1. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
2. École Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), Lausanne CH-1015, Switzerland
3. MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 01239, USA
4. Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Lisbon 1049-001, Portugal