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Abstract: Use of carbon in tokamaks leads to a major tritium retention issue due to co-deposition. To investigate this process a low power L-mode experiment was performed on DIII-D in which 13CH4 was puffed into the main vessel through the toroidally-symmetric pumping plenum at the top of lower single-null discharges. Subsequently, the 13C content of tiles taken from the vessel wall was measured. The interpretive OEDGE code was used to model the results. It was found that the 13C deposition pattern is controlled by: (a) source strength of 13C+, (b) Δr s, radial location of the 13C+ source, (c) D ⊥, (d) M ∥, the scrape-off layer parallel Mach number. Best agreement was found for (a) ∼50% conversion efficiency 13CH4 → 13C+, (b) Δr s, ∼+3.5cm (outboard of separatrix) near 13CH4 injection location, (c) D ⊥ ∼0.3m2/s, (d) M ∥ ∼0.4 toward inside. [Copyright &y& Elsevier]

Abstract: The radial particle flux in the scrape-off-layer (SOL) during ELMing H-mode is examined in DIII-D as a function of density. The global radial particle flux in the outboard far SOL is determined by a window frame technique. Between ELMs, the outboard far SOL particle flux increases strongly with density and remains similar to the particle flux across the separatrix as estimated by the pedestal density and temperature gradients. At low density the steep density gradient of the pedestal extends up to 2cm outside the separatrix, while at high density the density gradient flattens just outside the separatrix. During ELMs the far SOL particle flux becomes localized to the outboard midplane and the assumptions for the window frame analysis break down. Implications for scaling of outboard main chamber wall particle flux and pedestal fueling are explored. [Copyright &y& Elsevier]

Abstract: Use of carbon in tokamaks leads to a serious tritium retention issue due to co-deposition. To further investigate the processes involved, a detached ELMy H-mode (6.5MW NBI) experiment was performed on DIII-D in which 13CH4 was puffed into the main vessel through the toroidaly symmetric pumping plenum at the top of lower single-null discharges. Subsequently, the 13C content of tiles taken from the vessel wall was measured. The interpretive OEDGE code was used to model the results. The 13C deposition pattern could be reproduced, in general shape and magnitude, by assuming in the code the existence of a parallel flow and a radial pinch in the scrape-off layer. Parallel flows of Mach ∼0.3 toward the inner divertor and a radial pinch ∼10–20m/s (+R-direction) were found to yield 13C deposition comparable to the experiment. [Copyright &y& Elsevier]

Abstract: Experiments are described which examine the transport and deposition of carbon entering the main plasma scrape-off layer in DIII-D. 13CH4 was injected from a toroidally symmetric source into the crown of lower single-null partially detached ELMy H-mode plasmas. 13C deposition, mapped by nuclear reaction analysis of tiles, was high at the inner divertor but absent at the outer divertor, as found previously for low-density L-mode plasmas. This asymmetry indicates that ionized carbon is swept towards the inner divertor by a fast flow in the scrape-off layer. In the private flux region between inner and outer strike points, carbon deposition was low for L-mode but high for the H-mode plasmas. OEDGE modeling reproduces observed deposition patterns and indicates that neutral carbon dominates deposition in the divertor from detached H-mode plasmas. [Copyright &y& Elsevier]

Abstract: A self-contained gas injection system for the Divertor Material Evaluation System (DiMES) on DIII-D has been employed for in situ study of chemical erosion in the tokamak divertor environment. The porous plug injector (PPI) releases methane into the plasma at a controlled rate through a porous graphite surface flush to a tile. In this way, the perturbation to the local plasma can be minimized, while also simulating the immediate environment of methane molecules released from a solid graphite surface. Photon efficiencies of CH4 for measured local plasma conditions are reported. The contribution of chemical vs physical sputtering to the source of C+ at the target can, in principle, be assessed through measurement of CII and CD/CH band emissions during release of CH4 from the PPI, and due to intrinsic emission. These first results from this new experimental tool demonstrate the potential for the PPI to provide definitive results in future applications in DIII-D and indicate the improvements required to obtain firm quantitative conclusions. [Copyright &y& Elsevier]

Abstract: Far scrape-off layer (SOL) plasma parameters in DIII-D depend strongly on the discharge density and confinement regime. In L-mode, cross-field transport increases with the average discharge density and elevates the far SOL density, thus increasing plasma-wall contact. Far SOL density near the low field side (LFS) of the main chamber wall also increases with decreasing plasma current and with decreasing outer wall gap. In H-mode, between edge localized modes (ELMs), plasma-wall contact is weaker than in L-mode. During ELMs plasma fluxes to the LFS wall increase to, or above the L-mode levels. A large fraction of the net cross-field fluxes is convected through the SOL by large amplitude intermittent transport events. In high-density L-mode and during ELMs in H-mode, intermittent events propagate all the way to the LFS wall and may cause sputtering. [Copyright &y& Elsevier]

Abstract: For some divertor aspects, such as detached plasmas or the private flux zone, it is not clear that the controlling physics has been fully identified. This is a particular concern when the details of the plasma are likely to be important in modeling the problem – for example, modeling co-deposition in detached inner divertors. An empirical method of ‘reconstructing’ the plasma based on direct experimental measurements may be useful in such situations. It is shown that a detached plasma in the outer divertor leg of DIII-D can be reconstructed reasonably well using spectroscopic and probe data as input to a simple onion-skin model and the Monte Carlo hydrogenic code, EIRENE. The calculated 2D distributions of n e and T e in the detached divertor were compared with direct measurements from the divertor Thomson scattering system, a diagnostic capability unique to DIII-D. [Copyright &y& Elsevier]

Abstract: 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, τ c, is not high, only ∼5ms, 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. [Copyright &y& Elsevier]

Abstract: 13CH4 was injected with a toroidally-symmetric gas system into 22 identical lower-single-null L-mode discharges on DIII-D. The injection level was adjusted so that it did not significantly perturb the core or divertor plasmas, with a duration of ∼3s on each shot, for a total of ∼300TL of injected particles. The plasma shape remained very constant; the divertor strike points were controlled to ∼1cm at the divertor plate. At the beginning of the subsequent machine vent, 29 carbon tiles were removed for nuclear reaction analysis of 13C content to determine regions of carbon deposition. It was found that only the tiles inboard of the inner strike point had appreciable 13C above background. Visible spectroscopy measurements of the carbon injection and comparisons with modeling are consistent with carbon transport by means of scrape-off layer flow. [Copyright &y& Elsevier]