1. CCFE, UK Atomic Energy Authority, Culham Science Centre, Oxfordshire OX14 3DB, United Kingdom
2. Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom
3. DEN-Service de Recherches de Metallurgie Physique, CEA, Universite Paris-Saclay, F-91191 Gif-sur-Yvette, France
4. Metallurgie Physique et Genie des Materiaux, UMET, CNRS, UMR 8207, Universite de Lille, F-59000 Lille, France
5. Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland