1. Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3. School of Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan
4. Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA
5. Centro de Fisica de Materiales (CFM-MPC), Centro Mixto CSIC-UPV/EHU, Manuel de Lardizabal 4, E-20018 San Sebastian, Spain
6. Donostia International Physics Center (DIPC), Manuel de Lardizabal 5, E-20018 San Sebastian, Spain
7. Physics Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA