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Material migration in tokamaks: Erosion-deposition patterns and transport processes
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.ORCID iD: 0000-0003-1062-8101
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Controlled thermonuclear fusion may become an attractive future electrical power source. The most promising of all fusion machine concepts is called a tokamak. The fuel, a plasma made of deuterium and tritium, must be confined to enable the fusion process. It is also necessary to protect the wall of tokamaks from erosion by the hot plasma. To increase wall lifetime, the high-Z metal tungsten is foreseen as wall material in future fusion devices due to its very high melting point. This thesis focuses on the following consequences of plasma impact on a high-Z wall: (i) erosion, transport and deposition of high-Z wall materials; (ii) fuel retention in tokamak walls; (iii) long term effects of plasma impact on structural machine parts; (iv) dust production in tokamaks.

An extensive study of wall components has been conducted with ion beam analysis after the final shutdown of the TEXTOR tokamak. This unique possibility offered by the shutdown combined with a tracer experiment led to the largest study of high-Z metal migration and fuel retention ever conducted. The most important results are:

 

- transport is greatly affected by drifts and flows in the plasma edge;

- stepwise transport along wall surfaces takes place mainly in the toroidal direction;

- fuel retention is highest on slightly retracted wall elements;

- fuel retention is highly inhomogeneous.

 

A broad study on structural parts of a tokamak has been conducted on the TEXTOR liner. The plasma impact does neither degrade mechanical properties nor lead to fuel diffusion into the bulk after 26 years of duty time. Peeling deposition layers on the liner retain fuel in the order of 1g and represent a dust source. Only small amounts of dust are found in TEXTOR with overall low deuterium content. Security risks in future fusion devices due to dust explosions or fuel retention in dust are hence of lesser concern.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 109
Series
TRITA-EE, ISSN 1653-5146 ; 2017:060
Keywords [en]
TEXTOR, fusion, plasma physics, transport, migration, tracers, tokamak, limiter, divertor, high-Z, ion beam analysis, Rutherford backscattering, Nuclear reaction analysis, Elastic recoil detection analysis
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-209758ISBN: 978-91-7729-461-0 (print)OAI: oai:DiVA.org:kth-209758DiVA, id: diva2:1114187
Public defence
2017-09-19, F3, Lindstedtsvägen 26, Stockholm, 14:59 (English)
Opponent
Supervisors
Note

QC 20170630

Available from: 2017-06-30 Created: 2017-06-22 Last updated: 2017-06-30Bibliographically approved
List of papers
1. Global material migration in tokamaks: patterns, material balance and transport mechanisms in TEXTOR
Open this publication in new window or tab >>Global material migration in tokamaks: patterns, material balance and transport mechanisms in TEXTOR
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2017 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326Article in journal, Editorial material (Refereed) Submitted
Abstract [en]

As the last experiment before the final shutdown of the TEXTOR tokamak, equipped with all-graphite plasma-facing components (PFCs), MoF6 had been injected into the vacuum vessel. During decommissioning all PFCs became available for surface studies, enabling detailed mapping of previously injected Mo, W and intrinsic 625 Inconel metals. As a result, detailed deposition patterns for these metals were obtained, revealing a number of findings: a) High-Z metals are mainly globally deposited, with concentrations decaying exponentially with distance from the injection source; b) the decay length is of the order of 0.1 m on the main PFC and 1.0 m on the receded PFC; c) ion flow velocities co-decide the position of maximum deposition. Modelling with ERO shows exponential decay. Simulated decay lengths are between 0.15 – 1.30 m, depending on the anomalous cross field diffusion coefficient. Extensive measurements of Mo have been undertaken in order to quantify the amounts deposited on the graphite PFCs, showing that the ratio of local to global deposited high-Z metals is 0.3-0.4. However, only up to 20% of the injected Mo could be detected on all the PFCs. A large fraction of injected Mo may have been pumped out before being deposited. The bumper limiter is found to be the major repository for all probed elements. For W and F, this is solely due to the limiter size – the areal concentration is not enhanced. For Mo, Inconel metals, 15N and D the areal concentration on the bumper limiter is higher than on the toroidal belt limiter ALTII acting as main PFC.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-210414 (URN)
Note

QC 20170630

Available from: 2017-06-30 Created: 2017-06-30 Last updated: 2017-06-30Bibliographically approved
2. 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, article id 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
3. Ageing of structural materials in tokamaks: TEXTOR liner study
Open this publication in new window or tab >>Ageing of structural materials in tokamaks: TEXTOR liner study
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2017 (English)In: Physica scripta. T, ISSN 0281-1847, Vol. T170, article id 014053Article in journal, Meeting abstract (Refereed) Published
Abstract [en]

After the final shut-down of the tokamak TEXTOR, all of its machine parts became accessible for comprehensive studies. This unique chance enabled the study of the Inconel 625 liner by a wide range of methods. The aim was to evaluate eventual alteration of surface and bulk characteristics from recessed wall elements that may influence the machine performance. The surface was covered with stratified layers consisting mainly of boron, carbon, oxygen, and in some cases also silicon. Wall conditioning and limiter materials hence predominantly define deposition on the liner. Deposited layers on recessed wall elements reach micrometre thickness within decades, peel off and may contribute to the dust inventory in tokamaks. Fuel retention was about 4 at% of the overall layers, with no evidence for diffusion into the Inconel substrate. Inconel 625 retained its mechanical strength despite 26 years of cyclic heating, stresses and particle bombardment.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2017
Keywords
Fusion, Plasma-Wall Interaction, Fuel Retention, Inconel, Dust
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-210415 (URN)10.1088/1402-4896/aa8bee (DOI)000415066700001 ()
Conference
16th International Conference on Plasma-Facing Materials and Components for Fusion Applications, 16th - 19th May 2017, Neuss/Düsseldorf, Germany
Note

QC 20170630

Available from: 2017-06-30 Created: 2017-06-30 Last updated: 2017-11-29Bibliographically approved
4. 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, p. 280-284Article 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
5. Modelling of deposition and erosion of injected WF6 and MoF6 in TEXTOR
Open this publication in new window or tab >>Modelling of deposition and erosion of injected WF6 and MoF6 in TEXTOR
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2016 (English)In: Nuclear Materials and Energy, ISSN 2352-1791Article in journal (Refereed) Published
Abstract [en]

Tracer injection experiments in TEXTOR with MoF 6 and WF 6 lead to local deposition of about 6% for Mo and about 1% for W relative to the injected amount of Mo and W atoms. Modelling of these experiments has been done with ERO applying updated data for physical sputtering. The dissociation of the injected molecules has been treated in a simplified manner due to the lack of dissociation rate coefficients. How- ever, with this it was possible to reproduce the observed radial penetration of Mo and W atoms into the plasma. The modelled local deposition efficiencies are about 50% for Mo and 60% for W assuming typical plasma parameters for the experimental conditions used. To reproduce the measured deposition efficien- cies an enhancement factor for the erosion of deposited Mo and W has to be assumed ( ∼10 for Mo and ∼25 for W). Due to the rather low electron temperature T e of these plasma conditions (T e ∼15 eV at the location of injection), Mo and W are mostly sputtered by impurities whereas sputtering due to deuterium is negligible. A parameter study applying larger electron temperature leads to increased sputtering and thus to reduced local deposition efficiencies of about 30% for Mo and 5% for W. Though, even under these conditions enhanced erosion, albeit with reduced enhancement factors, is needed in the modelling to obtain the small measured deposition efficiencies.

Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-210416 (URN)10.1016/j.nme.2016.10.022 (DOI)000417445800003 ()2-s2.0-85007022634 (Scopus ID)
Conference
22nd International Conference on Plasma Surface Interactions in Controlled Fusion Devices, 30th May - 3rd June 2016, Rome, Italy
Note

QC 20170630

Available from: 2017-06-30 Created: 2017-06-30 Last updated: 2017-12-29Bibliographically approved
6. 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, article id 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
Keywords
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

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Citation style
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  • modern-language-association-8th-edition
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  • Other style
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  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
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