[1] UKAEA Euratom Fus Assoc, Culharn Sci Ctr, Abingdon OX14 3DB, Oxon, England
[2] EURATOM, Max Planck Inst Plasmaphys, Garching, Germany
[3] Univ London, Technol & Med, Imperial Coll Sch, London SW7 2BZ, England
[4] Univ York, York YO10 5DD, N Yorkshire, England