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The dynamics of Alfvén eigenmodes excited by energetic ions in toroidal plasmas
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.ORCID iD: 0000-0002-3262-1958
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Experiments for the development of fusion power that are based on magnetic confinement deal with plasmas that inevitably contain energetic (non-thermal) particles. These particles come e.g. from fusion reactions or from external heating of the plasma. Ensembles of energetic ions can excite plasma waves in the Alfvén frequency range to such an extent that the resulting wave fields redistribute the energetic ions, and potentially eject them from the plasma. The redistribution of ions may cause a substantial reduction heating efficiency, and it may damage the inner walls and other components of the vessel. Understanding the dynamics of such instabilities is necessary to optimise the operation of fusion experiments and of future fusion power plants.

A Monte Carlo model that describes the nonlinear wave-particle dynamics in a toroidal plasma has been developed to study the excitation of the abovementioned instabilities. A decorrelation of the wave-particle phase is added in order to model stochasticity of the system (e.g. due to collisions between particles). Based on the nonlinear description with added phase decorrelation, a quasilinear version of the model has been developed, where the phase decorrelation has been replaced by a quasilinear diffusion coefficient in particle energy. When the characteristic time scale for macroscopic phase decorrelation becomes similar to or shorter than the time scales of nonlinear wave-particle dynamics, the two descriptions quantitatively agree on a macroscopic level. The quasilinear model is typically less computationally demanding than the nonlinear model, since it has a lower dimensionality of phase space.

In the presented studies, several effects on the macroscopic wave-particle dynamics by the presence of phase decorrelation have been theoretically and numerically analysed, e.g. effects on the growth and saturation of the wave amplitude, and on the so called frequency chirping events with associated hole-clump pair formation in particle phase space. Several effects coming from structures of the energy distribution of particles around the wave-particle resonance has also been studied.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xiv, 71 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:014
Keyword [en]
Fusion plasma physics, tokamak, wave-particle interactions, toroidal Alfvén eigenmodes, bump-on-tail instabilities, magnetohydrodynamics, nonlinear dynamics, quasilinear dynamics, Hamiltonian mechanics, Monte Carlo method
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-163127ISBN: 978-91-7595-499-8 (print)OAI: oai:DiVA.org:kth-163127DiVA: diva2:798745
Presentation
2015-05-22, Seminar Room, 3rd floor, Teknikringen 31, Stockholm, 13:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2011-5387
Note

QC 20150330

Available from: 2015-03-30 Created: 2015-03-27 Last updated: 2015-03-30Bibliographically approved
List of papers
1. Monte-Carlo model for nonlinear interactions of Alfvén eigenmodes with energetic ions
Open this publication in new window or tab >>Monte-Carlo model for nonlinear interactions of Alfvén eigenmodes with energetic ions
2012 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 401, no 1, 012024- p.Article in journal (Refereed) Published
Abstract [en]

A Monte-Carlo model for interactions between a single Alfvén eigenmode and energetic ions in a tokamak is presented. A phenomenological decorrelation of the wave-particle phase is introduced to mimic decorrelation by collisions or by other waves. Analysis is dedicated to how the strength of the phase decorrelation affects the nonlinear wave-particle interactions. Several of the phenomena that have been observed in some earlier models describing the nonlinear dynamics of Alfvén eigenmodes have been verified, such as the growth and saturation of the wave mode amplitude giving rise to a localized flattening of the distribution function, as well as the generation of coherent structures in the distribution function. The degree of phase decorrelation is shown to have a strong effect on the dynamics of the Alfvén eigenmode excitation.

Keyword
Coherent structure, De correlations, Eigen modes, Energetic ion, Monte-carlo models, Nonlinear interactions, Wave modes, Wave-particle interactions
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-118913 (URN)10.1088/1742-6596/401/1/012024 (DOI)000312549900024 ()2-s2.0-84874163425 (Scopus ID)
Conference
13th Joint Varenna-Lausanne International Workshop on Theory of Fusion Plasmas; Varenna; Italy; 27 August 2012 through 31 August 2012
Note

QC 20130304

Available from: 2013-03-04 Created: 2013-03-04 Last updated: 2017-12-06Bibliographically approved
2. Modeling the dynamics of toroidal Alfvén eigenmodes
Open this publication in new window or tab >>Modeling the dynamics of toroidal Alfvén eigenmodes
2013 (English)Conference paper, Published paper (Other academic)
Abstract [en]

A model describing nonlinear dynamics of a single Alfvén eigenmode excited by an inverted energy distribution of energetic ions is presented, suitable for drift orbit averaged Monte Carlo codes. The nonlinear dynamics of the wave mode is modeled with a complex wave amplitude, and is characterized by the formation of coherent structures in phase space, caused by wave-particle interaction. The transition to a quasilinear regime is modeled with a phenomenological decorrelation of the wave-particle phase. As the decorrelation is increased the coherent phase-space structures diminishes, and frequency chirping events in the marginal stability region is limited. The strength of the decorrelation modifies the saturation level and saturation time of the eigenmode amplitude.

Place, publisher, year, edition, pages
International Atomic Energy Agency, 2013
Keyword
Monte Carlo model, wave-particle interactions, bump-on-tail instabilities, toroidal Alfvén eigenmodes, nonlinear dynamics
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-161926 (URN)
Conference
13th IAEA Technical Meeting on “Energetic Particles in Magnetic Confinement Systems”, Beijing, China, September 17 - 20, 2013
Note

QC 20150319

Available from: 2015-03-19 Created: 2015-03-19 Last updated: 2016-09-27Bibliographically approved
3. Comparisons of the nonlinear and the quasilinear model for the bump-on-tail instability with phase decorrelation
Open this publication in new window or tab >>Comparisons of the nonlinear and the quasilinear model for the bump-on-tail instability with phase decorrelation
2014 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The dynamics of discrete global modes in a toroidal plasma interacting with an energetic particle distribution is studied, and in particular when the dynamics of the system using the nonlinear and quasilinear descriptions are macroscopically similar. The dynamics can be described with a nonlinear bump-on-tail model in a two-dimensional phase space of particles. A Monte Carlo framework is developed for this model with an included decorrelation of the wave-particle phase, which is used to model extrinsic stochastisation of the wave-particle interactions. From this description, a quasilinear version of the model is also developed, which is described by a diffusive process in energy space due to the added phase decorrelation. Due to the reduced dimensionality of phase space, the quasilinear description is typically less computationally demanding than the nonlinear description. The purpose of the studies is to find conditions when a quasilinear model sufficiently describes the same phenomena of the wave-plasma interactions as a nonlinear model does. Via numerical and theoretical parameter studies, regimes where the two models overlap macroscopically are found. These regimes exist above a given threshold of the strength of the decorrelation, where coherent phase space structures are destroyed on time scales shorter than characteristic time scales of nonlinear particle motion in phase space close to the wave-particle resonance. Specifically for the quasilinear model, a theoretical value of the time scale of quasilinear flattening is derived and numerically verified.

Series
Journal of Physics: Conference Series, ISSN 1742-6588
Keyword
Dynamics, Time measurement, Wave plasma interactions
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-159056 (URN)10.1088/1742-6596/561/1/012019 (DOI)000346423600019 ()2-s2.0-84919338077 (Scopus ID)
Conference
Joint Varenna-Lausanne International Workshop on the Theory of Fusion Plasmas, SEP 01-05, 2014, Varenna, ITALY
Note

QC 20150122

Available from: 2015-01-22 Created: 2015-01-20 Last updated: 2017-03-24Bibliographically approved
4. The effects of phase decorrelation on the dynamics of the bump-on-tail instability
Open this publication in new window or tab >>The effects of phase decorrelation on the dynamics of the bump-on-tail instability
2015 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, no 8, 082106Article in journal (Refereed) Published
Abstract [en]

The dynamics of the bump-on-tail instability has been studied. The novelty of the work is the analysis of how the bump-on-tail dynamics is affected by an extrinsic stochastisation of the phase of the wave-particle interaction; here referred to as phase decorrelation. For this purpose, a nonlinear Monte Carlo model has been developed. When the characteristic time scale for macroscopic phase decorrelation becomes shorter than time scales of nonlinear wave-particle dynamics, the system may be described quasilinearly, with the phase decorrelation being replaced by a quasilinear diffusion coefficient in particle energy. A purely quasilinear Monte Carlo model, which is typically less computationally demanding than the fully nonlinear description due to the reduced dimensionality of phase space, has been developed for comparison. In this paper, parameter regimes, where the nonlinear and the quasilinear descriptions quantitatively agree on a macroscopic level, have been investigated, using combined theoretical and numerical analyses. Qualitative effects on the macroscopic dynamics by the presence of phase decorrelation and/or by structures of the energy distribution function in the proximity of the wave-particle resonance are also studied.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
Keyword
Monte Carlo model, wave-particle interactions, bump-on-tail instabilities, nonlinear dynamics, quasilinear dynamics, toroidal Alfvén eigenmodes
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-161935 (URN)10.1063/1.4928094 (DOI)000360647600010 ()2-s2.0-84938790389 (Scopus ID)
Funder
Swedish Research Council, 621-2011-5387
Note

QC 20151005. Updated from submitted to published.

Available from: 2015-03-27 Created: 2015-03-19 Last updated: 2017-12-04Bibliographically approved

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