1. Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark
2. ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St Paul-lez-Durance, France
3. Ioffe Institute, Polytechnicheskaya St. 26, St. Petersburg 194021, Russian Federation
4. Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland