1. College of Science, Donghua University, 201620 Shanghai, People's Republic of China
2. Max Planck Institute for Extraterrestrial Physics, 85748 Garching, Germany
3. Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft-und Raumfahrt (DLR), 82234 Weßling, Germany
4. Member of Magnetic Confinement Fusion Research Centre, Ministry of Education, 201620 Shanghai, People's Republic of China