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Fusion Plasma Observations at JET with the TOFOR Neutron Spectrometer: Instrumental Challenges and Physics Results
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The neutron spectrometer TOFOR was installed at JET in 2005 for high-rate observation of neutrons from reactions between two deuterium (D) ions. Neutron spectrometry as a fusion plasma diagnostic technique is invoked to obtain information about the velocity states of fusion fuel ions. Based on neutron spectrometry data, conclusions can be drawn on the efficiency of plasma heating schemes as well as optimization of fuel ion confinement. The quality of TOFOR analysis is found to depend on how well the instrument response function is known; discriminator threshold levels, detector time alignment and electronics broadening are identified as crucial issues.

About 19 percent of the neutrons observed with TOFOR have scattered off the JET vessel wall or other structures in the line-of-sight before reaching the instrument, as established through simulations and measurements. A method has been developed to take these neutrons into account in the analysis. TOFOR measurements of fast deuterium distributions are seen to agree with distributions deduced from NPA data, obtained based on an entirely different principle. This serves as validation of the modeling and analysis.

Extraordinary statistics in the TOFOR measurements from JET pulses heated with 3rd harmonic RF heating on D beams allow for study of instabilities using neutron emission spectrometry. At ITER, similar studies should be possible on a more regular basis due to higher neutron rates.

Observations of neutrons from Be+3He reactions in the TOFOR spectrum from D plasmas heated with fundamental RF tuned to minority 3He raise the question of beryllium neutrons at JET after installation of the ITER-like wall, and at ITER, with beryllium as the plasma facing component. This is especially important for the first few years of ITER operation, where the machine will not yet have been certified as a nuclear facility and should be run in zero-activation mode.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2010. , p. 110
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 715
Keywords [en]
Fusion, Plasma diagnostics, Neutron Spectrometry, TOFOR, JET, ITER, time-of-flight, plasma heating
National Category
Subatomic Physics
Research subject
Applied Nuclear Physics
Identifiers
URN: urn:nbn:se:uu:diva-114077ISBN: 978-91-554-7724-0 (print)OAI: oai:DiVA.org:uu-114077DiVA, id: diva2:293215
Public defence
2010-03-26, Polhemsalen, Ångströmlaboratoriet, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2010-03-04 Created: 2010-02-09 Last updated: 2010-06-08Bibliographically approved
List of papers
1. The TOFOR neutron spectrometer and its first use at JET
Open this publication in new window or tab >>The TOFOR neutron spectrometer and its first use at JET
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2006 In: Review of Scientific Instruments, Vol. 77, no 10E702, p. 3-Article in journal (Refereed) Published
Identifiers
urn:nbn:se:uu:diva-96184 (URN)
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2010-02-11Bibliographically approved
2. The 2.5-MeV neutron time-of-flight spectrometer TOFOR for experiments at JET
Open this publication in new window or tab >>The 2.5-MeV neutron time-of-flight spectrometer TOFOR for experiments at JET
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2008 (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. 591, no 2, p. 417-430Article in journal (Refereed) Published
Abstract [en]

A time-of-flight (TOF) spectrometer for measurement of the 2.5-MeV neutron emission from fusion plasmas has been developed and put into use at the JET tokamak. It has been optimized for operation at high rates (TOFOR) for the purpose of performing advanced neutron emission spectroscopy (NES) diagnosis of deuterium plasmas with a focus on the fuel ion motional states for different auxiliary heating scenarios. This requires operation over a large dynamic range, including high rates of > 100 kHz with a maximum value of 0.5 MHz for the TOFOR design. This paper describes the design principles and their technical realization. The performance is illustrated with recent neutron TOF spectra recorded for plasmas subjected to different heating scenarios. A true event count rate of 39 kHz has been achieved at about a tenth of the expected neutron yield limit of JET, giving a projected maximum of 400 kHz at peak JET plasma yield. This means that the count rate capability for NES diagnosis of D plasmas has been improved more than an order of magnitude. Another important performance factor is the spectrometer bandwidth, where data have been acquired and analyzed successfully with a response function for neutrons over the energy range 1 to > 5 MeV. The implications of instrumental advancement represented by TOFOR are discussed.

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-16750 (URN)10.1016/j.nima.2008.03.010 (DOI)000257529700011 ()
Note

Conference Information: 21st IAEA Fusion Energy Conference Chengdu, PEOPLES R CHINA, OCT 16-21, 2006

Available from: 2008-06-05 Created: 2008-06-05 Last updated: 2017-12-08Bibliographically approved
3. Modeling and TOFOR measurements of scattered neutrons at JET
Open this publication in new window or tab >>Modeling and TOFOR measurements of scattered neutrons at JET
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper, the scattered and direct neutron fluxes in the line-of-sight of the TOFOR neutron spectrometer at JET are simulated and the simulations compared with measurement results. The Monte Carlo code MCNPX is used in the simulations, with a vessel material composition obtained from the JET drawing office and neutron emission profiles calculated from TRANSP simulations of beam ion density profiles. The MCNPX simulations show that the material composition of the scattering wall has a large effect on the shape of the scattered neutron spectrum. Neutron source profile shapes as well as radial and vertical source displacements in the TOFOR line-of-sight are shown to only marginally affect the scatter, while having a larger impact on the direct neutron flux. A matrix of simulated scatter spectra for mono-energetic source neutrons is created which is folded with an approximation of the source spectrum for each JET pulse studied to obtain a scatter component for use in the data analysis. The scatter components thus obtained are shown to describe the measured data. It is also demonstrated that the scattered flux is approximately constant relative to the total neutron yield as measured with the JET fission chambers, while there is a larger spread in the direct flux, consistent with simulations. The simulated effect on the integrated scattered/direct ratio of an increase with movements outward along the radial direction and a drop at higher values of the vertical plasma position is also reproduced in the measurements. Finally, the quantitative agreement found in scatter/direct ratios between simulations (0.185±0.005) and measurements (0.187±0.050) serves as a solid benchmark of the MCNPX model used.

Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-114036 (URN)
Note
To be submitted to Plasma Physics and Controlled FusionAvailable from: 2010-02-08 Created: 2010-02-08 Last updated: 2010-02-11
4. Cross-validation of JET fast deuterium results from TOFOR and NPA
Open this publication in new window or tab >>Cross-validation of JET fast deuterium results from TOFOR and NPA
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2009 (English)In: 36th EPS Conference on Plasma Physics, Sofia, June 29 - July 3, 2009 ECA Vol.33E, 2009, p. P-2.151-Conference paper, Published paper (Other academic)
National Category
Physical Sciences
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-114038 (URN)
Conference
36th EPS Conference on Plasma Physics
Available from: 2010-02-08 Created: 2010-02-08 Last updated: 2010-02-18Bibliographically approved
5. Neutron emission generated by fast deuterons accelerated with ion cyclotron heating at JET
Open this publication in new window or tab >>Neutron emission generated by fast deuterons accelerated with ion cyclotron heating at JET
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2010 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, no 2, p. 022001-Article in journal, Letter (Refereed) Published
Abstract [en]

For the first time, the neutron emission from JET plasmas heated with combined deuterium neutral beam injection and third harmonic ion cyclotron radio frequency heating have been studied with neutron emission spectroscopy (NES). Very high DD neutron rates were observed with only modest external heating powers, which was attributed to acceleration of deuterium beam ions to energies of about 2-3 MeV, where the DD reactivity is on a par of that of the DT reaction. Fast deuterium energy distributions were derived from analysis of NES data and confirm acceleration of deuterium beam ions up to energies around 3 MeV, in agreement with theoretical predictions. The high neutron rates allowed for observations of changes in the fast deuterium populations on a time scale of 50 ms. Correlations were seen between fast deuterium ions at different energies and magnetohydrodynamic activities, such as monster sawtooth crashes and toroidal Alfvén eigenmodes.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-114441 (URN)10.1088/0029-5515/50/2/022001 (DOI)000275322200002 ()
Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2017-12-12Bibliographically approved
6. Neutron emission from beryllium reactions in JET deuterium plasmas with 3He minority
Open this publication in new window or tab >>Neutron emission from beryllium reactions in JET deuterium plasmas with 3He minority
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2010 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, no 4, p. 045005-Article in journal (Refereed) Published
Abstract [en]

Recent fast ion studies at JET involve ion cyclotron resonance frequency (ICRF) heating tuned to minority He-3 in cold deuterium plasmas, with beryllium evaporation in the vessel prior to the session. During the experiments, the high-resolution neutron spectrometer TOFOR was used to study the energy spectrum of emitted neutrons. Neutrons of energies up to 10MeV, not consistent with the neutron energy spectrum expected from d(d,n)He-3 reactions, were observed. In this paper, we interpret these neutrons as a first-time observation of a Be-9(He-3, n)C-11 neutron spectrum in a tokamak plasma, a conclusion based on a consistent analysis of experimental data and Monte Carlo simulations. Be-9(a, n)C-12 and Be-9(p, n)B-9 reactions are also simulated for p and a fusion products from d(He-3, a) p reactions; these two-step processes are seen to contribute on a level of about 10% of the single-step process in Be-9(He-3, n) C-11. Contributions to the total neutron yield from the Be-9(3He, n)C-11 reaction are found to be in the range 13 +/- 3 to 57 +/- 5%. We demonstrate how TOFOR can be used to simultaneously (i) probe the deuterium distribution, providing reliable measurements of the bulk deuterium temperature, here in the range 3.2 +/- 0.4 to 6.3 +/- 1.0 keV and (ii) provide an estimate of the beryllium concentration (in the range 0.48 +/- 0.17 to 6.4 +/- 1.7% of n(e) assuming T-3He = 300 keV). The observation of Be-9 related neutrons is relevant in view of the upcoming installation of a beryllium-coated ITER-like wall on JET and for ITER itself. An important implication is possible neutron-induced activation of the ITER vessel during the low-activation phase with ICRF heating tuned to minority He-3 in hydrogen plasmas.

Keywords
PACS codes: 29.30.Hs, 29.25.Dz, 52.55.-s, 52.55.Fa, 25.55.-e, 52.55.Pi
National Category
Physical Sciences
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-114040 (URN)10.1088/0029-5515/50/4/045005 (DOI)000276475600009 ()
Available from: 2010-02-08 Created: 2010-02-08 Last updated: 2017-12-12Bibliographically approved
7. Neutron emission levels during the ITER zero activation phase
Open this publication in new window or tab >>Neutron emission levels during the ITER zero activation phase
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2010 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, no 8, p. 084020-Article in journal (Refereed) Published
Abstract [en]

In recent experiments at JET, a contribution to the neutron emission from reactions between beryllium and 3He, 4He and H has been identified. With the beryllium walled planned for ITER, this raises the question of possible neutron activation during the ITER zero activation phase. Here, we estimate the neutron emission rates for various heating scenarios foreseen for this ITER phase using Monte Carlo simulations. The emission is seen to be strongly dependent on the scenario chosen and the assumptions involved. We find that fundamental minority heating can contribute on the scale of low temperature deuterium plasmas, depending on minority concentration and ICRH power applied. Harmonic ICRH leads to production of tails that can give rise to significant neutron emission rates, while rates from hydrogen beams will be near zero. Better knowledge of the zero activation phase conditions, and more sophisticated ICRH codes, would be needed to give exact rate predictions. We conclude that rates from so-called zero activation plasmas will be significantly lower than expected for the DD or DT phases, but far from zero.

Keywords
PACS codes: 29.30.Hs, 29.25.Dz, 52.55.-s, 52.55.Fa, 25.55.-e, 52.55.Pi
National Category
Natural Sciences
Research subject
Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-114041 (URN)10.1088/0029-5515/50/8/084020 (DOI)000280505800021 ()
Available from: 2010-02-08 Created: 2010-02-08 Last updated: 2017-12-12Bibliographically approved

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Citation style
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  • modern-language-association-8th-edition
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Output format
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