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Fuel ion densities and distributions in fusion plasmas: Modeling and analysis for neutron emission spectrometry
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Neutrons produced in fusion reactions in a magnetically confined plasma carry information about the distributions and densities of the fuel ions in the plasma. This thesis presents work where various theoretical models of different fuel ion distributions in the plasma are used to calculate modeled components of the neutron energy spectrum. The calculated components can then be compared with measured data, either to benchmark and validate the model or to derive various plasma parameters from the experimental data. Neutron spectra measured with the spectrometers TOFOR and the MPR, which are both installed at the JET tokamak in England, are used for this purpose. The thesis is based on three papers.

The first paper presents the analysis of TOFOR data from plasmas heated with neutral beams and radio frequency waves tuned to the third harmonic of the deuterium cyclotron frequency, which creates fast (supra thermal) ions in the MeV range. It is found that effects of the finite Larmor radii of the fast ions need to be included in the modeling in order to understand the data. These effects are important for fast ion measurements if there is a gradient in the fast ion distribution function with a scale length that is comparable to - or smaller than - the width of the field of view of the measuring instrument, and if this scale length is comparable to - or smaller than - the Larmor radii of the fast ions.

The second paper presents calculations of the neutron energy spectrum from the T(t,n)4He reaction, for JET relevant fuel ion distributions. This is to to form a starting point for the investigation of the possibility to obtain fast ion information from the t-t neutron spectrum, in a possible future deuterium-tritium campaign at JET. The t-t spectrum is more challenging to analyze than the d-d and d-t cases, since this reaction has three (rather than two) particles in the final state, which results in a broad continuum of neutron energies rather than a peak. However, the presence of various final state interactions - in particular between the neutron and the 4He - might still allow for spectrometry analysis.

Finally, in Paper III, a method to derive the fuel ion ratio, nt/nd, is presented and applied to MPR data from the JET d-t campaign in 1997. The trend in the results are consistent with Penning trap measurements of the fuel ion ratio at the plasma edge, but the absolute numbers are not the same. Measuring the fuel ion ratio in the core plasma is an important task for fusion research, and also a very complicated one. Future work should aim at measuring this quantity in several independent ways, which should then be cross checked against each other.

Place, publisher, year, edition, pages
Uppsala: Department of Physics and Astronomy, Uppsala University , 2012. , 44 p.
National Category
Fusion, Plasma and Space Physics Subatomic Physics
Research subject
Applied Nuclear Physics
Identifiers
URN: urn:nbn:se:uu:diva-192530OAI: oai:DiVA.org:uu-192530DiVA: diva2:599854
Supervisors
Available from: 2013-01-23 Created: 2013-01-22 Last updated: 2016-03-29Bibliographically approved
List of papers
1. Finite Larmor radii effects in fast ion measurements with neutron emission spectrometry
Open this publication in new window or tab >>Finite Larmor radii effects in fast ion measurements with neutron emission spectrometry
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2013 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 55, no 1, 015008- p.Article in journal (Refereed) Published
Abstract [en]

When analysing data from fast ion measurements it is normally assumed that the gyro-phase distribution of the ions is isotropic within the field of view of the measuring instrument. This assumption is not valid if the Larmor radii of the fast ions are comparable to—or larger than—the gradient scale length in the spatial distribution of the ions, and if this scale length is comparable to—or smaller than—the width of the field of view of the measuring instrument. In this paper the effect of such an anisotropy is demonstrated by analysing neutron emission spectrometry data from a JET experiment with deuterium neutral beams together with radiofrequency heating at the third harmonic of the deuterium cyclotron frequency. In the experiment, the neutron time-of-flight spectrometer TOFOR was used to measure the neutrons from the d(d,n) 3 He-reaction. Comparison of the experimental data with Monte Carlo calculations shows that the finite Larmor radii of the fast ions need to be included in the modelling to get a good description of the data. Similar effects are likely to be important for other fast ion diagnostics, such as γ -ray spectroscopy and neutral particle analysis, as well.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-192528 (URN)10.1088/0741-3335/55/1/015008 (DOI)000312586000028 ()
Conference
18th International Stellarator Hellotron Workshop and 10th Asia Pacific Plasma Theory Conference; Australian Natl Univ, Canberra, AUSTRALIA; 30 Jan-3 Feb 2012
Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2017-12-06
2. Neutron emission from a tritium rich fusion plasma: simulations in view of a possible future d-t campaign at JET
Open this publication in new window or tab >>Neutron emission from a tritium rich fusion plasma: simulations in view of a possible future d-t campaign at JET
2012 (English)In: 39th EPS Conference on Plasma Physics 2012 (EPS 2012): Europhysics Conference Abstracts, 2012Conference paper, Published paper (Other academic)
Abstract [en]

 Neutron energy spectra from the t(t,2n)4He (t-t) reaction has been calculated for different fuel ion distributions, in order to assess the possibility to use this reaction when analyzing neutron spectrometry data from plasmas with very high tritium fraction. The shape of the neutron spectrum is determined by three-body kinematics, and is modified by interactions between the reaction products, primarily between the neutron and the 4He. The results indicate that the analysis of a t-t spectrum will be more challenging than for the d-t and d-d reactions. However, for fast ions in the MeV range, produced e.g. by harmonic radiofrequency heating and neutral beam injection, it should still be possible to obtain fast ion information from the neutron spectrum.

Keyword
nuclear fusion, neutron spectrometry, fast ions
National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-192532 (URN)
Conference
39th EPS Conference on Plasma Physics 2012, EPS 2012, 2 - 6 July 2012, Stockholm, Sweden
Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2016-01-25

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