[1] EURATOM, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England
[2] EURATOM, FOM, Inst Plasma Phys Rijnhuizen, NL-3439 MN Nieuwegein, Netherlands
[3] Univ London Imperial Coll Sci Technol & Med, London SW7 2BZ, England
[4] Univ Calif Los Angeles, Dept Phys & Astron, Los Angeles, CA 90095 USA
[5] Walsh Sci Ltd, Culham Sci Ctr, Abingdon OX14 3EB, Oxon, England