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Publications (10 of 223) Show all publications
Sias, G., Frassinetti, L., Hellsten, T., Petersson, P., Rubel, M., Fridström, R., . . . Zychor, I. (2019). A locked mode indicator for disruption prediction on JET and ASDEX upgrade. Fusion engineering and design, 138, 254-266
Open this publication in new window or tab >>A locked mode indicator for disruption prediction on JET and ASDEX upgrade
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2019 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 138, p. 254-266Article in journal (Refereed) Published
Abstract [en]

The aim of this paper is to present a signal processing algorithm that, applied to the raw Locked Mode signal, allows us to obtain a disruption indicator in principle exploitable on different tokamaks. A common definition of such an indicator for different machines would facilitate the development of portable systems for disruption prediction, which is becoming of increasingly importance for the next tokamak generations. Moreover, the indicator allows us to overcome some intrinsic problems in the diagnostic system such as drift and offset. The behavior of the proposed indicator as disruption predictor, based on crossing optimized thresholds of the signal amplitude, has been analyzed using data of both JET and ASDEX Upgrade experiments. A thorough analysis of the disruption prediction performance shows how the indicator is able to recover some missed and tardy detections of the raw signal. Moreover, it intervenes and corrects premature or even wrong alarms due to, e.g., drifts and/or offsets.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
Tokamak, Disruption prediction, Locked mode signal, Disruption indicators, Feature extraction
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-244550 (URN)10.1016/j.fusengdes.2018.11.021 (DOI)000457663100032 ()2-s2.0-85057571211 (Scopus ID)
Note

QC 20190315

Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-03-15Bibliographically approved
Ström, P., Petersson, P., Rubel, M., Fortuna-Zaleśna, E., Widdowson, A. & Sergienko, G. (2019). Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wall. Journal of Nuclear Materials, 516, 202-213
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: 2019-03-15Bibliographically approved
Drenik, A., Bergsåker, H., Bykov, I., Frassinetti, L., Garcia Carrasco, A., Hellsten, T., . . . Zychor, I. (2019). Analysis of the outer divertor hot spot activity in the protection video camera recordings at JET. Fusion engineering and design, 139, 115-123
Open this publication in new window or tab >>Analysis of the outer divertor hot spot activity in the protection video camera recordings at JET
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2019 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 139, p. 115-123Article in journal (Refereed) Published
Abstract [en]

Hot spots on the divertor tiles at JET result in overestimation of the tile surface temperature which causes unnecessary termination of pulses. However, the appearance of hot spots can also indicate the condition of the divertor tile surfaces. To analyse the behaviour of the hot spots in the outer divertor tiles of JET, a simple image processing algorithm is developed. The algorithm isolates areas of bright pixels in the camera image and compares them to previously identified hot spots. The activity of the hot spots is then linked to values of other signals and parameters in the same time intervals. The operation of the detection algorithm was studied in a limited pulse range with high hot spot activity on the divertor tiles 5, 6 and 7. This allowed us to optimise the values of the controlling parameters. Then, the wider applicability of the method has been demonstrated by the analysis of the hot spot behaviour in a whole experimental campaign.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
JET, ITER-like wall, Plasma-wall interaction, Image analysis
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-245939 (URN)10.1016/j.fusengdes.2018.12.079 (DOI)000458939100016 ()2-s2.0-85059687937 (Scopus ID)
Note

QC 20190318

Available from: 2019-03-18 Created: 2019-03-18 Last updated: 2019-03-18Bibliographically approved
Neverov, V. S., Bergsåker, H., Bykov, I., Frassinetti, L., Garcia Carrasco, A., Hellsten, T., . . . Zychor, I. (2019). Determination of isotope ratio in the divertor of JET-ILW by high-resolution H alpha spectroscopy: H-D experiment and implications for D-T experiment. Nuclear Fusion, 59(4), Article ID 046011.
Open this publication in new window or tab >>Determination of isotope ratio in the divertor of JET-ILW by high-resolution H alpha spectroscopy: H-D experiment and implications for D-T experiment
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2019 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, no 4, article id 046011Article in journal (Refereed) Published
Abstract [en]

The data of the H alpha high-resolution spectroscopy, collected on the multiple lines of sight, which cover the entire divertor space in poloidal cross-section, during the recent hydrogen-deuterium experiments in JET-ILW (ITER-like wall), are processed. A strong spatial inhomogeneity of the hydrogen concentration, H/(H + D), in divertor is found in many pulses. Namely, the H/(H + D) ratio may be lower in the inner divertor than that in the outer divertor by the values of 0.15-0.35, depending on the conditions of gas puffing and plasma heating. This effect suggests the necessity of spatially-resolved measurements of isotope ratio in the divertor in the upcoming deuterium-tritium experiments. Also, separation of the overlapped T alpha and D alpha spectral lines is shown to be a challenging task especially when the local Doppler-broadened (Gaussian) line shapes are noticeably distorted by the net inward flux of fast non-Maxwellian neutral atoms. We use the respective, formerly developed model of an asymmetric spectral line shape, while analysing the data of the first deuterium-tritium experiment in JET-C (carbon wall), and test the model via comparing the isotope ratio results with another diagnostic's measurements. This model is shown to increase the accuracy of tritium concentration measurements in the divertor.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
tokamak diagnostics, spectral line shapes, inverse problems, isotope ratio
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-245124 (URN)10.1088/1741-4326/ab0000 (DOI)000458371300001 ()
Note

QC 20190318

Available from: 2019-03-18 Created: 2019-03-18 Last updated: 2019-03-18Bibliographically approved
Vasilopoulou, T., Bergsåker, H., Bykov, I., Frassinetti, L., Garcia Carrasco, A., Hellsten, T., . . . Zychor, I. (2019). Improved neutron activation dosimetry for fusion. Fusion engineering and design, 139, 109-114
Open this publication in new window or tab >>Improved neutron activation dosimetry for fusion
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2019 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 139, p. 109-114Article in journal (Refereed) Published
Abstract [en]

Neutron activation technique has been widely used for the monitoring of neutron fluence at the Joint European Torus (JET) whereas it is foreseen to be employed at future fusion plants, such as ITER and DEMO. Neutron activation provides a robust tool for the measurement of neutron fluence in the complex environment encountered in a tokamak. However, activation experiments previously performed at JET showed that the activation foils used need to be calibrated in a real fusion environment in order to provide accurate neutron fluence data. Triggered by this challenge, an improved neutron activation method for the evaluation of neutron fluence at fusion devices has been developed. Activation assemblies similar to those used at JET were irradiated under 14 MeV neutrons at the Frascati Neutron Generator (FNG) reference neutron field. The data obtained from the calibration experiment were applied for the analysis of activation foil measurements performed during the implemented JET Deuterium-Deuterium (D-D) campaign. The activation results were compared against thermoluminescence measurements and a satisfactory agreement was observed. The proposed method provides confidence on the use of activation technique for the precise estimation of neutron fluence at fusion devices and enables its successful implementation in the forthcoming JET Deuterium-Tritium (D-T) campaign.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Neutron activation, Neutron dosimetry, JET, Fusion
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-245937 (URN)10.1016/j.fusengdes.2019.01.002 (DOI)000458939100015 ()2-s2.0-85059692263 (Scopus ID)
Note

QC 201900318

Available from: 2019-03-18 Created: 2019-03-18 Last updated: 2019-03-18Bibliographically approved
Coad, J. P., Rubel, M., Likonen, J., Bekris, N., Brezinsek, S., Matthew, G. F., . . . Widdowson, A. M. (2019). Material migration and fuel retention studies during the JET carbon divertor campaigns. Fusion engineering and design, 138, 78-108
Open this publication in new window or tab >>Material migration and fuel retention studies during the JET carbon divertor campaigns
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2019 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 138, p. 78-108Article in journal (Refereed) Published
Abstract [en]

The first divertor was installed in the JET machine between 1992 and 1994 and was operated with carbon tiles and then beryllium tiles in 1994-5. Post-mortem studies after these first experiments demonstrated that most of the impurities deposited in the divertor originate in the main chamber, and that asymmetric deposition patterns generally favouring the inner divertor region result from drift in the scrape-off layer. A new monolithic divertor structure was installed in 1996 which produced heavy deposition at shadowed areas in the inner divertor corner, which is where the majority of the tritium was trapped by co-deposition during the deuterium-tritium experiment in 1997. Different divertor geometries have been tested since such as the Gas-Box and High-Delta divertors; a principle objective has been to predict plasma behaviour, transport and tritium retention in ITER. Transport modelling experiments were carried out at the end of four campaigns by puffing C-13-labelled methane, and a range of diagnostics such as quartz-microbalance and rotating collectors have been installed to add time resolution to the post-mortem analyses. The study of material migration after D-D and D-T campaigns clearly revealed important consequences of fuel retention in the presence of carbon walls. They gave a strong impulse to make a fundamental change of wall materials. In 2010 the carbon divertor and wall tiles were removed and replaced with tiles with Be or W surfaces for the ITER-Like Wall Project.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
Fusion, JET, Divertor, Carbon, Plasma-facing components
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-244549 (URN)10.1016/j.fusengdes.2018.10.002 (DOI)000457663100013 ()2-s2.0-85056661344 (Scopus ID)
Note

QC 20190313

Available from: 2019-03-13 Created: 2019-03-13 Last updated: 2019-03-13Bibliographically approved
Lawson, K. D., Bergsåker, H., Bykov, I., Frassinetti, L., Garcia Carrasco, A., Hellsten, T., . . . et al., . (2019). Population modelling of the He II energy levels in tokamak plasmas: I. Collisional excitation model. Paper presented at ARK REH, 1982, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, V49, P545. JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 52(4), Article ID 045001.
Open this publication in new window or tab >>Population modelling of the He II energy levels in tokamak plasmas: I. Collisional excitation model
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2019 (English)In: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol. 52, no 4, article id 045001Article in journal (Refereed) Published
Abstract [en]

Helium is widely used as a fuel or minority gas in laboratory fusion experiments, and will be present as ash in DT thermonuclear plasmas. It is therefore essential to have a good understanding of its atomic physics. To this end He II population modelling has been undertaken for the spectroscopic levels arising from shells with principal quantum number n = 1-5. This paper focuses on a collisional excitation model; ionisation and recombination will be considered in a subsequent article. Heavy particle collisional excitation rate coefficients have been generated to supplement the currently-available atomic data for He II, and are presented for proton, deuteron, triton and alpha-particle projectiles. The widely-used criterion for levels within an n shell being populated in proportion to their statistical weights is reassessed with the most recent atomic data, and found not to apply to the He II levels at tokamak densities (10(18)-10(21) m(-3)). Consequences of this and other likely sources of errors are quantified, as is the effect of differing electron and ion temperatures. Line intensity ratios, including the so-called 'branching ratios' and the fine-structure beta(1), beta(2), beta(3), and gamma ratios, are discussed, the latter with regard to their possible use as diagnostics.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
He II, population modelling, tokamak plasmas, spectral line intensity ratios
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-243932 (URN)10.1088/1361-6455/aaf703 (DOI)000456847500001 ()
Conference
ARK REH, 1982, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, V49, P545
Note

QC 20190212

Available from: 2019-02-12 Created: 2019-02-12 Last updated: 2019-02-12Bibliographically approved
Oya, Y., Masuzaki, S., Tokitani, M., Azuma, K., Oyaidzu, M., Isobe, K., . . . Contributors, J. (2018). Correlation of surface chemical states with hydrogen isotope retention in divertor tiles of JET with ITER-Like Wall. Fusion engineering and design, 132, 24-28
Open this publication in new window or tab >>Correlation of surface chemical states with hydrogen isotope retention in divertor tiles of JET with ITER-Like Wall
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2018 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 132, p. 24-28Article in journal (Refereed) Published
Abstract [en]

To understand the fuel retention mechanism correlation of surface chemical states and hydrogen isotope retention behavior determined by XPS (X-ray photoelectron spectroscopy) and TDS (Thermal desorption spectroscopy), respectively, for JET ITER-Like Wall samples from operational period 2011–2012 were investigated. It was found that the deposition layer was formed on the upper part of the inner vertical divertor area. At the inner plasma strike point region, the original surface materials, W or Mo, were found, indicating to an erosion-dominated region, but deposition of impurities was also found. Higher heat load would induce the formation of metal carbide. At the outer horizontal divertor tile, mixed material layer was formed with iron as an impurity. TDS showed the H and D desorption behavior and the major D desorption temperature for the upper part of the inner vertical tile was located at 370 °C and 530 °C. At the strike point region, the D desorption temperature was clearly shifted toward higher release temperatures, indicating the stabilization of D trapping by higher heat load.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Hydrogen isotope behavior, JET-ILW, Plasma wall interactions
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-228730 (URN)10.1016/j.fusengdes.2018.04.124 (DOI)000436887700005 ()2-s2.0-85046629650 (Scopus ID)
Funder
EU, European Research Council
Note

QC 20180529

Available from: 2018-05-29 Created: 2018-05-29 Last updated: 2018-07-17Bibliographically approved
Rubel, M., Widdowson, A., Grzonka, J., Fortuna-Zalesna, E., Moon, S., Petersson, P., . . . Tokitani, M. (2018). Dust generation in tokamaks: Overview of beryllium and tungsten dust characterisation in JET with the ITER-like wall. Paper presented at 13th International Symposium on Fusion Nuclear Technology (ISFNT), SEP 25-29, 2017, Kyoto, Japan. Fusion engineering and design, 136, 579-586
Open this publication in new window or tab >>Dust generation in tokamaks: Overview of beryllium and tungsten dust characterisation in JET with the ITER-like wall
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2018 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 136, p. 579-586Article in journal (Refereed) Published
Abstract [en]

Operation of the JET tokamak with beryllium and tungsten ITER-like wall provides unique opportunity for detailed studies on dust generation: quantity, morphology, location, etc. The programme carried out in response to ITER needs for safety assessment comprises: (i) remotely controlled vacuum cleaning of the divertor; (ii) local sampling of loosely bound matter from plasma-facing components (PFC); (iii) collection of mobilized dust on various erosion-deposition probes located in the divertor and in the main chamber. Results of comprehensive analyses performed by a number of complementary techniques, e.g. a range of microscopy methods, electron and ion spectroscopy, liquid scintillography and thermal desorption, are summarized by following points: (a) Total amount of dust collected by vacuum cleaning after three campaigns is about 1-1.4 g per campaign (19.1-23.5 h plasma operation), i.e. over 100 times smaller than in JET operated with carbon walls (i.e. in JET-C). (b) Two major categories of Be dust are identified: flakes of co-deposits formed on PFC and droplets (2-10 mu m in diameter). Small quantifies, below 1 g, of Be droplets and splashes are associated mainly with melting of beryllium limiters.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
JET tokamak, ITER-like wall, Dust, Beryllium, Tungsten
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-240780 (URN)10.1016/j.fusengdes.2018.03.027 (DOI)000452583700110 ()2-s2.0-85044008512 (Scopus ID)
Conference
13th International Symposium on Fusion Nuclear Technology (ISFNT), SEP 25-29, 2017, Kyoto, Japan
Note

QC 20190109

Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2019-01-09Bibliographically approved
Ström, P., Petersson, P., Rubel, M. & Possnert, G. (2018). 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)]. Review of Scientific Instruments, 89(4), Article ID 049901.
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
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9901-6296

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