• Energy Research

A refined version of the Fundamenksi-Moulton 'free-streaming' model (FSM) for the dynamics of divertor density, particle flux, and heat flux during edge localized modes (ELMs) is presented. This model depends only on inter-ELM pedestal and divertor conditions and, crucially, incorporates particle recycling: a FSM with recycling model, FSRM. The effective particle recycling coefficient, Reff, is the only empirical fitting parameter in the FSRM. The predictions of the FSRM are systematically tested against a DIII-D database of ELM ion and energy fluence measurements and are shown to be consistent with the model across a wide range of pedestal and divertor conditions using a constant value of 0.96 for Reff . Predictions for W sputtering during ELMs are developed based on the FSRM. It is concluded that energetic free-streaming D+ ions and C6+ impurities are the dominant contributors to the intra-ELM gross erosion of W in the DIII-D divertor, i.e., recycling ions and impurities have relatively little impact on the total W sputtering rate. These calculations are also shown to be consistent with spectroscopic measurements of W gross erosion for three different pedestal conditions after incorporating the strong electron density dependence of the WI 400.8 nm ionizations/photon (S/XB) coefficient. Keywords: Tungsten sputtering, Tungsten Erosion, WI spectroscopy, Edge Localized Modes, Recycling

The tungsten erosion process for an H-mode discharge from the DIII-D Metal Rings Campaign is modeled using OEDGE and TRIM.SP. The OEDGE code is employed to calculate tungsten erosion between edge-localized modes (ELMs). Then a newly developed semi-analytical carbon–tungsten mixed material model based on TRIM.SP is used to simulate the intra-ELM tungsten gross erosion profiles. The tungsten erosion is found to be dominated by carbon, with different origin for carbon between ELMs and during ELMs. For inter-ELM, the tungsten is mainly eroded by locally redeposited low charge state carbon, while for intra-ELM, the C6+ originated from the pedestal region is found to dominate the tungsten erosion in the near separatrix region, whereas the locally redeposited low charge state C fluxes lead to a nonnegligible tungsten erosion in the outer SOL region. These results suggest that modeling of W erosion during ELMs needs to include impurity transport from the pedestal to the divertor during an ELM. In addition, for both inter- and intra-ELM simulation, a carbon coverage of 30% on the tungsten surface is needed to reproduce the measured erosion at the divertor target. Keywords: Plasma material interaction, Tungsten, Carbon, Erosion

Experiments using the V-shaped closed slot tungsten (W) coated SAS-VW divertor in DIII-D studied the effects of the BT direction on core contamination of eroded tungsten from a closed slot divertor configuration. Core W content is inferred using soft-X ray tomography (SXR) and vacuum ultraviolet spectroscopy (SPRED), while W divertor erosion is inferred from visible spectroscopy of W emission (400.9 nm) measured by in-slot filterscopes (filtered photo-multipliers). Post-mortem analysis from the campaign discovered tile misalignment leading to suspected pronounced leading-edge erosion in the unfavorable BT direction (ion B→×∇B→ drift away from divertor) likely not captured by diagnostics. However, empirical findings show up to ∼ 2-3x larger core contamination in the favorable BT direction even considering no additional W erosion from leading edges. A “source-to-core efficiency factor” is derived to estimate the effects of leading-edge erosion and compare W contamination for two pairs of H-mode discharges in opposite BT directions. While having differing absolute parameters, similar core impurity density gradients suggest comparable core impurity transport. These results show that favorable BT may have stronger source-to-core pathways for W impurities sourced from the outer divertor region. Possible explanations could include the effects of E→×B→ drifts on W transport in the scrape-off-layer (SOL) as well as previously determined fast SOL inner target directed flows in favorable BT.

A series of L-mode plasma discharges was performed in the DIII-D tokamak to assess the impact of outer strike point (OSP) position and toroidal magnetic field direction on erosion and core contamination potential of the recently-installed, tungsten-coated Small Angle Slot (SAS-VW) divertor. In one discharge, in-slot emission spectroscopy measured an < 48 % increase in the W gross erosion rate when the OSP was moved 3 cm outwards, away from the V-shaped vertex of the slot divertor. However, the effective W yield (erosion rate divided by the incident D flux) was, overall, insensitive to changes in OSP location. Consistently low estimates of the effective W yield based on measurements taken a few cm outwards from the vertex suggest potentially significant C surface contamination. No W emission signal was detected when orienting the toroidal magnetic field such that the ion B×∇B drift direction is pointed away from the X-point. However, measurements of W content in the plasma core for both toroidal magnetic field directions suggest the presence of additional, unmeasured sources of erosion. The difference in the measured core W density with OSP position is much greater than the difference in the measured erosion rates, which may suggest that the leakage of eroded impurities out of the divertor is governed primarily through the parallel ion temperature gradient and friction forces.

We report measurements of a+/− 5 mm toroidal variation of the outer strike point radial position using an array of three identical Langmuir probes distributed at 90° intervals around the torus (90°, 180°, 270°). The strike point radial location is determined from the profiles of floating potential (Vf) measured by the three 6 mm diameter domed Langmuir probes as the strike point is swept radially on a horizontal tile surface just outside of the upper small angle slot (SAS1) divertor. Based on the three probe measurements, the strike point variation is consistent with previous error field measurements by Schaffer [1,2] and estimates by Luxon [3] which indicated the strike point error could appear as an n = 1 radial variation of 4.5 mm at the outer mid plane and thus could be effectively described with a three point measurement. The results are also consistent with field line tracing calculations using the MAFOT code [4]. The small angle slot (SAS1) divertor performance is particularly sensitive to a misalignment with the divertor plasma since enhanced neutral confinement and recycling in the slot and distribution of neutrals along the slot surfaces are important for achieving divertor detachment at the lowest possible core plasma separatrix density. These strike point measurements are discussed with regard to the slot divertor alignment.

Surfacing Eroding Thermocouples (SETC) have been used to provide high spatial and temporal resolution heat flux measurement in DIII-D. SETCs were first tested in the lower divertor of DIII-D using the Divertor Material Evaluation System (DiMES), then an array of SETCS was permanently installed in the upper SAS-VW divertor for studying the heat flux mitigation in closed divertor geometry. SETCs proved that inducing divertor detachment with various methods is an effective way to reduce the peak heat flux at the divertor targets. In the investigated discharges the heat flux is reduced to less than 30% under detachment compared to attached conditions. A new method of using the combination of recessed SETC and flush SETC was demonstrated to be able to measure the heat flux from volumetric radiation and the charge exchange neutrals. It was observed that the magnitude of the heat flux from radiation and charge exchange neutrals increased during the process of detachment. While 30% of the total incident heat flux is attributable to the volumetric radiation and the charge exchange neutrals in the fully detached divertor condition with B×∇B drift into the divertor, it could reach more than 60% with B×∇B drift away from the divertor.

Dedicated experiments in DIII-D find that magnetic shaping and divertor target geometry significantly affect the divertor plasma conditions and divertor detachment process in the small-angle-slot (SAS) divertor. The compact SAS divertor in DIII-D provides a good testbed for understanding the effects of a tightly closed divertor on particle and power dissipation, and for application to core–edge integration solutions. A longer outer leg facilitates the achievement of divertor dissipation in the SAS divertor, while a shorter leg leads to higher electron temperatures near the divertor target plate and requires higher upstream densities to achieve the same level of divertor detachment. In addition, with the ion B × ∇B drift away from the SAS divertor and the outer strike point (OSP) near the outer corner, the target temperature is lower for a particular upstream density than with the OSP on a slanted or flat surface, leading to lower heat flux even when the particle flux remains similar. In contrast, with the ion B × ∇B drift into the SAS divertor, a strike point at the inner slanted surface exhibits a lower upstream density to achieve divertor detachment than a strike point either at the outer corner or the outer slanted target. Experimental results and SOLPS-ITER simulations with full drifts suggest the strong interplay between drift flows and the neutral distribution resulted from target shaping. Furthermore, in-slot gas puffing has been shown to achieve global divertor detachment with an onset density about 10 % lower than that using main-chamber gas puffing when the outer strike point is placed at the inner slanted surface. Corresponding modelling reveals that the local gas puffing enhances the neutral ionization which potentially facilitates the achievement of divertor dissipation. However, such improvement diminishes when the strike point is at the outer corner, which also indicates the geometric dependence on divertor performance in the SAS divertor. Even with different strike point locations, complete divertor detachment with very low particle and heat fluxes at the divertor targets and a high confinement core with normalized energy confinement factor H98 > 1.0 can be simultanesouly achieved with the SAS divertor with ion B × ∇B drift into SAS divertor, demonstrating the benefit of a closed divertor for exploration of core–edge integration.

Recent DIII-D experiments on Small Angle Slot (SAS) divertors have confirmed that a combination of divertor closure and target shaping can enhance cooling across the divertor target and increase energy dissipation, but with significant dependence on BT (toroidal magnetic field) direction. In these novel divertors, the roles of closure, target shaping, drifts, and scale lengths are all interconnected in optimizing dissipation, with the separatrix electron density neSEP being the key parameter associated with the level of dissipation/detachment. After modifying the original flat-targeted graphite SAS to include a V shape with a tungsten coating on the outer side of the divertor (SAS-VW), matched series of discharges were run to compare to detailed SOLPS-ITER modeling. Experimentally, when run as designed with the outer strike point at the slot vertex, SAS-VW requires nearly identical neSEP for detachment as the original SAS, with little difference in dissipation for the new geometry. This is in contrast to (1) earlier modeling predictions that a small change of the SAS geometry to a V shape should enhance dissipation at the same neSEP for magnetic configurations having better H-mode access (ion B × ∇B drift directed into the divertor), and (2) despite the achievement of significantly higher (2-7x) neutral pressures and compression in the SAS-VW slot. Comparisons of experimental density scans to the most recent SOLPS-ITER modeling with ExB drifts show reasonable agreement for dissipation/detachment onset when using separatrix density as the independent parameter. In order to help understand the discrepancy in modeled vs actual performance for the new configuration, additional measurements varying gas injection location and impurity injection were undertaken. In-slot D2 gas fueling is more effective (5–22 %) in promoting detachment, in accord with modeling. In-slot impurity injection (N2 or Ne) can yield 30 % lower core Zeff and 15 % less confinement degradation after detachment compared to main chamber puffing, as well as relatively lower tungsten leakage from the divertor. Modeling can also reproduce the improved detachment seen as the strike point moves inboard of the slot vertex.While we can explain the effects of the most important parameters causing energy dissipation in these slot divertors, it remains that many aspects of their behavior cannot be accurately modeled using state-of-art codes such as SOLPS-ITER. This is of concern for future model-driven designs utilizing similar V-shaped geometries.