approximately 60-75% of the CH4 gets ionized to C+, C2+, etc., and is efficiently transported along the SOL to the inner divertor, trapping hydrogen by co-deposition there.