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Material migration in tokamaks: Studies of deposition processes and characterisation of dust particles
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.ORCID iD: 0000-0003-1062-8101
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Thermonuclear fusion may become an attractive future power source. The most promising of all fusion machine concepts is the tokamak. Despite decades of active research, still huge tasks remain before a fusion power plant can go online. One of these important tasks deals with the interaction between the fusion plasma and the reactor wall. This work focuses on how eroded wall materials of different origin and mass are transported in a tokamak device. Element transport can be examined by injection of certain species of unique and predetermined origin, so called tracers. Tracer experiments were conducted at the TEXTOR tokamak before its final shutdown. This offered an unique opportunity for studies of the wall and other internal components: For the first time it was possible to completely dismantle such a machine and analyse every single part of reactor wall, obtaining a detailed pattern of material migration. Main focus of this work is on the high-Z metals tungsten and molybdenum, which were introduced by WF6 and MoF6 injection into the TEXTOR tokamak in several material migration experiments. It is shown that Mo and W migrate in a similar way around the tokamak and that Mo can be used as tracer for W transport. It is further shown how other materials - medium-Z (Ni), low-Z (N-15 and F), fuel species (D) - migrate and get deposited. Finally, the outcome of dust sampling studies is discussed. It is shown that dust appearance and composition depends on origin, formation conditions and that it can originate even from remote systems like the NBI system. Furthermore, metal splashes and droplets have been found, some of them clearly indicating boiling processes.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 80 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:102
Keyword [en]
tokamak, fusion, plasma, material migration, particle transport, TEXTOR, PWI, plasma-wall interaction, plasma facing components, PFC, PFM, plasma facing materials, ALT limiter, MoF6, tracer experiment, molybdenum hexafluoride, future energy source, fuel retention, deuterium retention
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-178026ISBN: 978-91-7595-766-1 (print)OAI: oai:DiVA.org:kth-178026DiVA: diva2:876116
Presentation
2015-12-15, Seminarierummet på plan 3, Teknikringen 31, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20151203

Available from: 2015-12-03 Created: 2015-12-02 Last updated: 2016-10-26Bibliographically approved
List of papers
1. Local Migration Studies of High-Z Metals in the TEXTOR Tokamak
Open this publication in new window or tab >>Local Migration Studies of High-Z Metals in the TEXTOR Tokamak
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2016 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T167, 014058Article in journal (Refereed) Published
Abstract [en]

Volatile compounds of tungsten (WF6) and molybdenum (MoF6) were used as tracers of high-Z metal migration in the TEXTOR tokamak in several gas injection experiments when puffing was done through a test limiter. The experiments with W were performed prior major shut-downs, while the MoF6 was followed by the final shutdown in connection with TEXTOR decommissioning. In all cases a set of various surface probes and limiter tiles were retrieved and analysed with electron and ion beam techniques. The focus was on the local deposition in the vicinity of the gas inlet and in the inlet system. Depth profiles in the deposits and metal distribution maps clearly shown that only near the gas inlet significant amounts of Mo are deposited along the scrape-off layer flow and E×B drift directions, which could be reproduced by ERO-code modelling. Correlation between the plasma operation scenario and the deposition patterns is presented.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-178044 (URN)10.1088/0031-8949/T167/1/014058 (DOI)000383504700059 ()2-s2.0-84959870422 (Scopus ID)
Note

QC 20170630

Available from: 2015-12-03 Created: 2015-12-03 Last updated: 2017-11-29Bibliographically approved
2. Dust Survey Following the Final Shutdown of TEXTOR: Metal Particles and Fuel Retention
Open this publication in new window or tab >>Dust Survey Following the Final Shutdown of TEXTOR: Metal Particles and Fuel Retention
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2016 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T167, 014059Article in journal (Refereed) Published
Abstract [en]

The work presents results of a broad TEXTOR dust survey in terms of its composition, structure, distribution and fuel content. The dust particles were collected after final shutdown of TEXTOR in December 2013. Fuel retention, as determined by thermal desorption, varied significantly, even by two orders of magnitude, dependent on the dust location in the machine. Dust structure was examined by means of scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, focused ion beam and scanning transmission electron microscopy. Several categories of dust have been identified. Carbon-based stratified and granular deposits were dominating, but the emphasis in studies was on metal dust. They were found in the form of small particles, small spheres, flakes and splashes which formed “comet”-like structures clearly indicating directional effects in the impact on surfaces of plasma-facing components. Nickel-rich alloys from the Inconel liner and iron-based ones from various diagnostic holders were the main components of metal-containing dust, but also molybdenum and tungsten debris were detected. Their origin is discussed.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016
Keyword
TEXTOR, dust, fuel retention, high-Z metals
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-178047 (URN)10.1088/0031-8949/T167/1/014059 (DOI)000383504700060 ()2-s2.0-84959896914 (Scopus ID)
Note

QC 20160127

Available from: 2015-12-03 Created: 2015-12-03 Last updated: 2017-06-30Bibliographically approved
3. Tracer techniques for the assessment of material migration and surface modification of plasma-facing components
Open this publication in new window or tab >>Tracer techniques for the assessment of material migration and surface modification of plasma-facing components
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2015 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 463, 280-284 p.Article in journal (Refereed) Published
Abstract [en]

Tracer techniques were used in the TEXTOR tokamak to determine high-Z metal migration and the deposition of species used for plasma edge cooling or wall conditioning under different types of operation conditions. Volatile molybdenum hexa-fluoride, nitrogen-15 and oxygen-18 were used as markers in tokamak or ion cyclotron wall conditioning discharges (ICWC). The objective was to obtain qualitative and quantitative of a global and local deposition pattern and material mixing effects. The deposition and retention was studied on plasma-facing components, collector probes and test limiters. Optical spectroscopy and ex-situ analysis techniques were used to determine the plasma response to tracer injection and the modification of surface composition. Molybdenum and light isotopes were detected on all types of limiters and short-term probes retrieved from the vessel showing that both helium and nitrogen are trapped following wall conditioning and edge cooling. Only small amounts below 1 x 10(19) m(-2) of O-18 were detected on surfaces treated by oxygen-assisted ICWC.

Place, publisher, year, edition, pages
Elsevier, 2015
National Category
Materials Engineering Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-172689 (URN)10.1016/j.jnucmat.2014.11.074 (DOI)000358467200052 ()2-s2.0-84937517699 (Scopus ID)
Funder
Swedish Research Council, 621-2009-4138
Note

QC 20150901

Available from: 2015-09-01 Created: 2015-08-27 Last updated: 2017-06-30Bibliographically approved
4. Characterisation of surface layers formed on plasma-facing components in controlled fusion devices: Role of heavy ion elastic recoil detection
Open this publication in new window or tab >>Characterisation of surface layers formed on plasma-facing components in controlled fusion devices: Role of heavy ion elastic recoil detection
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2015 (English)In: Vacuum, ISSN 0042-207X, E-ISSN 1879-2715, Vol. 122, 260-267 p.Article in journal (Refereed) Published
Abstract [en]

Wall components retrieved from the TEXTOR tokamak after tracer experiments with nitrogen-15 and molybdenum hexafluoride (MoF6) injection were studied to determine deposition patterns and, by this, to conclude on material migration. Toroidal limiter tiles made of carbon fibre composites and fine grain graphite were examined using time-of-flight heavy ion elastic recoil detection analysis. Molybdenum deposition patterns indicated migration based on erosion and prompt re-deposition. Nitrogen-15 was trapped together with the deposited molybdenum. Some information on the depth distribution of species in the top 400 nm layer of the limiters was obtained; however surface roughness of the samples strongly limited resolution. In the case of molybdenum, the largest concentration was found in the 100 nm outermost layer, whereas fluorine and nitrogen-15 displayed more irregular profiles. Other species, besides deuterium fuel and carbon-12, were also identified: boron-10 and boron-11 originating from boronisations, carbon-13 from earlier tracer experiments, nitrogen-14 from plasma edge cooling and metals eroded from the Inconel wall.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-178042 (URN)10.1016/j.vacuum.2015.04.019 (DOI)2-s2.0-84945489472 (Scopus ID)
Note

QC 20150203

Available from: 2015-12-03 Created: 2015-12-03 Last updated: 2017-12-01Bibliographically approved

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