Inspection of W 7-X plasma-facing components after the operation phase OP1.2b: observations and first assessments

Dhard, C. P.; Äkäslompolo, S.; Balden, M.; Baldzuhn, J.; Biedermann, C.; Bräuer, T.; Brezinsek, S.; Endler, M.; Hayashi, Y.; Hwangbo, Dogyun; Kajita, Shin; Krause, Marco; Kornejew, P.; Masuzaki, S.; Mayer, M.; Motojima, G.; Naujoks, D.; Otte, M.; Rohde, V.

doi:10.1088/1402-4896/ab4b3f

Cited by 15 publications

(12 citation statements)

References 14 publications

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“…This is a pronounced difference to tokamaks, where strong carbon deposition in the inner divertor or in remote divertor areas is observed [31,32,33]. Carbon-containing thin layers are observed at various places inside the main chamber of W7-X [34], but total amounts of deposited carbon are not yet available. Pump-out of carbon-containing molecules (CO, CO2, CH4) is also observed, but has not been quantified yet.…”

Section: Discussionmentioning

confidence: 92%

Material erosion and deposition on the divertor of W7-X

et al. 2020

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The net erosion and deposition pattern of carbon from the Test Divertor Unit (TDU) of the stellarator W7-X was determined. Special target elements with marker layers consisting of about 300 nm molybdenum and 5-10 µm carbon on top were used during the operation phase OP 1.2a. The thicknesses of the marker layers were determined by elastic backscattering spectrometry (EBS) using 2.5 MeV protons before and after plasma exposure and laser-induced breakdown spectroscopy (LIBS) on selected target elements after exposure. Scanning electron microscopy (SEM) was used for investigating the surface morphology before and after exposure. Massive erosion of up to 20 µm carbon was observed at the strike line, in total 48±14 g carbon were eroded from the 10 TDUs. The erosion was laterally nonuniform on the micro-scale. Strongly eroded surfaces were considerably smoother as compared to the original material. Only very little deposition of carbon is observed on the TDU: This means that the TDU is a large net erosion source.

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Section: Discussionmentioning

confidence: 92%

Material erosion and deposition on the divertor of W7-X

et al. 2020

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“…If GDC was the reason for the discretized Fe and Mo deposition peaks, larger depositions should be seen at larger depth. During OP 1.2b, two steel components melting events were observed [1,16]. These results show that the discretized deposition peaks of Fe and Mo are quite likely occurred during OP 1.2b.…”

Section: Distribution Patterns Of Deposits On Target Elementsmentioning

confidence: 60%

“…W7-X completed the island divertor operation using test divertor units (TDUs) in operation phase 1.2a (OP 1.2a) and OP 1.2b [1]. These inertially cooled TDUs are distributed along the helical edge of the plasma contour and alternatively located on the upper and lower side of plasma in the five nearly identical modules (see figure 1 of reference [2]).…”

Section: Introductionmentioning

confidence: 99%

Distributions of deposits and hydrogen on the upper and lower TDUs3 target elements of Wendelstein 7-X

Zhao¹,

Masuzaki²,

Motojima³

et al. 2022

Nucl. Fusion

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Distributions of deposits and hydrogen (H) on the graphite divertor target elements TM4h4 and TM3v5 in the test divertor units 3 (TDUs3) of Wendelstein 7-X (W7-X) are studied. The TM4h4 and TM3v5 are located at the magnetically symmetric positions in the upper and lower divertor. The microstructure of the deposition layer is characterized by a transmission electron microscope (TEM) combined with a focused ion beam (FIB). Metallic deposits such as iron (Fe), molybdenum (Mo), chromium (Cr) are detected in the deposition layer by energy-dispersive X-ray spectroscopy (EDS). The depth-resolved distribution patterns of boron (B) and metallic deposits on upper and lower horizontal (h) divertor target elements TM4h4 as well as upper and lower vertical (v) divertor target elements TM3v5 are clarified by glow discharge optical emission spectrometry (GDOES). Results for both TM4h4 and TM3v5 show that the B deposition regions exhibit higher H retention due to the co-deposition with deposits. On the other hand, up-down asymmetries in B deposition caused by particle drift exist on both TM4h4 and TM3v5. The B deposition amount on upper TM4h4 is 40% smaller than that on lower TM4h4. While for the vertical target elements, the B deposition amount on upper TM3v5 is 35% larger than that on lower TM3v5. Meanwhile, a shift of around 3 cm in B deposition peaks is observed on upper and lower TM4h4 and TM3v5. Results of numerical simulation of carbon deposition/erosion profiles on the target elements using ERO2.0 code and power flux measured by infrared cameras are shown and compared with the above mentioned B profiles.

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“…A majority of the inboard side and other areas that are predicted to be exposed to significant heat loads are covered by the graphite heat shield. The other areas with low heat fluxes are covered with steel panels [23].…”

Section: Interference-filtered 2d Cameras In the Visible Rangementioning

confidence: 99%

Analysis of hydrogen fueling, recycling, and confinement at Wendelstein 7-X via a single-reservoir particle balance

et al. 2022

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A single-reservoir particle balance for the main plasma species hydrogen has been established for Wendelstein 7-X (W7-X). This has enabled the quantitative characterization of the particle sources in the standard island divertor configuration for the first time. Findings from attached scenarios with two different island sizes with a boronized wall and turbo molecular pumping are presented. Fueling efficiencies, particle flows and source locations were measured and used to infer the total particle confinement time $\tau_{\rm{p}}$. Perturbative gas injection experiments served to measure the effective particle confinement time $\tau_{\rm{p}}^*$. Combining both confinement times provides access to the global recycling coefficient $\bar{R}$. Hydrogen particle inventories have been addressed and the knowledge of particle sources and sinks reveals the core fueling distribution and provides insight into the capability of the magnetic islands to control exhaust features. Measurements of hydrogen fueling efficiencies were sensitive to the precise fueling location and measured between 12~\% and 31~\% with the recycling fueling at the strike line modeled at only 6~\%, due to much higher densities. 15~\% of the total \SI{5.2E+22}{a/s} recycling flow ionizes far away from the recycling surfaces in the main chamber. It was shown that 60~\% of recycled particles ionize above the horizontal and 18~\% above the vertical divertor target, while the remainder of the recycling flow ionizes above the baffle (7~\%). Combining these source terms with their individual fueling efficiencies resolves the core fueling distribution. Due to the higher fueling efficiency in the main chamber, up to 51~\% of the total \SI{5.1E+21}{1/s} core fueling particles are entering the confined plasma from the main chamber. $\tau_{\rm{p}}$ values in the range of 260 ms were extracted for these discharges. Together with $\tau_{\rm{p}}$, the global recycling coefficient $\bar{R}$ was resolved for every $\tau_{\rm{p}}^*$ measurement and a typical value close to unity was obtained. An increase of the island size, resulted in no change of $\tau_{\rm{p}}$, but doubled $\tau_{\rm{p}}^*$, indicating the feasibility of the control coils as an actuator to control exhaust features without affecting core confinement properties.

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Inspection of W 7-X plasma-facing components after the operation phase OP1.2b: observations and first assessments

Cited by 15 publications

References 14 publications

Material erosion and deposition on the divertor of W7-X

Material erosion and deposition on the divertor of W7-X

Distributions of deposits and hydrogen on the upper and lower TDUs3 target elements of Wendelstein 7-X

Analysis of hydrogen fueling, recycling, and confinement at Wendelstein 7-X via a single-reservoir particle balance

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