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Gatu Johnson, Maria
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Publications (10 of 39) Show all publications
Gatu Johnson, M. (2010). Fusion Plasma Observations at JET with the TOFOR Neutron Spectrometer: Instrumental Challenges and Physics Results. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Fusion Plasma Observations at JET with the TOFOR Neutron Spectrometer: Instrumental Challenges and Physics Results
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
Fusion, Plasma diagnostics, Neutron Spectrometry, TOFOR, JET, ITER, time-of-flight, plasma heating
National Category
Subatomic Physics
Research subject
Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-114077 (URN)978-91-554-7724-0 (ISBN)
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
Hellesen, C., Gatu Johnson, M., Andersson Sundén, E., Conroy, S., Ericsson, G., Eriksson, J., . . . Weiszflog, M. (2010). Measurements of fast ions and their interactions with MHD activity using neutron emission spectroscopy. Nuclear Fusion, 50(8), 084006
Open this publication in new window or tab >>Measurements of fast ions and their interactions with MHD activity using neutron emission spectroscopy
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2010 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, no 8, p. 084006-Article in journal (Refereed) Published
Abstract [en]

Ion cyclotron radio frequency (ICRF) heating can produce fast ion populations with energies reaching up to several megaelectronvolts. Here, we present unique measurements of fast ion distributions from an experiment with 3rd harmonic ICRF heating on deuterium beams using neutron emission spectroscopy (NES). From the experiment, very high DD neutron rates were observed, using only modest external heating powers. This was attributed to acceleration of deuterium beam ions to energies up to about 2-3 MeV, where the DD reactivity is on a par with that of the DT reaction. The high neutron rates allowed for observations of changes in the fast deuterium energy distribution on a time scale of 50 ms. Clear correlations were seen between fast deuterium ions in different energy ranges and magnetohydrodynamic activities, such as monster sawteeth and toroidal Alfven eigen modes (TAE). Specifically, NES data showed that the number of deuterons in the region between 1 and 1.5 MeV were decaying significantly during strong TAE activity, while ions with lower energies around 500 keV were not affected. This was attributed to resonances with the TAE modes.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-114442 (URN)10.1088/0029-5515/50/8/084006 (DOI)000280505800007 ()
Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2018-04-23
Gatu Johnson, M., Hellesen, C., Andersson Sundén, E., Cecconello, M., Conroy, S., Ericsson, G., . . . Weiszflog, M. (2010). Neutron emission from beryllium reactions in JET deuterium plasmas with 3He minority. Nuclear Fusion, 50(4), 045005
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
Hellesen, C., Gatu Johnson, M., Andersson Sundén, E., Conroy, S., Ericsson, G., Ronchi, E., . . . Sharapov, S. (2010). Neutron emission generated by fast deuterons accelerated with ion cyclotron heating at JET [Letter to the editor]. Nuclear Fusion, 50(2), 022001
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
Gatu Johnson, M., Hellesen, C., Andersson Sundén, E., Cecconello, M., Conroy, S., Ericsson, G., . . . Weiszflog, M. (2010). Neutron emission levels during the ITER zero activation phase. Nuclear Fusion, 50(8), 084020
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
Gatu Johnson, M., Cecconello, M., Hellesen, C., Andersson Sundén, E., Conroy, S., Ericsson, G., . . . Tardocchi, M. (2009). Cross-validation of JET fast deuterium results from TOFOR and NPA. In: 36th EPS Conference on Plasma Physics, Sofia, June 29 - July 3, 2009 ECA Vol.33E. Paper presented at 36th EPS Conference on Plasma Physics (pp. P-2.151).
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
Ronchi, E., Conroy, S., Andersson Sundén, E., Ericsson, G., Hellesen, C., Gatu Johnson, M., . . . Hjalmarsson, A. (2008). A Neural Networks Framework for Real-Time Unfolding of Neutron Spectroscopic Data at JET. Review of Scientific Instruments, 79(10), 10E513
Open this publication in new window or tab >>A Neural Networks Framework for Real-Time Unfolding of Neutron Spectroscopic Data at JET
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2008 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 79, no 10, p. 10E513-Article in journal (Refereed) Published
Abstract [en]

A determination of fast ion population parameters such as intensity and kinetic temperature is important for fusion reactors. This becomes more challenging with finer time resolution of the measurements, since the limited data in each time slice cause increasing statistical variations in the data. This paper describes a framework using Bayesian-regularized neural networks (NNs) designed for such a task. The method is applied to the TOFOR 2.5 MeV fusion neutron spectrometer at JET. NN training data are generated by random sampling of variables in neutron spectroscopy models. Ranges and probability distributions of the parameters are chosen to match the experimental data. Results have shown good performance both on synthetic and experimental data. The latter was assessed by statistical considerations and by examining the robustness and time consistency of the results. The regularization of the training algorithm allowed for higher time resolutions than simple forward methods. The fast execution time makes this approach suitable for real-time analysis with a time resolution limit in the microsecond time scale.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-16763 (URN)10.1063/1.2953492 (DOI)000260573500095 ()
Note
JET-EFDA Contributors. Conference information: 17th Topical Conference High Temperature Plasma Diagnostics, May 2008, Albaquerque, New Mexico, USAAvailable from: 2008-06-05 Created: 2008-06-05 Last updated: 2017-12-08Bibliographically approved
Andersson Sundén, E., Conroy, S., Ericsson, G., Gatu Johnson, M., Giacomelli, L., Hellesen, C., . . . Tardocchi, M. (2008). Evaluation of Spectral Unfolding for Neutron Spectroscopy. In: AIP Conference Proceedings 988 (pp. 315).
Open this publication in new window or tab >>Evaluation of Spectral Unfolding for Neutron Spectroscopy
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2008 (English)In: AIP Conference Proceedings 988, 2008, p. 315-Conference paper, Published paper (Refereed)
Identifiers
urn:nbn:se:uu:diva-16773 (URN)
Note
International Workshop on Burning Plasma Diagnostics, September 2007, Varenna, ItalyAvailable from: 2008-06-05 Created: 2008-06-05 Last updated: 2017-01-25
Giacomelli, L., Hjalmarsson, A., Källne, J., Hellesen, C., Tardocchi, M., Goroni, G., . . . Weiszflog, M. (2008). Neutron Emission Spectroscopy Results for ITB and Mode Conversion ICRH Experiments at JET. In: Rev. Sci. Instr., submitted.
Open this publication in new window or tab >>Neutron Emission Spectroscopy Results for ITB and Mode Conversion ICRH Experiments at JET
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2008 (English)In: Rev. Sci. Instr., submitted, 2008Conference paper, Published paper (Refereed)
Identifiers
urn:nbn:se:uu:diva-16765 (URN)
Note
17th Topical Conference High Temperature Plasma Diagnostics, May 2008, Albaquerque, New Mexico, USAAvailable from: 2008-06-05 Created: 2008-06-05 Last updated: 2017-01-25Bibliographically approved
Conroy, S., Weiszflog, M., Andersson Sundén, E., Ericsson, G., Gatu Johnson, M., Hellesen, C., . . . Sjöstrand, H. (2008). Neutron spectrometer for ITER using silicon detectors. In: Rev. Sci. Instr., submitted.
Open this publication in new window or tab >>Neutron spectrometer for ITER using silicon detectors
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2008 (English)In: Rev. Sci. Instr., submitted, 2008Conference paper, Published paper (Refereed)
Identifiers
urn:nbn:se:uu:diva-16753 (URN)
Note
17th Topical Conference High Temperature Plasma Diagnostics, May 2008, Albaquerque, New Mexico, USAAvailable from: 2008-06-05 Created: 2008-06-05 Last updated: 2017-01-25
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