Magnetohydrodynamic stability analyses of tokamak edge plasmas

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Doctoral thesis (article-based)
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84, [50]
Helsinki University of Technology publications in engineering physics. A, 836
Edge Locilised Modes (ELMs) are edge phenomena in fusion plasmas that cause small bursts of energy and particles out of the plasma. In a fusion devices such as a tokamak, ELMs affect the plasma confinement and can cause divertor plate erosion. Therefore, for the operation of a tokamak fusion plasma it is important to understand the physical mechanisms behind the ELM phenomenon and to be able to minimise the detrimental effects of the ELMs. In this thesis, the ELMs are modelled using magnetohydrodynamic stability analysis. First an accurate equilibrium of the experimental plasma is created and then the stability of the equilibrium is analysed. The stability analyses show that the Type I or 'giant' ELMs in ASDEX Upgrade and JET plasmas are triggered by peeling-ballooning modes with low to intermediate toroidal mode number (n). The radial struture of these modes is relatively wide and is localised near the edge of the plasma. The ASDEX Upgrade plasmas with smaller Type II or 'grassy' ELMs are found to have narrower mode structure of the triggering instability. The stability against low-n modes is improved as well causing the triggering instability to shift to higher n. Increased plasma pressure in the core region is found to improve the stability of the edge against low-n instabilities. This can explain the easier access to Type II ELMs observed in such plasmas. The Type III ELMs in JET plasmas are found to be deep in the stable region against the low- to intermediate-n peeling-ballooning modes and are likelyto triggered by a different mechanism than other ELMs. Of various ELM-control methods, pellet triggering is studied and it is found that pellet-triggered ELMs are destabilised by the same mechanism as intrinsic Type I ELMs. In quiescent H-mode where no ELMs are observed, the plasma edge stability is significantly better than in similar ELMy plasmas. This can explain the absence of ELMs.
ELM, stability, modelling, edge plasma, MHD
Other note
  • S. Saarelma, S. Günter, T. Kurki-Suonio, and H.-P. Zehrfeld. 2000. ELM phenomenon as an interaction between bootstrap-current driven peeling modes and pressure-driven ballooning modes. Plasma Physics and Controlled Fusion, Vol. 42, number 5A, pages A139-A145. [article1.pdf] © 2000 Institute of Physics Publishing Ltd. By permission.
  • S. Saarelma, S. Günter, T. Kiviniemi, T. Kurki-Suonio, and ASDEX Upgrade Team. 2002. MHD stability analysis of type II ELMs in ASDEX Upgrade. Contributions to Plasma Physics, Vol. 42, pages 277-282.
  • S. Saarelma, S. Günter, L. D. Horton, and ASDEX Upgrade Team. 2003. MHD stability analysis of type II ELMs in ASDEX Upgrade. Nuclear Fusion, Vol. 43, pages 262-267. [article3.pdf] © 2003 International Atomic Energy Agency (IAEA). By permission.
  • S. Saarelma and S. Günter. 2004. Edge stability analysis of high β<sub>p</sub> plasmas. Plasma Physics and Controlled Fusion, Vol. 46, pages 1259-1270. [article4.pdf] © 2004 Institute of Physics Publishing Ltd. By permission.
  • S. Saarelma, V. Parail, Y. Andrew, E. de la Luna, A. Kallenbach, M. Kempenaars, A. Korotkov, A. Loarte, J. Lönnroth, P. Monier-Garbet, J. Stober, W. Suttrop, and Contributors to the EFDA-JET workprogramme. 2005. MHD stability analysis of diagnostic optimized configuration shots in JET. Plasma Physics and Controlled Fusion, Vol. 47, pages 713-731. [article5.pdf] © 2005 Institute of Physics Publishing Ltd. By permission.
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