[en] Predictive modeling of the closed Small Angle Slot (SAS) divertor with toroidal tungsten (W) rings in different poloidal locations is conducted to evaluate the impact of divertor closure on high-Z impurity sourcing and transport. These simulations utilize the DIVIMP code for W erosion, transport and deposition/redeposition, with background plasma solutions provided by SOLPS5.1. It is found that the level of W leakage is mostly determined by the divertor plasma conditions, the location of the W source and redeposition processes. High density plasmas in the SAS slot result in shorter ionization mean free paths than in the far-target region, resulting in higher redeposition rate. The induced friction force dominates in the near-target region, driving W impurities downstream towards the target surface. As a result, a W source at the strike point in the SAS divertor demonstrates lower net erosion and divertor leakage, despite higher gross erosion, compared to W sources in the far-target region. (topical issue article)

[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] Full text: A central concern in the design of burning plasma devices, such as ITER or DEMO, is that they represent extrapolations in dimensionless parameters from existing devices. The only means by which one can confidently predict their operation is via a well validated simulation based upon a first principles, or as close as possible, model. Accurate atomic physics data are essential to this task, not only for these predictive applications, but also for the validation of the model against the available data. In this talk, we present examples illustrating the sensitivity of such simulations to uncertainties in atomic physics data. The Gas Puff Imaging (GPI) diagnostic provides spatially and temporally resolved data ideal for validating plasma turbulence simulations. To date, those validation tests have been based on statistical characterizations of the turbulence, such as correlation lengths and times. In using those data, one must account for the spatial extent of the neutral gas cloud that is being “lit” by the plasma turbulence. Neutral transport simulations of that gas cloud have essentially no free parameters and are themselves amenable to quantitative validation tests. We will describe a couple of these, utilizing deuterium or helium gas puffs, and examine the sensitivity of the results to the atomic physics data for those systems. The use of tungsten in ITER as a principal plasma facing material has led to its introduction in a growing number of existing devices. Experiments conducted on those surfaces are frequently targeted at assessing tungsten erosion and its transport into the tokamak core. We will highlight two such recent investigations. In both cases, factor-of-a-few variations in the tungsten atomic physics data result in order of magnitude, or more, changes in the analysis results. Finally, the drive towards truly first principles simulations entails, at least in some regimes, a shift from fluid descriptions of the plasma to ones that are fully kinetic. The development of ever more capable computers and simulation algorithms has been steadily relaxing restrictions on the dimensionality and spatial coverage of such simulations. As they become more detailed and practical, additional atomic physics data will be required, e.g., doubly differential ionization cross sections. (author)

[en] We analyze a DIII-D tokamak experiment where two tungsten spots on the removable DiMES divertor probe were exposed to 12 s of attached plasma conditions, with moderate strike point temperature and density (~20 eV, ~4.5 x10"1"9 m"-"3), and 3% carbon impurity content. Both very small (1 mm diameter) and small (1 cm diameter) deposited samples were used for assessing gross and net tungsten sputtering erosion. The analysis uses a 3-D erosion/redeposition code package (REDEP/WBC), with input from a diagnostic-calibrated near-surface plasma code (OEDGE), and with focus on charge state resolved impinging carbon ion flux and energy. The tungsten surfaces are primarily sputtered by the carbon, in charge states +1 to +4. We predict high redeposition (~75%) of sputtered tungsten on the 1 cm spot–with consequent reduced net erosion–and this agrees well with post-exposure DiMES probe RBS analysis data. As a result, this study and recent related work is encouraging for erosion lifetime and non-contamination performance of tokamak reactor high-Z plasma facing components

[en] Modeling with OEDGE was carried out to assess the initial and long-term plasma contamination efficiency of Ar puffing from different divertor locations, i.e. the inner divertor, the outer divertor and the dome, in the EAST superconducting tokamak for typical ohmic plasma conditions. It was found that the initial Ar contamination efficiency is dependent on the local plasma conditions at the different gas puff locations. However, it quickly approaches a similar steady state value for Ar recycling efficiency >0.9. OEDGE modeling shows that the final equilibrium Ar contamination efficiency is significantly lower for the more closed lower divertor than that for the upper divertor. (paper)

[en] Previous studies (Carpentier et al 2011 J. Nucl. Mater. 415 S165-S169) carried out with the LIM code of the ITER first wall (FW) on beryllium (Be) erosion, re-deposition and tritium retention by co-deposition under steady-state burning plasma conditions have shown that, depending on input plasma parameter assumptions and sputtering yields, the erosion lifetime and fuel retention on some parts of the FW can be a serious concern. The importance of the issue is such that a benchmark of this previous work is sought and has been provided by the ERO code (Pitts et al 2011 J. Nucl. Mater. 415 S957-S964) simulations described in this paper. Provided that inputs to the codes are carefully matched, excellent agreement is found between the erosion/deposition profiles from both codes for a given ITER-shaped FW panel. Issues regarding the difficult problem of the correct treatment of Be sputtering are discussed in relation to the simulations. The possible influence of intrinsic Be impurity is investigated.

[en] Trace ¹³CH₄ injection experiments into the main scrape-off layer (SOL) of low density L-mode and high-density H-mode plasmas have been performed in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] 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 ¹³C 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 high surface concentration was found 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 divertor X-point region due to balance of the friction force and the ion temperature gradient force

[en] A substantial reduction of net compared to gross erosion of molybdenum and tungsten was observed in experiments conducted in the lower divertor of DIII-D using the divertor material evaluation system. Post-exposure net erosion of molybdenum and tungsten films was measured by Rutherford backscattering (RBS) yielding net erosion rates of 0.4–0.7 nm s⁻¹ for Mo and ∼0.14 nm s⁻¹ for W. Gross erosion was estimated using RBS on a 1 mm diameter sample, where re-deposition is negligible. Net erosion on a 1 cm diameter sample was reduced compared to gross erosion by factors of ∼2 for Mo and ∼3 for W. The experiment was modeled with the REDEP/WBC erosion/re-deposition code package coupled to the Ion Transport in Materials and Compounds—DYNamics mixed-material code, with plasma conditions supplied by the Onion skin modeling + Eirene + Divimp for edGE modeling code with input from divertor Langmuir probes. The code-calculated net/gross erosion rate ratios of 0.46 for Mo and 0.33 for W are in agreement with the experiment. (paper)

[en] Gross and net erosion rates of silicon from silicon carbide (SiC) coatings were measured in the divertor of DIII-D under well diagnosed reactor-relevant plasma conditions. Amorphous and crystalline SiC coatings on graphite with thickness of ∼80 nm and ∼250 μm, respectively, were exposed near an attached outer strike point of lower single null L-mode plasmas using the Divertor Material Evaluation System (DiMES). Plasma density and electron temperature near the center of the coatings were n _e ∼ 4 × 10¹⁹ m⁻³ and T _e ∼ 23 eV. Gross erosion of Si from all samples was measured spectroscopically using the Si II 636 nm line. It was found to be a factor of ∼4 higher for the amorphous coatings compared to the crystalline one. The thin amorphous coatings allowed measurements of net Si erosion with Rutherford backscattering. Net average Si erosion rate of ∼3 × 10¹⁶ cm⁻² s⁻¹ was measured on the amorphous coatings with toroidal extent of 1 mm, where, according to ERO code modeling, the local redeposition of Si was about 30%. Using this rate, spectroscopic measurements, measured D⁺ ion fluxes, and corrections from ERO-OEDGE modeling, effective SXB coefficient for the Si II 636 nm line of ∼52 and Si sputtering yield of ∼0.017 Si/D were calculated. Deuterium retention on SiC coatings was measured by 2.5 MeV ³He nuclear reaction analysis at 0.5–2.5 × 10¹⁷ atoms cm⁻², consistent with retention due to implantation into a surface undergoing net erosion. (topical issue article)