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Material characterization for magnetically confined fusion: Surface analysis and method development
KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics. (Plasma-Wall interactions)ORCID iD: 0000-0001-9299-3262
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The dream of abundant clean energy has brought scientists and laypeople alike to ponder the possibilities of nuclear fusion since it was established as the energy source of the stars in 1939. Starting from the mid the 20th century, significant effort has been put into overcoming the technological challenges related to the construction of a power plant, but initial optimism has faded somewhat due to a notable absence of practical outcomes. Nevertheless, the research continues and progress is made slowly but surely.

The present work deals with a small part of the fusion puzzle, namely the materials to be used in the first wall surrounding a magnetically confined plasma. Carbon, which has historically been considered as the most viable element for this role, has been ruled out due to issues with plasma-induced erosion, hydrocarbon formation and a buildup of thick deposited material layers on wall components. The latter two lead to an unacceptable accumulation of radioactive tritium, both in the deposited layers and in dust particles. A metal wall, which would alleviate these particular problems but increase the severity of others, is therefore envisioned for a future demonstration reactor.

Three contributions to the overall research effort are made though this thesis. First, an increased understanding of plasma-induced erosion of so-called reduced activation ferritic-martensitic steels and preferential sputtering of light material components is provided. High-resolution ion beam analysis and microscopy methods are used to examine samples of such a steel after exposure to plasma under controlled circumstances. Model films consisting of a mixture of iron and tungsten deposited on silicon substrates are also studied as they constitute simpler systems where the effects of interest may be simulated. The knowledge obtained is necessary for an assessment of the possibility to use reduced activation steel as a plasma-facing material in specific regions of a reactor wall.

The second contribution consists of reports on the composition of deposited material layers on wall components retrieved from the plasma confinement experiments JET and TEXTOR. These provide limited conclusions on the range and rate of material erosion, transport and deposition in two cases.

Finally, a detection system for the ion beam technique elastic recoil detection analysis has been assembled, tested and put into operation. In addition to improving the quality of analyses performed on fusion-related materials, the system has become an established tool available for users of the 5 MV electrostatic pelletron accelerator at Uppsala University’s Tandem Laboratory.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 189
Series
TRITA-EECS-AVL ; 2019:4
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-241093ISBN: 978-91-7873-055-1 (print)OAI: oai:DiVA.org:kth-241093DiVA, id: diva2:1277095
Public defence
2019-02-13, F3, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20190110

Available from: 2019-01-16 Created: 2019-01-09 Last updated: 2019-01-18Bibliographically approved
List of papers
1. 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, p. 260-267Article 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: 2019-01-09Bibliographically approved
2. A combined segmented anode gas ionization chamber and time-of-flight detector for heavy ion elastic recoil detection analysis
Open this publication in new window or tab >>A combined segmented anode gas ionization chamber and time-of-flight detector for heavy ion elastic recoil detection analysis
2016 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 87, no 10, article id 103303Article in journal (Refereed) Published
Abstract [en]

A dedicated detector system for heavy ion elastic recoil detection analysis at the Tandem Laboratory of Uppsala University is presented. Benefits of combining a time-of-flight measurement with a segmented anode gas ionization chamber are demonstrated. The capability of ion species identification is improved with the present system, compared to that obtained when using a single solid state silicon detector for the full ion energy signal. The system enables separation of light elements, up to Neon, based on atomic number while signals from heavy elements such as molybdenum and tungsten are separated based on mass, to a sample depth on the order of 1 μm. The performance of the system is discussed and a selection of material analysis applications is given. Plasma-facing materials from fusion experiments, in particular metal mirrors, are used as a main example for the discussion. Marker experiments using nitrogen-15 or oxygen-18 are specific cases for which the described improved species separation and sensitivity are required. Resilience to radiation damage and significantly improved energy resolution for heavy elements at low energies are additional benefits of the gas ionization chamber over a solid state detector based system.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016
National Category
Accelerator Physics and Instrumentation
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-193638 (URN)10.1063/1.4963709 (DOI)000387661900024 ()2-s2.0-84990211387 (Scopus ID)
Funder
EU, Horizon 2020, 633053
Note

QC 20161007

Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2019-01-09Bibliographically approved
3. Erratum: "A combined segmented anode gas ionization chamber and time-of-flight detector for heavy ion elastic recoil detection analysis" [Rev. Sci. Instrum. 87, 103303 (2016)]
Open this publication in new window or tab >>Erratum: "A combined segmented anode gas ionization chamber and time-of-flight detector for heavy ion elastic recoil detection analysis" [Rev. Sci. Instrum. 87, 103303 (2016)]
2018 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 89, no 4, article id 049901Article in journal (Refereed) Published
Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-227781 (URN)10.1063/1.5030502 (DOI)000431139400078 ()29716341 (PubMedID)2-s2.0-85045335587 (Scopus ID)
Note

QC 201805014

Available from: 2018-05-14 Created: 2018-05-14 Last updated: 2019-01-09Bibliographically approved
4. Ion beam analysis of tungsten layers in EUROFER model systems and carbon plasma facing components
Open this publication in new window or tab >>Ion beam analysis of tungsten layers in EUROFER model systems and carbon plasma facing components
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2016 (English)In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 371, p. 355-359Article in journal (Refereed) Published
Abstract [en]

The tungsten enriched surface layers in two fusion-relevant EUROFER steel model samples, consisting of an iron-tungsten mixture exposed to sputtering by deuterium ions, were studied by Rutherford backscattering spectrometry and medium energy ion scattering. Exposure conditions were the same for the two samples and the total amount of tungsten atoms per unit area in the enriched layers were similar (2e15 and 2.4e15 atoms/cm2 respectively), despite slightly different initial atomic compositions. A depth profile featuring exponential decrease in tungsten content towards higher depths with 10-20 atomic percent of tungsten at the surface and a decay constant between 0.05 and 0.08 Å-1 was indicated in one sample, whereas only the total areal density of tungsten atoms was measured in the other. In addition, two different beams, iodine and chlorine, were employed for elastic recoil detection analysis of the deposited layer on a polished graphite plate from a test limiter in the TEXTOR tokamak following experiments with tungsten hexafluoride injection. The chlorine beam was preferred for tungsten analysis, mainly because it (as opposed to the iodine beam) does not give rise to problems with overlap of forward scattered beam particles and recoiled tungsten in the spectrum.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
ERDA, EUROFER, Fusion, MEIS, Tungsten, Atoms, Carbon, Chlorine, Forward scattering, Fusion reactions, Iodine, Ion beams, Ions, Magnetoplasma, Rutherford backscattering spectroscopy, Elastic recoil detection analysis, Exposure parameters, Medium energy ion scattering, Orders of magnitude, Rutherford back-scattering spectrometry
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-177225 (URN)10.1016/j.nimb.2015.09.024 (DOI)000373412000071 ()2-s2.0-84960241915 (Scopus ID)
Note

QC 20171027

Available from: 2015-11-25 Created: 2015-11-17 Last updated: 2019-01-09Bibliographically approved
5. Compositional and morphological analysis of FeW films modified by sputtering and heating
Open this publication in new window or tab >>Compositional and morphological analysis of FeW films modified by sputtering and heating
2017 (English)In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 12, p. 472-477Article in journal (Refereed) Published
Abstract [en]

Surface compositional changes of iron-tungsten films by deuterium (D) ion bombardment were studied by means of medium energy ion scattering, elastic recoil detection analysis and Rutherford backscattering spectrometry. The energy of the bombarding ions was 200 eV/D and the fluence was varied from 1e21 D/m2 to 1e24 D/m2. A significant increase of the tungsten concentration within the 20 nm closest to the sample surface, caused by preferential sputtering of iron, was seen for the films exposed 1e23 D/m2 or more. In the sample exposed to the highest fluence, 1e24 D/m2, the concentration of tungsten was increased from an initial 1.7 at. % up to approximately 24 at. % averaged over the 5 nm closest to the surface. The analysis was complicated by the presence of oxygen on the sample surfaces. In order to study the thermal stability of the tungsten enriched layer, the sample initially exposed to 1e23 D/m2 at room temperature was heated to 400 °C in the measurement chamber for medium energy ion scattering and several spectra were recorded at intermediate temperatures. The obtained data showed that the layer was relatively stable below 200 °C whereas a drastic change in the film composition occurred between 200 °C and 250 °C due to interdiffusion of iron and silicon, the latter of which was the substrate material. The surface morphologies of the films were probed with atomic force microscopy showing that protrusions of 10–100 nm width appeared after deuterium bombardment at fluences higher than 1e22 D/m2.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-216945 (URN)10.1016/j.nme.2017.03.002 (DOI)000417293300073 ()2-s2.0-85015302256 (Scopus ID)
Funder
Swedish Foundation for Strategic Research
Note

QC 20171025

Available from: 2017-10-24 Created: 2017-10-24 Last updated: 2019-01-28Bibliographically approved
6. Sputtering of polished EUROFER97 steel: Surface structure modification and enrichment with tungsten and tantalum
Open this publication in new window or tab >>Sputtering of polished EUROFER97 steel: Surface structure modification and enrichment with tungsten and tantalum
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2018 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 508, p. 139-146Article in journal (Refereed) Published
Abstract [en]

Surface structure modification and enrichment with tungsten and tantalum were measured for polished EUROFER97 samples after exposure to a deuterium ion beam. Time-of-flight medium energy ion scattering and time-of-flight elastic recoil detection analysis were implemented for measuring atomic composition profiles. Atomic force microscopy and optical microscopy were used to investigate surface morphology. The deuterium particle fluence was varied between 1021 D/m2 and 1024 D/m2, projectile energy was 200 eV/D and exposure temperatures up to 1050 K were applied. The average fraction of tungsten plus tantalum to total metal content in the 2 nm closest to the sample surface was increased from an initial 0.0046 to 0.12 for the sample exposed to the highest fluence at room temperature. The enrichment was accompanied by an increase in surface roughness of one order of magnitude and grain dependent erosion of the material. The appearance of protrusions with heights up to approximately 40 nm after ion beam exposure at room temperature was observed on individual grains. Samples exposed to 1023 D/m2 at temperatures of 900 K and 1050 K displayed recrystallization and cracking while changes to the total surface fraction of tungsten and tantalum were limited to less than a factor of two compared to the sample exposed to the same fluence at room temperature.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
AFM, Erosion, EUROFER, Sputtering, ToF-ERDA, ToF-MEIS
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-229259 (URN)10.1016/j.jnucmat.2018.05.031 (DOI)000439134500016 ()2-s2.0-85047329344 (Scopus ID)
Funder
Swedish Research Council, 821-2012-5144Swedish Foundation for Strategic Research , RIF14-0053EU, European Research Council
Note

QC 20180601

Available from: 2018-06-01 Created: 2018-06-01 Last updated: 2019-01-09Bibliographically approved
7. Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wall
Open this publication in new window or tab >>Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wall
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2019 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 516, p. 202-213Article in journal (Refereed) Published
Abstract [en]

Inconel-600 blocks and stainless steel covers for quartz microbalance crystals from remote corners in the JET-ILW divertor were studied with time-of-flight elastic recoil detection analysis and nuclear reaction analysis to obtain information about the areal densities and depth profiles of elements present in deposited material layers. Surface morphology and the composition of dust particles were examined with scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analyzed components were present in JET during three ITER-like wall campaigns between 2010 and 2017. Deposited layers had a stratified structure, primarily made up of beryllium, carbon and oxygen with varying atomic fractions of deuterium, up to more than 20%. The range of carbon transport from the ribs of the divertor carrier was limited to a few centimeters, and carbon/deuterium co-deposition was indicated on the Inconel blocks. High atomic fractions of deuterium were also found in almost carbon-free layers on the quartz microbalance covers. Layer thicknesses up to more than 1 micrometer were indicated, but typical values were on the order of a few hundred nanometers. Chromium, iron and nickel fractions were less than or around 1% at layer surfaces while increasing close to the layer-substrate interface. The tungsten fraction depended on the proximity of the plasma strike point to the divertor corners. Particles of tungsten, molybdenum and copper with sizes less than or around 1 micrometer were found. Nitrogen, argon and neon were present after plasma edge cooling and disruption mitigation. Oxygen-18 was found on component surfaces after injection, indicating in-vessel oxidation. Compensation of elastic recoil detection data for detection efficiency and ion-induced release of deuterium during the measurement gave quantitative agreement with nuclear reaction analysis, which strengthens the validity of the results.

Keywords
Fusion, Tokamak, Plasma-wall interactions, ToF-ERDA, NRA, SEM
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-240616 (URN)10.1016/j.jnucmat.2018.11.027 (DOI)000458897100020 ()2-s2.0-85060313456 (Scopus ID)
Note

QC 20190125

Available from: 2018-12-20 Created: 2018-12-20 Last updated: 2019-08-08Bibliographically approved

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