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Liquid scintillators as neutron diagnostic tools for fusion plasmas: System characterization and data analysis
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics. (Nuclear Fusion)
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

The neutrons produced in fusion devices carry information about various properties of the ions that are reacting in the machine. Measurements of the neutron flux and energy distribution can therefore be used to study the behaviour of the plasma ions under different experimental conditions.

Several neutron detection techniques are available, each having advantages and disadvantages compared to the others. In this thesis we study neutron measurements performed with NE213 liquid scintillators. One advantage of NE213s compared to other neutron detection techniques is that they are simple to use, small and cheap. On the other hand, their response to neutrons makes the extraction of information about the neutron energy less precise.

In the thesis we present the development of methods for the characterization and the data analysis of NE213 detectors. The work was performed using two instruments installed at the Joint European Torus (JET) tokamak in the UK: the “Afterburner” detector, which is an NE213 installed on a tangential line of sight, and the neutron camera, which is a system composed of 19 NE213 detectors installed on different lines of sight (10 horizontal and 9 vertical).The analysis of data from the Afterburner detector was focused on resolving different features of the neutron energy spectra which are related to different properties of the ion velocity distribution.

The analysis of data from the neutron camera was directed towards the investigation of the spatial distribution of ions in the plasma. However, the individual characterization of the camera detectors allowed the inclusion of information about the energy distribution of the ions in the analysis.

The outcomes of the studies performed indicate that the methods developed give reliable results and can therefore be applied to extract information about the plasma ions. In particular, the possibility of performing neutron emission spectroscopy analysis in each line of sight of a neutron camera is of great value for future studies.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. , p. 87
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1646
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-345983ISBN: 978-91-513-0275-1 (print)OAI: oai:DiVA.org:uu-345983DiVA, id: diva2:1190103
Public defence
2018-05-04, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2018-04-11 Created: 2018-03-13 Last updated: 2018-04-11
List of papers
1. Generation of the neutron response function of an NE213 scintillator for fusion applications
Open this publication in new window or tab >>Generation of the neutron response function of an NE213 scintillator for fusion applications
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2017 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 866, p. 222-229Article in journal (Refereed) Published
Abstract [en]

In this work we present a method to evaluate the neutron response function of an NE213 liquid scintillator. This method is particularly useful when the proton light yield function of the detector has not been measured, since it is based on a proton light yield function taken from literature, MCNPX simulations, measurements of gammarays from a calibration source and measurements of neutrons from fusion experiments with ohmic plasmas. The inclusion of the latter improves the description of the proton light yield function in the energy range of interest (around 2.46 MeV). We apply this method to an NE213 detector installed at JET, inside the radiation shielding of the magnetic proton recoil (MPRu) spectrometer, and present the results from the calibration along with some examples of application of the response function to perform neutron emission spectroscopy (NES) of fusion plasmas. We also investigate how the choice of the proton light yield function affects the NES analysis, finding that the result does not change significantly. This points to the fact that the method for the evaluation of the neutron response function is robust and gives reliable results.

Keywords
NE213 scintillator, Neutron spectroscopy, Response function, Proton light yield
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-330537 (URN)10.1016/j.nima.2017.04.023 (DOI)000407863700029 ()
Available from: 2017-10-04 Created: 2017-10-04 Last updated: 2018-04-23Bibliographically approved
2. Forward fitting of experimental data from a NE213 neutron detector installed with the magnetic proton recoil upgraded spectrometer at JET
Open this publication in new window or tab >>Forward fitting of experimental data from a NE213 neutron detector installed with the magnetic proton recoil upgraded spectrometer at JET
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2014 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 85, no 11, p. 11E123-Article in journal (Refereed) Published
Abstract [en]

In this paper, we present the results obtained from the data analysis of neutron spectra measured with a NE213 liquid scintillator at JET. We calculated the neutron response matrix of the instrument combining MCNPX simulations, a generic proton light output function measured with another detector and the fit of data from ohmic pulses. For the analysis, we selected a set of pulses with neutral beam injection heating (NBI) only and we applied a forward fitting procedure of modeled spectral components to extract the fraction of thermal neutron emission. The results showed the same trend of the ones obtained with the dedicated spectrometer TOFOR, even though the values from the NE213 analysis were systematically higher. This discrepancy is probably due to the different lines of sight of the two spectrometers (tangential for the NE213, vertical for TOFOR). The uncertainties on the thermal fraction estimates were from 4 to 7 times higher than the ones from the TOFOR analysis.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-240136 (URN)10.1063/1.4895565 (DOI)000345646000143 ()25430302 (PubMedID)
Available from: 2015-01-07 Created: 2015-01-05 Last updated: 2018-04-23
3. Dual sightline measurements of MeV range deuterons with neutron and gamma-ray spectroscopy at JET
Open this publication in new window or tab >>Dual sightline measurements of MeV range deuterons with neutron and gamma-ray spectroscopy at JET
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2015 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 55, no 12, article id 123026Article in journal (Refereed) Published
Abstract [en]

Observations made in a JET experiment aimed at accelerating deuterons to the MeV range by third harmonic radio-frequency (RF) heating coupled into a deuterium beam are reported. Measurements are based on a set of advanced neutron and gamma-ray spectrometers that, for the first time, observe the plasma simultaneously along vertical and oblique lines of sight. Parameters of the fast ion energy distribution, such as the high energy cut-off of the deuteron distribution function and the RF coupling constant, are determined from data within a uniform analysis framework for neutron and gamma-ray spectroscopy based on a one-dimensional model and by a consistency check among the individual measurement techniques. A systematic difference is seen between the two lines of sight and is interpreted to originate from the sensitivity of the oblique detectors to the pitch-angle structure of the distribution around the resonance, which is not correctly portrayed within the adopted one dimensional model. A framework to calculate neutron and gamma-ray emission from a spatially resolved, two-dimensional deuteron distribution specified by energy/pitch is thus developed and used for a first comparison with predictions from ab initio models of RF heating at multiple harmonics.

The results presented in this paper are of relevance for the development of advanced diagnostic techniques for MeV range ions in high performance fusion plasmas, with applications to the experimental validation of RF heating codes and, more generally, to studies of the energy distribution of ions in the MeV range in high performance deuterium and deuterium-tritium plasmas.

Keywords
fusion, tokamak, fast ions, neutron spectrometry, gamma-ray spectroscopy
National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-247990 (URN)10.1088/0029-5515/55/12/123026 (DOI)000366534500028 ()
Available from: 2015-03-25 Created: 2015-03-25 Last updated: 2018-04-23Bibliographically approved
4. Absolute calibration of the JET neutron profile monitor
Open this publication in new window or tab >>Absolute calibration of the JET neutron profile monitor
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(English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623Article in journal (Refereed) Submitted
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-345980 (URN)
Available from: 2018-03-13 Created: 2018-03-13 Last updated: 2018-03-13
5. Calculation of the profile-dependent neutron backscatter matrix for the JET neutron camera system
Open this publication in new window or tab >>Calculation of the profile-dependent neutron backscatter matrix for the JET neutron camera system
2017 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 123, p. 865-868Article in journal (Refereed) Published
Abstract [en]

We investigated the dependence of the backscatter component of the neutron spectrum on the emissivity profile. We did so for the JET neutron camera system, by calculating a profile-dependent backscatter matrix for each of the 19 camera channels using a MCNP model of the JET tokamak. We found that, when using a low minimum energy for the summation of the counts in the neutron pulse height spectrum, the backscatter contribution can depend significantly on the emissivity profile. The maximum variation in the backscatter level was 24% (8.0% when compared to the total emission). This effect needs to be considered when a correction for the backscatter contribution is applied to the measured profile.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2017
Keywords
Neutron, Profile monitor, Backscatter, mcnp
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-341822 (URN)10.1016/j.fusengdes.2017.03.124 (DOI)000418992000181 ()
Conference
29th Symposium on Fusion Technology (SOFT), SEP 05-09, 2016, Prague, CZECH REPUBLIC
Available from: 2018-02-15 Created: 2018-02-15 Last updated: 2018-03-13Bibliographically approved
6. Study of the energy-dependent fast ion redistribution during sawtooth oscillations with the neutron camera at JET
Open this publication in new window or tab >>Study of the energy-dependent fast ion redistribution during sawtooth oscillations with the neutron camera at JET
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(English)Manuscript (preprint) (Other academic)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-345982 (URN)
Available from: 2018-03-13 Created: 2018-03-13 Last updated: 2018-04-23

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