DiMES PMI research at DIII-D in support of ITER and beyond
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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Advances in Understanding of High-Z Material Erosion and Re-deposition in Low-Z Wall Environment in DIII-D
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