DiMES PMI research at DIII-D in support of ITER and beyond

doi:10.1016/j.fusengdes.2017.03.007

Fusion Engineering and Design

Volume 124, November 2017, Pages 196-201

https://doi.org/10.1016/j.fusengdes.2017.03.007 Get rights and content

Highlights

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Net erosion of high-Z PFC materials in DIII-D divertor is reduced by short scale re-deposition.
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Positive electrical biasing locally suppresses erosion of high-Z PFC materials.
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Local injection of methane gas suppressed Mo erosion by forming in-situ carbon coating.
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Measurements of Mo and W erosion on DiMES samples are well reproduced by ERO-OEDGE modeling.
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Migration of W in and out of DIII-D divertor was studied using DiMES and MiMES collector probes.

Abstract

An overview of recent Plasma-Material Interactions (PMI) research at the DIII-D tokamak using the Divertor Material Evaluation System (DiMES) is presented. The DiMES manipulator allows for exposure of material samples in the lower divertor of DIII-D under well-diagnosed ITER-relevant plasma conditions. Plasma parameters during the exposures are characterized by an extensive diagnostic suite including a number of spectroscopic diagnostics, Langmuir probes, IR imaging, and Divertor Thomson Scattering. Post-mortem measurements of net erosion/deposition on the samples are done by Ion Beam Analysis, and results are modelled by the ERO and REDEP/WBC codes with plasma background reproduced by OEDGE/DIVIMP modelling based on experimental inputs. This article highlights experiments studying sputtering erosion, re-deposition and migration of high-Z elements, mostly tungsten and molybdenum, as well as some alternative materials. Results are generally encouraging for use of high-Z PFCs in ITER and beyond, showing high redeposition and reduced net sputter erosion. Two methods of high-Z PFC surface erosion control, with (i) external electrical biasing and (ii) local gas injection, are also discussed. These techniques may find applications in the future devices.

Introduction

Control of plasma-material interactions (PMI) is one of the main challenges facing ITER and future magnetic fusion devices. The Divertor Material Evaluation System (DiMES) [1] has been the main diagnostic for PMI studies in the lower divertor of DIII-D tokamak [2] since 1992. The Midplane Material Evaluation Station (MiMES) [3], [4] was added in 2007 to study PMI at the outer main chamber wall. Since plasma-facing components (PFCs) in DIII-D are made of graphite, a lot of effort in 1990s and 2000s was devoted to studies of erosion and re-deposition of carbon (see, e.g. [4] and references therein). When the decision was made to go with an all-W divertor in ITER, the DiMES program shifted focus to strengthen studies of PMI with metallic PFCs in support of ITER. Since background levels of metallic impurities, particularly those of high-Z elements like tungsten (W) and molybdenum (Mo), are typically very low in DIII-D, it is possible to study erosion/re-deposition and migration of these elements without background contamination. Generally, erosion of high-Z materials in DIII-D is significantly due to low-Z ions, such as carbon (C) and nitrogen (N), similar to the earlier report from ASDEX Upgrade [5]. Here we provide an overview of DiMES experiments and analysis accomplished in the last four years.

Section snippets

Experimental arrangement on DIII-D

Fig. 1 shows a poloidal cross-section of DIII-D with a typical Lower Single Null (LSN) last closed flux surface (LCFS) and the outer strike point (OSP) on top of DiMES. Poloidal locations of DiMES, MiMES and the main diagnostics used in the experiments described below are also shown. DiMES is imaged by the DiMES TV filtered camera (employing commercial analog or digital CMOS, or specialized radiation-hardened CMOS detectors over the years) and a high resolution Multi-chord Divertor Spectrometer

Sample preparation and analysis

Fig. 2 shows top views of a few DiMES heads used in recent experiments. Samples employed for studies of erosion/re-deposition feature thin films of metals deposited either on silicon (Si) substrate (some over a C inter-layer as in Fig. 2(a)) or on polished graphite (Fig. 2(b and c)). Metallic coatings are usually deposited in a magnetron sputter deposition system and pre-characterized by Rutherford Backscattering (RBS) at Sandia National Laboratories (SNL Albuquerque). By comparing pre- and

Reduction of net erosion of Mo and W by short-scale redeposition

A total of five experiments have been performed in reproducible LSN L-mode discharges with Mo and W coatings [6], [7], [8]. All samples featured 1 cm diameter Mo or W films 15–40 nm thick deposited on a Si substrate. Net erosion and redeposition of Mo and W were measured by RBS. In the first two experiments, samples with 1 cm diameter Si disk coated with Mo were used and gross erosion of Mo was measured spectroscopically by monitoring the Mo I, 386 nm emission line and applying an S/XB

Summary

Significant progress in understanding of erosion/re-deposition and migration of high-Z PFC materials in a carbon environment was made at DIII-D in the last 4 years. Carefully designed, well-diagnosed experiments were used to benchmark ITER-relevant modelling. One important conclusion is that high-Z PFC materials appear to function well, showing high sputter redeposition and low net erosion. We will continue supporting ITER PMI needs in collaboration with ITPA and ITER Organization.

Acknowledgments

This work was supported in part by the US DOE under DE-FG02-07ER54917, DE-AC05-06OR23100, DE AC05-00OR22725, DE-FC02-04ER54698, DE-AC52-07NA27344, DE-AC04-94AL85000, GA-DE-SC0008698, and Collaborative Research Opportunities Grant from the National Sciences and Engineering Research Council of Canada. 1DIII-D1 data shown in this paper can be obtained in digital format by following the links at https://fusion.gat.com/global/D3D_DMP.

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Cited by (23)

Investigation of W-SiC compositionally graded films as a divertor material
2024, Journal of Nuclear Materials
W-SiC composite material is a promising plasma-facing material candidate alternative to pure W due to the low neutron activation, low impurity radiation, and low tritium diffusivity of SiC while leveraging the high erosion resistance of the W armor. Additionally, W and SiC have high thermomechanical compatibility given their similar thermal expansion rates. The present study addresses the synthesis and performance of compositionally graded W-SiC films fabricated by pulsed-DC magnetron sputtering. Compositional gradients were characterized using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS), and crystallographic information was obtained using electron diffraction and X-ray diffraction (XRD). Samples were exposed to L-mode deuterium plasma discharges in the DIII-D tokamak using the Divertor Material Evaluation System (DiMES). Post-mortem characterizations were performed using scanning electron microscopy (SEM) and XRD. Electron diffraction and XRD showed that the compositionally graded W-SiC films were composed of polycrystalline W and amorphous SiC with amorphous W+SiC interlayers. No macroscopic delamination or microstructural changes were observed under mild exposure conditions. This study serves as a preliminary examination of W-SiC compositionally graded composites as a potential candidate divertor material in future tokamak devices.
Collector probe analysis of tungsten transport to the far-SOL from the DIII-D SAS-VW divertor experiment
2024, Nuclear Materials and Energy
Experimental results from the 2022 tungsten (W)-coated Small Angle Slot (SAS-VW) divertor campaign at DIII-D coupled with interpretive 3DLIM modelling show opposing trends for core impurity content when compared to impurity deposition on far-Scrape Off Layer (SOL) Collector Probes (CPs) with increasing main ion density. SAS-VW is a closed, W-coated divertor designed to more easily facilitate divertor detachment while reducing impurity leakage. An experiment performed a series of upper-single-null L-mode discharges in each toroidal magnetic field (B_T) direction, with increasing main ion density (line-averaged density = 3.15–4.35e19 m⁻³) that approaches and slightly exceeds the divertor detachment threshold. The results indicate: a) increased radial W transport with decreasing peak $T_{e, t}^{LP}$ ; and b) negligible change in W content in the far-SOL at the outer mid-plane with the onset of divertor detachment.
Preliminary W deposition measurements using double-sided, graphite CPs inserted at the Midplane Materials Evaluation System (MiMES) reveal a 75% decrease over the density scan when operating in the unfavorable (ion B×∇B out of the divertor) B_T direction. In contrast, soft X-ray (SXR) radiation from the same discharges is used as a proxy for W core contamination, showing core W content that increases by 77% with increasing line-averaged density. Similar L-mode discharges conducted in the favorable B_T direction result in significantly less deposition on CPs.
Using an interpretive modeling workflow following Zamperini 2022 [1] for assessing the transport of W sputtered from the SAS-VW divertor, the analysis suggests that W migration to the main chamber surfaces during the campaign may also contribute to far-SOL deposition.
<sup>13</sup>C surface characterization of midplane and crown collector probes on DIII-D
2023, Nuclear Materials and Energy
A dual collector probe system has been implemented on DIII-D for scrape-off-layer (SOL) impurity transport studies. These experiments injected isotopically enriched methane (¹³CD₄) and sampled the impurities from this extrinsic, primary source with graphite collector probes at the outboard midplane and crown of upper single null L-mode plasmas. Using a stable isotopic mixing model, results suggest that ¹³C from methane injections prior to these experiments has built up on the walls of DIII-D to act as a secondary, intrinsic source of enriched ¹³C to the collector probes. This secondary source accounts for nearly 60 % of the deposits on the midplane collector probes and nearly 90 % of the deposition on the collector probes in the crown. These results lay the foundation for future impurity transport models and suggest that further simulation of impurity transport during the methane injection experiments will require two sources of enriched impurities in order to accurately model the SOL impurity profiles of ¹³C.
3D modeling of boron transport in DIII-D L-mode wall conditioning experiments
2021, Nuclear Materials and Energy
DIII-D L-mode experiments with local boron powder injection for real-time wall conditioning have been interpreted for the first time with the 3D plasma edge transport Monte Carlo code EMC3-EIRENE. Local B sourcing in plasma scenarios with upstream densities $1.5 \cdot 1 0^{19}$ m⁻³ and 2.2 MW heating results in a non-axisymmetric B distribution in the scrape-off layer (SOL) and on the divertor. The SOL frictional flows at high plasma density cause a strong inboard drag of injected impurities ( $\approx 90 %$ ), while lower background plasma densities tend to result in a more uniform distribution. The thermal forces prevent B deposition in the near SOL while the frictional force causes B fluxes to cover the divertor plasma-facing components in a region 7–10 cm beyond the strike line. Radiative dissipation occurs for B influxes above $1 \cdot 1 0^{20}$ s⁻¹ and causes a moderate, non-axisymmetric reduction of the far SOL divertor heat fluxes. A comparison of top and midplane B injection shows no substantial difference in inboard vs. outboard asymmetries of the B distribution. On the other hand, erosion or recycling at the strike line may distribute the boron more uniformly in the SOL.
Use of isotopic tungsten tracers and a stable-isotope-mixing model to characterize divertor source location in the DIII-D metal rings campaign
2019, Nuclear Materials and Energy
Citation Excerpt :
Each array has ∼ 0.4 m2 of exposed W surface area within the DIII-D vessel, which has a total surface area of ∼70 m2. The experimental setup was fully instrumented including source spectroscopy, insert-embedded Langmuir probes and thermocouples [13,14], but particular to this work, the setup employed the use of an upstream, dual-facing, collector probe (CP) in the far-SOL [15], also labeled in Fig. 1a. For this application the CP was made fully of fine-grained graphite. The CP included a graphite housing with a smaller diameter removable graphite insert that acted as the sampling piece.
A basic stable isotope mixing model (bSIMM) is presented that enables the first-time use of multiple isotopic tungsten (W) tracers in a fusion device. DIII-D installed two toroidally symmetric, but poloidally distinct, arrays of tiles in the outer region of the lower divertor that are each distinguishable by different stable-isotope signatures of W. This installation was called the metal rings campaign. Experiments were then carried out with this setup to assess the W source from each location and how the sourced W led to contamination of the main scrape-off layer (SOL). The bSIMM method is derived and shown to be in good agreement with benchmark tests using known mixtures of different isotopic W signatures. The method is applied to a set of dual-facing impurity Collector Probes (CP) exposed in H-mode discharges during this metal rings campaign. Using the bSIMM as the main analysis tool, CP radial profiles show that the divertor W sources follow the discharge's strike-point position. In addition, opposing faces of the CPs have different W deposition profiles indicating poloidal variation of W content in the SOL and the isotopic signatures are shown to follow these trends. This work demonstrates that this methodology for detecting isotopic W sourced from different plasma facing components in a fusion device is reliable and versatile.
Evidence of near-SOL tungsten accumulation using a far-SOL collector probe array and OEDGE modelling in the DIII-D metal rings L-mode discharges
2019, Nuclear Materials and Energy
Citation Excerpt :
Ionization sources in the near- and far-SOL resulting from plasma contact with the main walls can be an additional driver of fuel plasma flows; data on such sources could be obtained by more comprehensive deployment of wall Langmuir probes and D⊥ measurements. Deployment of additional Collector Probes e.g. closer to the crown of the plasma, would provide further constraints on the modelling of the transport of impurity ions in the near- and far-SOL; this could be achieved on DIII-D using upper single null discharges together with a CP mounted on DiMES [22]. The use of other isotopic trace impurities, e.g. injected 13CH4, would provide further information on impurity ion transport in the SOL.
Experimental evidence is presented of near-scrape off layer (SOL) tungsten accumulation near the crown of lower single-null L-mode discharges in the DIII-D Metal Rings Campaign, based on a peripheral-SOL collector probe (CP) array and OEDGE modelling. Such accumulation has been long-theorized due to parallel force balance in the SOL dominated by the ion temperature gradient force [1,2] but direct experimental evidence has been lacking. Impurity accumulation at this location is undesirable since it largely sets the boundary condition for impurity levels in the confined plasma. Toroidally symmetric rings of 5 cm wide tungsten-coated tiles were installed in the outer divertor of DIII-D. A CP array having multiple-diameter, dual-facing collector rods with axes in the radial direction, was inserted into the peripheral-SOL near the outer midplane to measure the plasma W content. In many cases, more W was deposited on the largest rod on the side facing toward the inner target along the field lines, despite the location of the W source being at the outer divertor strike point. Interpretation of the W deposition profiles on the CP rods support the long-theorized impurity accumulation hypothesis; however, more definitive conclusions will require further experimentation and modelling effort.

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DiMES PMI research at DIII-D in support of ITER and beyond

Highlights

Abstract

Introduction

Section snippets

Experimental arrangement on DIII-D

Sample preparation and analysis

Reduction of net erosion of Mo and W by short-scale redeposition

Summary

Acknowledgments

J. Nucl. Mater.

Nucl. Fusion

J. Nucl. Mater.

Phys. Scr.

Plasma Phys. Control. Fusion

J. Nucl. Mater.

Phys. Scr.

Fusion Eng. Des.

J. Phys. B: At. Mol. Opt. Phys.

Phys. Fluids

Nucl. Instrum. Methods B

J. Nucl. Mater.

Nucl. Fusion

Nucl. Fusion

J. Nucl. Mater.

Nucl. Fusion

Advances in Understanding of High-Z Material Erosion and Re-deposition in Low-Z Wall Environment in DIII-D