Divimp Modeling of the Toroidally-symmetrical Injection of 13 Ch 4 into the Upper Sol of Diii-d

As part of a study to elucidate carbon-tritium co-deposition, an experiment was carried out on DIII-D involving a toroidally symmetric injection of CH4 at the top of the vessel (LSN divertor). The focus here is on interpretation of the region near the gas injection point using a Monte Carlo code, DIVIMP-HC, which models molecular breakup. The interpretive analysis indicates a parallel flow in the SOL of M|| ~ 0.4 directed toward the inner divertor and D⊥ ~ 0.3 m/s. For such a wall injection location most of the CH4 gets ionized, becomes C, and is efficiently transported along the SOL as C-ions to deposit in the inner divertor. The CH4 is ionized deeply in the SOL and so the ‘contamination efficiency’ of the confined plasma is not high: the confinement time is only ~ 5 ms, about 4X lower than if the CH4 were ionized at the separatrix.