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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Divertor Plasma Conditions and Their Effect on Carbon Migration in the ASDEX Upgrade Tokamak

Leena Aho-Mantila

Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the School of Science for public examination and debate in Auditorium K216 at the Aalto University School of Science (Espoo, Finland) on the 4th of November 2011 at 12 noon.

Overview in PDF format (ISBN 978-951-38-7757-6)   [1735 KB]
VTT Publications 773, ISSN 1455-0849

Dissertation is also available in print (ISBN 978-951-38-7756-9)
Copyright © 2011 VTT Technical Research Centre of Finland
VTT Publications 773, ISSN 1235-0621
VTT-PUBS-773

Abstract

Divertors play a critical role in power and particle exhaust and plasma-material interaction in tokamaks. Of particular concern is the release of impurities from the material components, which reduces the lifetime of the vessel wall and affects the fusion performance. Furthermore, impurities with low charge number can migrate and re-deposit forming potentially tritium-rich layers in the tokamak. Sophisticated numerical simulations are required to understand how the various complex and often nonlinear processes in the plasma boundary affect the divertor performance.

This thesis investigates the plasma conditions and the migration of impurities in the divertor region, using numerical code packages with the most complete available description of the relevant physical processes. It is known from earlier studies that the codes have both weaknesses and uncertainties in their models, which limits our current predictive capabilities. In this thesis work, carbon injection experiments at the ASDEX Upgrade tokamak are modelled. The work assesses the present-day boundary plasma models and elucidates impurity migration processes by comparing the simulations with experimental data.

The results show that both the local migration and the net erosion of carbon are sensitive to the divertor plasma conditions. Good agreement between the modelled and measured divertor plasma parameters is obtained for a limited operational regime at low density. In this regime, the measured carbon migration is also reproduced in the simulations, and it is shown to be significantly affected by the electric field and the collisionality of the plasma in the divertor region. A significant improvement in code-experiment agreement is obtained after drifts arising from electric and magnetic fields are included in both plasma and impurity simulations. At higher plasma densities, collisionless hot electrons are a potential explanation for the deficiencies observed when benchmarking the plasma models against experiments.

This thesis consists of an overview and of the following 5 publications:

  1. Aho-Mantila L K, Kurki-Suonio T, Chankin A V, Coster D P and Sipilä S K, ASCOT simulations of electron energy distribution at the divertor targets in an ASDEX Upgrade H-mode discharge, Plasma Physics and Controlled Fusion 50 (2008) 065021 (13 pp).
  2. Aho-Mantila L, Airila M I, Wischmeier M, Krieger K, Pugno R, Coster D P, Chankin A V, Neu R, Rohde V and the ASDEX Upgrade Team, Modelling of 13CH4 injection and local carbon deposition at the outer divertor of ASDEX Upgrade, Physica Scripta T138 (2009) 014019 (4 pp).
  3. Aho-Mantila L, Wischmeier M, Airila M I, Chankin A V, Coster D P, Fuchs Ch, Groth M, Kirschner A, Krieger K, Müller H W, Wolfrum E, and the ASDEX Upgrade Team, Modelling of Carbon Transport in the Outer Divertor Plasma of ASDEX Upgrade, Contributions to Plasma Physics 50, No. 3-5, 439-444 (2010).
  4. Aho-Mantila L, Wischmeier M, Krieger K, Rohde V, Müller H W, Coster D P, Groth M, Kirschner A, Neu R, Potzel S, Sieglin B, Wolfrum E, The ASDEX Upgrade Team, Effect of E × B driven transport on the deposition of carbon in the outer divertor of ASDEX Upgrade, accepted for publication in Journal of Nuclear Materials (2010) (4 pp).
  5. Aho-Mantila L, Wischmeier M, Krieger K, Müller H W, Potzel S, Rohde V, Hakola A, Coster D P, Kirschner A, Bonnin X, Borodin D, Conway G D and the ASDEX Upgrade Team, Influence of magnetic field reversal on the divertor plasma and its impact on local carbon migration in ASDEX Upgrade, submitted for publication in Nuclear Fusion (2011) (35 pp).

Keywords: tokamak, plasma-material interaction, scrape-off layer, kinetic modelling, fluid modelling, ASDEX Upgrade, fusion energy

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© 2011 Aalto University

Last update 2011-11-07