[en] Here, we present new findings for the oxidative erosion of codeposits on divertor tiles removed from the DIII-D tokamak (General Atomics, San Diego). We have extended the previously (Ochoukov R et al 2006 Phys. Scr. T 124 27) studied temperature and O₂ pressure ranges to 673 K (400 deg. C) and 79 kPa (600 Torr), respectively. We have also examined the dependence of the amount of deuterium removed by oxidation on the impurity concentration in the specimen

No abstract available

[en] 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, a major component of molecular influx due to chemical sputtering of graphite, from the tile surface into the plasma at a controlled rate through a porous graphite surface. Perturbation to local plasma is minimized, while also simulating the immediate environment of methane molecules released from a solid graphite surface. The release rate was chosen to be of the same order of magnitude as natural sputtering. Photon efficiencies of CH₄ for measured local plasma conditions are reported. The contribution of chemical versus physical sputtering to the source of C⁺ at the target is assessed through measurement of CII and CD/CH band emissions during release of CH₄ from the PPI, and due to intrinsic emission

[en] The effects of addition of ∼3% He"+ in simultaneous D–He irradiation at various D and He ion energies were studied for polycrystalline W at 300 and 500 K. Combinations of 250–750 eV/D"+ and 500–1000 eV/He"+ were used to vary the D and He ion range relative to each other. Total D and He retention were measured by thermal desorption spectroscopy up to 1473 K, and select specimens implanted at 500 K were analyzed by nuclear reaction analysis and electron recoil detection analysis. At both 300 and 500 K, D retention was reduced and trapping changed due to the addition of He"+; however, consistent with the literature, D and He diffused well beyond the ion ranges. Furthermore, varying the ion ranges had little effect on D retention, depth profile, and trapping. D diffusion into the bulk was reduced from far beyond 7 μm to less than 2 μm with the addition of He"+. (paper)

[en] It was discovered several decades ago that wall cleaning procedures, primarily to remove the light impurities C and O, were an important part of preparing fusion devices for plasma operation. Since that time, conditioning procedures making use of low-temperature plasmas in different gasses have become more elaborate, and have also found application in other fields. However, there are still many questions remaining with regard to the cleaning efficiency, and the most appropriate method for reducing impurity concentrations in the shortest time. In this paper, a short review is given, outlining the current status of removing oxygen from metal surfaces by the application of low-temperature plasmas.

[en] An experiment was conducted in DIII-D to examine carbon deposition when a secondary separatrix is near the wall. The magnetic configuration for this experiment was a biased double-null, similar to that foreseen for ITER. ¹³C methane was injected toroidally symmetrically near the secondary separatrix into ELMy H-mode deuterium plasmas. The resulting deposition of ¹³C was determined by nuclear reaction analysis. These results show that very little of the injected ¹³C was deposited at the primary separatrix, whereas a large fraction of injected ¹³C was deposited close to the point of injection near the secondary separatrix. Six of the tiles were put back into DIII-D, where they were baked at 350-360 °C for 2 h at ∼1 kPa in a 20% O₂/80% He gas mixture. Subsequent ion beam analysis of these tiles showed that about 21% of the ¹³C and 54% of the deuterium were removed by the bake.

[en] A probe has been designed, constructed, and successfully used to inject methane into the DIII-D lower divertor in a manner imitating natural release by chemical erosion. This porous plug injector (PPI) probe consists of a self-contained gas reservoir with an integrated pressure gauge and a 3 cm diameter porous surface through which gas is injected into the lower divertor of the tokamak. The probe is positioned flush with the divertor target surface by means of the divertor materials evaluation system. Two gas delivery systems were developed: in the first, gas flow is regulated by a remotely controlled microvalve and in the second by a fixed micro-orifice flow restrictor. Because of the large area of the porous surface through which gas is admitted, the injected hydrocarbon molecules see a local carbon surface (>90% carbon) similar to that seen by hydrocarbons being emitted by chemical sputtering from surrounding carbon tiles. The distributed gas source also reduces the disturbance to the local plasma while providing sufficient signal for spectroscopic detection. In situ spectroscopic measurements with the PPI in DIII-D allow the direct calibration of response for measured plasma conditions from a known influx of gas.