1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China
2. HeFei University of Technology, Hefei 230009,
People's Republic of China
3. CEA, IRFM, F-13108 Saint Paul-Lez-Durance,
France
4. CRPP, EPFL, CH-1015 Lausanne,
Switzerland