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Non-linear dynamics of Alfvén eigenmodes excited by fast ions in tokamaks
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The tokamak is so far the most promising magnetic configuration for achieving a net production of fusion energy. The D-T fusion reactions result in 3.5 MeV alpha-particles, which may destabilize Alfvén eigenmodes through wave-particle interaction. These instabilities redistribute the alpha-particles from the central region of the plasma towards the edge, where they are thermalized, and hence result in a reduced heating efficiency. The high-energy alpha-particles may even be thrown out of the plasma and may damage the wall.

To investigate the destabilization of Alfvén eigenmodes by high-energy ions, ion cyclotron resonance heating (ICRH) and neutral beam injection (NBI) are often used to create a high-energy tail on the distribution function. The ICRH does not only produce high-energy anisotropic tails, it also decorrelates the wave-particle interaction with the Alfvén eigenmodes. Without decorrelation of the wave-particle interaction an ion will undergo a superadiabatic oscillation in phase space and there will be no net transfer of energy to the mode. For the thermal ions the decorrelation from collisions dominates while for the high-energy ions the decorrelation from ICRH dominates. As the unstable modes grow up, the gradients in phase space, which drive the mode, are reduced, resulting in a weaker drive. The dynamics of the system becomes non-linear due to a continuous restoration of the gradients by D-T reactions and ICRH.

In this thesis the non-linear dynamics of toroidal Alfvén eigenmodes (TAEs) during ICRH has been investigated using the SELFO code. The SELFO code, which calculates the distribution function during ICRH self-consistently using a Monte-Carlo metod, has been upgraded to include interactions with TAEs. The fast decay of the mode amplitude as the ICRH is switched off, which is seen in experiments, as well as the oscillation of the mode amplitude as the distribution function is repetetively built up by the ICRH and flattened by the TAE has been reproduced using numerical simulations. In the presence of several unstable modes the dynamics become more complicated. The redistribution of an alpha-particle slowing down distribution function as well as the reduced heating efficiency in the presence of several modes has also been investigated.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , xiv, 61 p.
Series
Trita-EE, ISSN 1653-5146 ; 2007:001
Keyword [en]
fusion plasma, tokamak, MHD, toroidal Alfvén eigenmodes, thermonuclear alpha particles, ion cyclotron resonance heating
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4320ISBN: 978-91-7178-564-0 (print)OAI: oai:DiVA.org:kth-4320DiVA: diva2:11791
Public defence
2007-04-23, Sal F3, KTH, Lindstedtsvägen 23, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100628Available from: 2007-03-29 Created: 2007-03-29 Last updated: 2010-06-28Bibliographically approved
List of papers
1. Self-consistent calculations of the distribution function and wave field during ICRF heating and global Alfvén wave excitation
Open this publication in new window or tab >>Self-consistent calculations of the distribution function and wave field during ICRF heating and global Alfvén wave excitation
2004 (English)In: Proceeding of Theory of Fusions Plasmas, 2004, 123-132 p.Conference paper, Published paper (Refereed)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-6934 (URN)
Conference
Theory of Fusions Plasmas, August 30-September 3 (2004), Varenna, Italy
Note
QC 20100628Available from: 2007-03-29 Created: 2007-03-29 Last updated: 2010-07-14Bibliographically approved
2. Non-linear study of fast particle excitation of global Alfvén eigenmodes during ICRH
Open this publication in new window or tab >>Non-linear study of fast particle excitation of global Alfvén eigenmodes during ICRH
2005 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 45, 485-493 p.Article in journal (Refereed) Published
Abstract [en]

High-power ion–cyclotron resonance heating (ICRH) can produce centrally peaked fast ion distributions with wide non-standard drift orbits exciting Alfvén eigenmodes (AEs). The dynamics of the AE excitation depends not only on the anisotropy and the peaking of the fast ion distribution but also on the decorrelation of the AE interactions and the renewal of the fast ions resonant with the AE by ion–cyclotron interactions. A method of self-consistently including the evolution of the distribution function of fast ions during excitation of AEs and ICRH has been developed and implemented in the SELFO code. Numerical simulations of the AE dynamics and ICRH give a variation of the AE amplitude consistent with the experimentally observed splitting of the mode frequency. The experimentally observed fast damping of the mode as the ICRH is switched off is also evident in the simulations.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-6935 (URN)10.1088/0029-5515/45/6/010 (DOI)000230409800010 ()2-s2.0-19944380153 (Scopus ID)
Note

QC 20100628

Available from: 2007-03-29 Created: 2007-03-29 Last updated: 2013-03-21Bibliographically approved
3. Self-consistent study of fast particle redistribution by Alfvén eigenmodes during Ion cyclotron resonance heating
Open this publication in new window or tab >>Self-consistent study of fast particle redistribution by Alfvén eigenmodes during Ion cyclotron resonance heating
2005 (English)In: Proceedings of the 9th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, 2005, 14-20 p.Conference paper, Published paper (Refereed)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-6936 (URN)
Conference
The 9th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, November 9-11 (2005), Takayama, Japan
Note
QC 20100628Available from: 2007-03-29 Created: 2007-03-29 Last updated: 2012-03-20Bibliographically approved
4. Non-linear dynamics of Alfvén eigenmodes excited by thermonulcear alpha particles in the presence of ion cyclotron resonance heating
Open this publication in new window or tab >>Non-linear dynamics of Alfvén eigenmodes excited by thermonulcear alpha particles in the presence of ion cyclotron resonance heating
2007 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 47, no 9, 1131-1141 p.Article in journal (Refereed) Published
Abstract [en]

Alfvén eigenmodes (AEs) excited by thermonuclear α-particles can degrade the heating efficiency by spatial redistribution of the resonant α-particles. Changes of the orbit invariants in phase space by collisions and interactions with other waves, such as magnetosonic waves during ion cyclotron resonance heating (ICRH), lead to changes in the phase between the α-particles and AEs, causing a decorrelation of the interactions and stronger redistribution of the α-particles. Cyclotron interactions increase the decorrelation of the AE interactions with the high-energy ions and hence a stronger radial redistribution of the high-energy α-particles by the AEs. Renewal of the distribution function by thermonuclear reactions and losses of α-particles to the wall lead to a continuous drive of the AEs and a radial redistribution of the α-particles. The condition for excitation of AEs is shown to depend on the heating scenario where heating at the low field side creates a significant population of high-energy non-standard orbits which drive the modes. The redistribution results in a reduction in the averaged α-particle energy and a degradation of the heating efficiency. The effect on the distribution function in the presence of several unstable modes is not additive and the particle redistribution is found to saturate with an increasing number of modes.

Keyword
Alpha particles; Cyclotron resonance; Distribution functions; High energy physics; Plasma heating; Thermonuclear reactions; Ion cyclotron resonance heating (ICRH); Magnetosonic waves; Excitation energy
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-6937 (URN)10.1088/0029-5515/47/9/009 (DOI)000249968800012 ()2-s2.0-34548217149 (Scopus ID)
Note
QC 20100628Available from: 2007-03-29 Created: 2007-03-29 Last updated: 2012-03-22Bibliographically approved

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