[1] MIT, Dept Phys, Cambridge, MA 02139 USA
[2] Indian Inst Technol Delhi, Dept Phys, New Delhi 110016, India
[3] Univ Calif Riverside, Dept Phys & Astron, Riverside, CA 92521 USA
[4] Hong Kong Univ Sci & Technol, Dept Phys, Hong Kong, Peoples R China
[5] MIT, Francis Bitter Magnet Lab, 77 Massachusetts Ave, Cambridge, MA 02139 USA
[6] MIT, Plasma Sci & Fus Ctr, 77 Massachusetts Ave, Cambridge, MA 02139 USA