The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Predictive Modelling of Edge Transport Phenomena in ELMy H-Mode Tokamak Fusion Plasmas

Johnny-Stefan Lönnroth

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium Ko213 at Helsinki University of Technology (Espoo, Finland) on the 1st of December, 2009, at 12 noon.

Overview in PDF format (ISBN 978-952-248-158-0)   [1131 KB]
Dissertation is also available in print (ISBN 978-952-248-157-3)

Abstract

This thesis discusses a range of work dealing with edge plasma transport in magnetically confined fusion plasmas by means of predictive transport modelling, a technique in which qualitative predictions and explanations are sought by running transport codes equipped with models for plasma transport and other relevant phenomena. The focus is on high confinement mode (H-mode) tokamak plasmas, which feature improved performance thanks to the formation of an edge transport barrier. H-mode plasmas are generally characterized by the occurrence of edge localized modes (ELMs), periodic eruptions of particles and energy, which limit confinement and may turn out to be seriously damaging in future large tokamak reactors.

The thesis introduces schemes and models for qualitative study of the ELM phenomenon in predictive transport modelling. It aims to shed new light on the dynamics of ELMs using these models. It tries, again making use of the ELM modelling schemes, to explain various experimental observations, notably the loss of performance and increased ELM frequency observed in experiments with enhanced toroidal magnetic field ripple and in situations with strong external neutral gas puffing. Finally, it also tries to establish more generally the potential effects of ripple-induced thermal ion losses on H-mode plasma performance and ELMs.

It is demonstrated that the ELM modelling schemes introduced in the thesis can qualitatively reproduce the experimental dynamics of a number of ELM regimes. Using a theory-motivated ELM model based on a linear model of instability, the dynamics of combined ballooning-peeling mode ELMs is studied. It is shown that the ELMs are most often triggered by a ballooning mode instability, which renders the plasma peeling mode unstable and causes the ELM to continue in a peeling mode phase. Understanding the dynamics of ELMs will be a key issue when it comes to controlling and mitigating the ELMs in future large tokamaks. By means of integrated modelling, it is shown that an experimentally observed increase in the ELM frequency and deterioration of plasma confinement triggered by external neutral gas puffing might be due to a transition from the second to the first ballooning stable region, as expressed in magnetohydrodynamics (MHD). The result may have implications on the control of ELMs and performance in future tokamaks. Modest pedestal performance and benign ELMs observed in the presence of toroidal magnetic field ripple in dimensionless pedestal identity experiments between the JET and JT-6OU tokamaks are explained through predictive transport modelling as resulting from ripple-induced thermal ion losses, more precisely from non-diffusive (direct) losses. It is shown that ripple losses need not necessarily have a detrimental influence on performance, but that there is a trade-off between performance and benignity of ELMs. The results may have widely felt implications, because ITER, the next major facility on the way towards a commercial fusion reactor, is foreseen to operate with a non-negligible level of toroidal magnetic field ripple.

This thesis consists of an overview and of the following 6 publications:

  1. J.-S. Lönnroth, V. V. Parail, G. Corrigan, D. Heading, G. Huysmans, A. Loarte, S. Saarelma, G. Saibene, S. Sharapov, J. Spence, and contributors to the EFDA-JET Workprogramme. 2003. Integrated predictive modelling of the effect of neutral gas puffing in ELMy H-mode plasmas. Plasma Physics and Controlled Fusion, volume 45, number 9, pages 1689-1711. © 2003 Institute of Physics Publishing. By permission.
  2. J.-S. Lönnroth, V. Parail, G. Huysmans, G. Saibene, H. Wilson, S. Sharapov, G. Corrigan, D. Heading, R. Sartori, M. Bécoulet, and JET-EFDA contributors. 2004. Predictive transport modelling and MHD stability analysis of mixed type I-II ELMy H-mode JET plasmas. Plasma Physics and Controlled Fusion, volume 46, number 5, pages 767-796. © 2004 Institute of Physics Publishing. By permission.
  3. J.-S. Lönnroth, V. Parail, C. Figarella, X. Garbet, G. Corrigan, D. Heading, and JET-EFDA Contributors. 2004. Predictive modelling of ELMy H-modes with a new theory-motivated model for ELMs. Plasma Physics and Controlled Fusion, volume 46, number 5A, pages A249-A256. © 2004 Institute of Physics Publishing. By permission.
  4. J.-S. Lönnroth, V. Parail, A. Dnestrovskij, C. Figarella, X. Garbet, H. Wilson, and JET-EFDA Contributors. 2004. Predictive transport modelling of type I ELMy H-mode dynamics using a theory-motivated combined ballooning–peeling model. Plasma Physics and Controlled Fusion, volume 46, number 8, pages 1197-1215. © 2004 Institute of Physics Publishing. By permission.
  5. J.-S. Lönnroth, V. Parail, V. Hynönen, T. Johnson, T. Kiviniemi, N. Oyama, M. Beurskens, D. Howell, G. Saibene, P. de Vries, T. Hatae, Y. Kamada, S. Konovalov, A. Loarte, K. Shinohara, K. Tobita, H. Urano, and JET EFDA contributors. 2007. Effects of ripple-induced ion thermal transport on H-mode plasma performance. Plasma Physics and Controlled Fusion, volume 49, number 3, pages 273-295. © 2007 Institute of Physics Publishing. By permission.
  6. J.-S. Lönnroth, G. Corrigan, W. Fundamenski, V. Parail, J. Spence, D. Tskhakaya, and JET-EFDA contributors. 2005. Analysis of ELM heat pulse propagation in the JET scrape-off layer with an integrated fluid-kinetic approach. In: C. Hidalgo and B. Ph. van Milligen (editors). Proceedings of the 32nd European Physical Society Conference on Plasma Physics and Controlled Fusion. Tarragona, Spain. 27 June - 1 July 2005. Europhysics Conference Abstracts, volume 29C, P-4.040. © 2005 European Physical Society (EPS). By permission.

Keywords: fusion, tokamak, transport, modelling, H-mode, ELM, magnetic ripple

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© 2009 Helsinki University of Technology

Last update 2011-05-26