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 13 CH 4 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 2 /s. For such a wall injection location most of the CH 4 gets ionized, becomes C + , and is efficiently transported along the SOL as C-ions to deposit in the inner divertor. The CH 4 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 CH 4 were ionized at the separatrix.2

We report observations from the DIII-D tokamak indicating that boron (B) powder injection in tokamak plasmas improves wall conditions similarly to glow discharge boronization (GDB). Isotopically enriched B powder (B11 > 95%) was introduced gravitationally in a sequence of H-mode plasma discharges at rates up to ∼160 mg s−1 for durations up to 3 s. Boron injection to cumulative amounts ≤0.1 g appeared to improve wall conditions similarly to boronization, with indications of reduced wall fueling, reduced recycling at the outer strike point and reduced impurity content at breakdown. Post-mortem analysis of graphite samples exposed to far scrape-off layer plasma fluxes during boron injection confirm the formation of a B-C layer, with average surface composition B:C ∼ 1. The results suggest that injecting boron-rich powders in tokamak plasmas can effectively replenish boron films on carbon plasma facing components to improve wall conditions and extend the duration of the beneficial effects of GDB.

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Trace 13 CH 4 injection experiments into the main scrape-off layer of low density L-mode and high-density H-mode plasmas have been performed in the DIII-D tokamak[Luxon_NF02] to mimic the transport and deposition of carbon arising from a main chamber sputtering source. These experiments indicated entrainment of the injected carbon in plasma flow in the main SOL, and transport toward the inner divertor. Ex-situ surface analysis showed enhanced 13 C surface concentration at the corner formed by the divertor floor and the angled target plate of the inner divertor in L-mode; in H-mode, both at the corner and along the surface bounding the private flux region inboard of the outer strike point. Interpretative modeling was made consistent with these experimental results by imposing a parallel carbon ion flow in the main SOL toward the inner target, and a radial pinch toward the separatrix. Predictive modeling carried out to better understand the underlying plasma transport processes suggests that the deuterium flow in the main SOL is related to the degree of detachment of the inner divertor leg. These simulations show that carbon ions are entrained with the deuteron flow in the main SOL via frictional coupling, but higher charge state carbon ions may be suspended upstream of the inner M. Groth et al.Scrape-off layer transport and deposition studies in DIII-D GA-A25625, DRAFT 3 APS/DPP Invited ms 2 divertor X-point region due to balance of the friction force and the ion temperature gradient.

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