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Impact of Surface Structures onDeposition and Erosion in a Tokamak
KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fusion is a potentially unlimited and environmentally friendly energy source for human society in the future. However, along the way towards the application of fusion energy there are still unresolved complications. Among them, deposition and erosion are two critical issues. Deposition of fuel and impurities brings potential long-term fuel retention which may generate safety issues and limit the economic efficiency of fusion devices. Moreover, the erosion of the vacuum vessel wall in a fusion device generates impurities which contaminate core plasma and can restrict the life time of plasma facing component. The work in this thesis focuses on deposition and erosion on tiles in the JET-ILW project, which consist of tungsten (or tungsten coating carbon fibre composited) in the divertor and beryllium in limiters.

For the deposition issue, micro ion beam analysis (µ-IBA) was used for observing deuterium and beryllium distributions over tile surfaces. The surface topography was obtained from SEM, optical microscope and confocal laser scan microscope. Distribution maps from IBA were compared with surface topography. To explain experimental results, modelling of ion trajectories was applied on real and artificial surfaces. Micro IBA results show that deuterium and beryllium accumulated in depressed areas, e.g. pits, cracks or craters. Modelling implies that ion gyration, surface roughness and inclination of the magnetic field could to some extent explain this non-uniform distribution of deuterium and beryllium. The same kind of issue, although on different scale length, occurs also for penetration of impurities into artificial castellation grooves, also studied experimentally in the thesis.

For the erosion issue, the thesis includes analysis of a limiter marker tile which is designed for observing material erosion in JET. A new method to acquire erosion data from such marker tiles is proposed, by combining micro IBA and SEM image.  This method could separate the influence on IBA from roughness, a problem in applying IBA on rough surface. Similar Technique is applied to improve the interpretation of IBA measurements of deep penetration of deuterium into layered surface structures.

Abstract [sv]

Fusion är en potentiellt obegränsad och miljövänlig energikälla för det mänskliga samhället i framtiden. Det återstår emellertid vissa problem att lösa. Bland dem är deposition och erosion vid ytor som är i kontakt med plasmat kritiska. Deposition av bränsle och föroreningar ger potentiellt långsiktig ackumulation av bränsle (tritium) som kan ge upphov till säkerhetsproblem och försämra bränsleekonomin. Erosion av vakuumkärlets väggar i in fusionsanläggning alstrar förorenar plasmat och kan begränsa livstiden för väggkomponenter. Arbetet i denna avhandling fokuserar på deposition och erosion på ytor i JET-ILW-projektet, som består av volfram (eller volframbelagd kolfiberkomposit) i divertor och beryllium i limiter.

För depositionsfrågorna användes mikroanalys för att observera hur deuterium och beryllium fördelas över ytorna efter plasmaexponering. Yttopografi erhölls från SEM, optisk mikroskopi och konfokal laserskanmikroskopi. Distributionskartor från IBA jämfördes med yttopografin. För att förklara experimentella resultat användes modellering av jontrajektorior, dels på verklig experimentell topografi, dels på förenklade modellytor. Micro IBA-resultat visar att deuterium och beryllium ansamlas i mikroskopiskt nedsänkta områden, t.ex. gropar och sprickor. Modelleringen visar att joners gyratation delvis kan förklara denna ojämna fördelning av deuterium och beryllium.

För erosionsproblemet gjordes mätningar på markerplattor, konstruerade för att observera materialosion i JET. En ny metod införs för att erhålla erosionsdata, genom att kombinera mikro IBA och SEM-bild. Denna metod kan skilja på inflytande på IBA-resultat från skrovlighet, vilket är annars är ett problem IBA för på skrovlig yta. På samma sätt används mikroanalys för att förbättra tolkningen av analyser av deuterium som trängt in i en skiktad struktur.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 65
Series
TRITA-EECS-AVL ; 2019:11
Keywords [en]
Deposition, erosion, JET, micro ion beam analysis
Keywords [sv]
Deposition, erosion, JET, mikrojonstråleanalys
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-244029ISBN: 978-91-7873-080-3 (print)OAI: oai:DiVA.org:kth-244029DiVA, id: diva2:1288582
Public defence
2019-03-08, F3, Lindstedtsvägen 26, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20190214

Available from: 2019-02-14 Created: 2019-02-13 Last updated: 2019-02-14Bibliographically approved
List of papers
1. Microanalysis of deposited layers in the inner divertor of JET with ITER-like wall
Open this publication in new window or tab >>Microanalysis of deposited layers in the inner divertor of JET with ITER-like wall
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2017 (English)In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 412-417Article in journal (Refereed) Published
Abstract [en]

In JET with ITER-like wall, beryllium eroded in the main chamber is transported to the divertor and deposited mainly at the horizontal surfaces of tiles 1 and 0 (high field gap closure, HFGC). These surfaces are tungsten coated carbon fibre composite (CFC). Surface sampleswere collected following the plasma operations in 2011-2012 and 2013-2014 respectively. The surfaces, as well as polished cross sections of the deposited layers at the surfaces have been studied with micro ion beam analysis methods (mu-IBA). Deposition of Beand other impurities, and retention of D is microscopically inhomogeneous. Impurities and trapped deuterium accumulate preferentially in cracks, pits and depressed regions, and at the sides of large pits in the substrate (e.g. arc tracks where the W coating has been removed). With careful overlaying of mu-NRA elemental maps with optical microscopy images, it is possible to separate surface roughness effects from depth profiles at microscopically flat surface regions.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-220633 (URN)10.1016/j.nme.2017.02.015 (DOI)000417293300062 ()2-s2.0-85015310440 (Scopus ID)
Note

QC 20180112

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2019-02-13Bibliographically approved
2. The effect of gyration on the deposition of beryllium and deuterium at rough surface on thedivertor tiles with ITER-like-wall in JET
Open this publication in new window or tab >>The effect of gyration on the deposition of beryllium and deuterium at rough surface on thedivertor tiles with ITER-like-wall in JET
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2019 (English)In: Nuclear Materials and Energy, E-ISSN 2352-1791Article in journal (Refereed) Submitted
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-243923 (URN)
Note

QC 20190306

Available from: 2019-02-13 Created: 2019-02-13 Last updated: 2019-03-06Bibliographically approved
3. Modelling of effect from rough surface on deuterium and beryllium deposition on divertor target
Open this publication in new window or tab >>Modelling of effect from rough surface on deuterium and beryllium deposition on divertor target
(English)Manuscript (preprint) (Other academic)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-244026 (URN)
Note

QC 20190215

Available from: 2019-02-13 Created: 2019-02-13 Last updated: 2019-02-15Bibliographically approved
4. Fuel inventory and deposition in castellated structures in JET-ILW
Open this publication in new window or tab >>Fuel inventory and deposition in castellated structures in JET-ILW
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2017 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 57, no 6, article id 066027Article in journal (Refereed) Published
Abstract [en]

Since 2011 the JET tokamak has been operated with a metal ITER-like wall (JET-ILW) including castellated beryllium limiters and lamellae-type bulk tungsten tiles in the divertor. This has allowed for a large scale test of castellated plasma-facing components (PFC). Procedures for sectioning the limiters into single blocks of castellation have been developed. This facilitated morphology studies of morphology of surfaces inside the grooves for limiters after experimental campaigns 2011-2012 and 2013-2014. The deposition in the 0.4-0.5 mm wide grooves of the castellation is 'shallow'. It reaches 1-2 mm into the 12 mm deep gap. Deuterium concentrations are small (mostly below 1 × 1018 cm-2). The estimated total amount of deuterium in all the castellated limiters does not exceed the inventory of the plasma-facing surfaces (PFS) of the limiters. There are only traces of Ni, Cr and Fe deposited in the castellation gaps. The same applies to the carbon content. Also low deposition of D, Be and C has been measured on the sides of the bulk tungsten lamellae pieces. Modelling clearly reflects: (a) a sharp decrease in the measured deposition profiles and(b) an increase in deposition with the gap width. Both experimental and modelling data give a strong indication and information to ITER that narrow gaps in the castellated PFC are essential. X-ray diffraction on PFS has clearly shown two distinct composition patterns: Be with an admixture of Be-W intermetallic compounds (e.g. Be22W) in the deposition zone, whilst only pure Be has been detected in the erosion zone. The lack of compound formation in the erosion zone indicates that no distinct changes in the thermo-mechanical properties of the Be PFC might be expected.

Place, publisher, year, edition, pages
Institute of Physics Publishing, 2017
Keywords
beryllium limiters, castellation, deposition, fuel inventory, ITER-like wall, JET, Beryllium, Carbon, Deuterium, Erosion, Facings, Fighter aircraft, Fusion reactor divertors, Intermetallics, Jets, Magnetoplasma, Tungsten, Tungsten compounds, X ray diffraction, Experimental campaign, Fuel inventories, Morphology of surfaces, Plasma facing surfaces, Plasma-facing components, Thermomechanical properties, Tokamak devices
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-216517 (URN)10.1088/1741-4326/aa6864 (DOI)000425870600001 ()2-s2.0-85019426928 (Scopus ID)
Note

QC 20171201

Available from: 2017-12-01 Created: 2017-12-01 Last updated: 2019-02-13Bibliographically approved
5. Deep deuterium retention and Be/W mixingat tungsten coated surfaces in the JETdivertor
Open this publication in new window or tab >>Deep deuterium retention and Be/W mixingat tungsten coated surfaces in the JETdivertor
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2016 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896Article in journal, Editorial material (Refereed) Published
Abstract [en]

Surface samples from a full poloidal set of divertor tiles exposed in JET through operations2010–2012 with ITER-like wall have been investigated using SEM, SIMS, ICP-AES analysisand micro beam nuclear reaction analysis (μ-NRA). Deposition of Be and retention of D ismicroscopically inhomogeneous. With careful overlaying of μ-NRA elemental maps with SEMimages, it is possible to separate surface roughness effects from depth profiles at microscopicallyflat surface regions, without pits. With (3He, p) μ-NRA at 3–5 MeV beam energy the accessibledepth for D analysis in W is about 9 μm, sufficient to access the W/Mo and Mo/W interfaces inthe coatings and beyond, while for Be in W it is about 6 μm. In these conditions, at all plasmawetted surfaces, D was found throughout the whole accessible depth at concentrations in therange 0.2–0.7 at% in W. Deuterium was found to be preferentially trapped at the W/Mo andMo/W interfaces. Comparison is made with SIMS profiling, which also shows significant Dtrapping at the W/Mo interface. Mixing of Be and W occurs mainly in deposited layers.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-243916 (URN)
Note

QCR 20190312

Available from: 2019-02-11 Created: 2019-02-11 Last updated: 2019-03-12Bibliographically approved
6. Micro ion beam analysis for the erosion of beryllium marker tiles in atokamak limiter
Open this publication in new window or tab >>Micro ion beam analysis for the erosion of beryllium marker tiles in atokamak limiter
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2019 (English)In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584Article in journal (Refereed) In press
Abstract [en]

Beryllium limiter marker tiles were exposed to plasma in the Joint European Torus to diagnose the erosion of main chamber wall materials. A limiter marker tile consists of a beryllium coating layer (7–9 μm) on the top of bulk beryllium, with a nickel interlayer (2–3 μm) between them. The thickness variation of the beryllium coating layer, after exposure to plasma, could indicate the erosion measured by ion beam analysis with backscattering spectrometry. However, interpretations from broad beam backscattering spectra were limited by the non-uniform surface structures. Therefore, micro-ion beam analysis (μ-IBA) with 3 MeV proton beam for Elastic backscattering spectrometry (EBS) and PIXE was used to scan samples. The spot size was in the range of 3–10 μm. Scanned areas were analysed with scanning electron microscopy (SEM) as well. Combining results from μ-IBA and SEM, we obtained local spectra from carefully chosen areas on which the surface structures were relatively uniform. Local spectra suggested that the scanned area (≈600 μm × 1200 μm) contained regions with serious erosion with only 2–3 μm coating beryllium left, regions with intact marker tile, and droplets with 90% beryllium. The nonuniform erosion, droplets mainly formed by beryllium, and the possible mixture of beryllium and nickel were the major reasons that confused interpretation from broad beam EBS.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-244028 (URN)10.1016/j.nimb.2018.08.028 (DOI)
Note

QC 20190218

Available from: 2019-02-13 Created: 2019-02-13 Last updated: 2019-02-18Bibliographically approved

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