1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
2. General Atomics, P. O. Box 85608, San Diego, California 92186, USA
3. School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China
4. Lawrence Livermore National Laboratory, Livermore, California 94550, USA
5. Association Euratom-Risø DTU, DK-4000 Roskilde, Denmark