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Possnert, Göran
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Publications (10 of 263) Show all publications
Sias, G., Cecconello, M., Klimek, I., Wodniak, I., Yadykin, D., Andersson Sundén, E., . . . 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.

Keywords
Tokamak, Disruption prediction, Locked mode signal, Disruption indicators, Feature extraction
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-377710 (URN)10.1016/j.fusengdes.2018.11.021 (DOI)000457663100032 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.fusengdes.2018.11.021

Available from: 2019-03-08 Created: 2019-03-08 Last updated: 2019-03-08Bibliographically approved
Ström, P., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (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 analysed 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 mu m were indicated, but typical values were on the order of a few hundred nm. 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 mu m 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
Identifiers
urn:nbn:se:uu:diva-379019 (URN)10.1016/j.jnucmat.2018.11.027 (DOI)000458897100020 ()
Funder
Swedish Foundation for Strategic Research , RIF14-0053Swedish Research Council, 821-2012-5144Swedish Research Council, 2015-04884Swedish Research Council, 2017-00643
Note

For complete list of authors see http://dx.doi.org/10.1016/j.jnucmat.2018.11.027

Available from: 2019-03-12 Created: 2019-03-12 Last updated: 2019-04-18Bibliographically approved
Drenik, A., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . 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.

Keywords
JET, ITER-like wall, Plasma-wall interaction, Image analysis
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-378736 (URN)10.1016/j.fusengdes.2018.12.079 (DOI)000458939100016 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.fusengdes.2018.12.079

Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-11Bibliographically approved
Neverov, V. S., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . 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:uu:diva-378618 (URN)10.1088/1741-4326/ab0000 (DOI)000458371300001 ()
Note

For complete list of authors see http://dx.doi.org/10.1088/1741-4326/ab0000

Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-11Bibliographically approved
Yeung, M. S. Y., Djelloul, M., Steiner, E., Bernard, S., Salehpour, M., Possnert, G., . . . Frisen, J. (2019). Dynamics of oligodendrocyte generation in multiple sclerosis. Nature, 566(7745), 538-+
Open this publication in new window or tab >>Dynamics of oligodendrocyte generation in multiple sclerosis
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2019 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 566, no 7745, p. 538-+Article in journal (Refereed) Published
Abstract [en]

Oligodendrocytes wrap nerve fibres in the central nervous system with layers of specialized cell membrane to form myelin sheaths(1). Myelin is destroyed by the immune system in multiple sclerosis, but myelin is thought to regenerate and neurological function can be recovered. In animal models of demyelinating disease, myelin is regenerated by newly generated oligodendrocytes, and remaining mature oligodendrocytes do not seem to contribute to this process(2-4). Given the major differences in the dynamics of oligodendrocyte generation and adaptive myelination between rodents and humans(5-9), it is not clear how well experimental animal models reflect the situation in multiple sclerosis. Here, by measuring the integration of C-14 derived from nuclear testing in genomic DNA(10), we assess the dynamics of oligodendrocyte generation in patients with multiple sclerosis. The generation of new oligodendrocytes was increased several-fold in normal-appearing white matter in a subset of individuals with very aggressive multiple sclerosis, but not in most subjects with the disease, demonstrating an inherent potential to substantially increase oligodendrocyte generation that fails in most patients. Oligodendrocytes in shadow plaques-thinly myelinated lesions that are thought to represent remyelinated areas-were old in patients with multiple sclerosis. The absence of new oligodendrocytes in shadow plaques suggests that remyelination of lesions occurs transiently or not at all, or that myelin is regenerated by pre-existing, and not new, oligodendrocytes in multiple sclerosis. We report unexpected oligodendrocyte generation dynamics in multiple sclerosis, and this should guide the use of current, and the development of new, therapies.

National Category
Neurology Neurosciences
Identifiers
urn:nbn:se:uu:diva-379035 (URN)10.1038/s41586-018-0842-3 (DOI)000459769100050 ()30675058 (PubMedID)
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Knut and Alice Wallenberg FoundationEU, European Research Council
Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-11Bibliographically approved
Vasilopoulou, T., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . 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.

Keywords
Neutron activation, Neutron dosimetry, JET, Fusion
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-378735 (URN)10.1016/j.fusengdes.2019.01.002 (DOI)000458939100015 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.fusengdes.2019.01.002

Available from: 2019-03-08 Created: 2019-03-08 Last updated: 2019-03-08Bibliographically approved
O'Hare, P., Mekhaldi, F., Adolphi, F., Raisbeck, G., Aldahan, A., Anderberg, E., . . . Muscheler, R. (2019). Multiradionuclide evidence for an extreme solar proton event around 2,610 BP (similar to 660 BC). Proceedings of the National Academy of Sciences of the United States of America, 116(13), 5961-5966
Open this publication in new window or tab >>Multiradionuclide evidence for an extreme solar proton event around 2,610 BP (similar to 660 BC)
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2019 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 116, no 13, p. 5961-5966Article in journal (Refereed) Published
Abstract [en]

Recently, it has been confirmed that extreme solar proton events can lead to significantly increased atmospheric production rates of cosmogenic radionuclides. Evidence of such events is recorded in annually resolved natural archives, such as tree rings [carbon-14 (C-14)] and ice cores [beryllium-10 (Be-10), chlorine-36 (Cl-36)]. Here, we show evidence for an extreme solar event around 2,610 years B.P. (similar to 660 BC) based on high-resolution Be-10 data from two Greenland ice cores. Our conclusions are supported by modeled C-14 production rates for the same period. Using existing Cl-36 ice core data in conjunction with Be-10, we further show that this solar event was characterized by a very hard energy spectrum. These results indicate that the 2,610-years B.P. event was an order of magnitude stronger than any solar event recorded during the instrumental period and comparable with the solar proton event of AD 774/775, the largest solar event known to date. The results illustrate the importance of multiple ice core radionuclide measurements for the reliable identification of short-term production rate increases and the assessment of their origins.

Place, publisher, year, edition, pages
NATL ACAD SCIENCES, 2019
Keywords
solar storms, radionuclides, ice cores, solar proton events
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-381578 (URN)10.1073/pnas.1815725116 (DOI)000462382800026 ()30858311 (PubMedID)
Funder
Swedish Research Council, DNR2016-00218Swedish Research Council, DNR2013-8421Swedish Research CouncilSwedish Polar Research Secretariat
Available from: 2019-04-12 Created: 2019-04-12 Last updated: 2019-04-12Bibliographically approved
Lawson, K. D., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). Population modelling of the He II energy levels in tokamak plasmas: I. Collisional excitation model. 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, ISSN 0953-4075, E-ISSN 1361-6455, 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
Institute of Physics Publishing (IOPP), 2019
Keywords
He II, population modelling, tokamak plasmas, spectral line intensity ratios
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-376712 (URN)10.1088/1361-6455/aaf703 (DOI)000456847500001 ()
Note

For complete list of authors see http://dx.doi.org/10.1088/1361-6455/aaf703

Available from: 2019-02-12 Created: 2019-02-12 Last updated: 2019-02-12Bibliographically approved
Hatano, Y., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). Tritium distributions on W-coated divertor tiles used in the third JET ITER-like wall campaign. Nuclear Materials and Energy, 18, 258-261
Open this publication in new window or tab >>Tritium distributions on W-coated divertor tiles used in the third JET ITER-like wall campaign
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2019 (English)In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 18, p. 258-261Article in journal (Refereed) Published
Abstract [en]

Tritium (T) distributions on tungsten (W)-coated plasma-facing tiles used in the third ITER-like wall campaign (2015-2016) of the Joint European Torus (JET) were examined by means of an imaging plate technique and beta-ray induced x-ray spectrometry, and they were compared with the distributions after the second (2013-2014) campaign. Strong enrichment of T in beryllium (Be) deposition layers was observed after the second campaign. In contrast, T distributions after the third campaign was more uniform though Be deposition layers were visually recognized. The one of the possible explanations is enhanced desorption of T from Be deposition layers due to higher tile temperatures caused by higher energy input in the third campaign.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Divertor, Tritium, Retention, Deposition, Radiation
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-379779 (URN)10.1016/j.nme.2019.01.001 (DOI)000460107500044 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.nme.2019.01.001

Available from: 2019-03-21 Created: 2019-03-21 Last updated: 2019-03-21Bibliographically approved
Apel, J., Wallin, P., Stora, J. & Possnert, G. (2018). Early Holocene human population events on the island of Gotland in the Baltic Sea (9200-3800 cal. BP). Quaternary International, 465, 276-286
Open this publication in new window or tab >>Early Holocene human population events on the island of Gotland in the Baltic Sea (9200-3800 cal. BP)
2018 (English)In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 465, p. 276-286Article in journal (Refereed) Published
Abstract [en]

The summed probability distribution of 162 radiocarbon dates from Gotland was analysed with reference to archaeological and environmental data in order to evaluate possible variations in settlement intensity on the island. The data indicated variations in demographic development on the island, with probably several different colonization events and external influences; the pioneer settlement reached the island around 9200 cal. BP. After the initial colonization, the radiocarbon dates were rather evenly distributed until around 7700-7600 cal. BP, then there was a drop in the number of dates between 8300 and 8000 cal. BP that may be associated with the 8200 cold event. A marked decline in the number of dates between 7600 and 6000 cal. BP may be associated initially with the Littorina I transgression, but this transgression cannot explain why the Late Mesolithic period is not well represented on Gotland: the climatic development was favourable but did not result in increased human activity. The number of radiocarbon dates indicated that the population size remained low until around 6000 cal. BP, after which there was a gradual increase that reached a first 'threshold' after 5600 cal. BP and a second 'threshold' after 4500 cal. BP. The first apparent population increase was associated with the appearance of the Funnel Beaker Culture (FBC) and the second with Pitted Ware Culture (PWC) complexes. A decline in the number of dates occurred after 4300 cal. BP, i.e. towards the Late Neolithic. There was an association between the frequency distributions of the radiocarbon dates and the number of stray finds from different time periods but any correlation was not straightforward. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2018
Keywords
Gotland, Mesolithic, Neolithic, Settlement intensity, Population events, Demography
National Category
Archaeology
Identifiers
urn:nbn:se:uu:diva-351059 (URN)10.1016/j.quaint.2017.03.044 (DOI)000427454900010 ()
Funder
Swedish Research Council
Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-05-30Bibliographically approved
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