1. College of Science, Donghua University, 201620 Shanghai, People's Republic of China
2. Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 51147 Cologne, Germany
3. Member of Magnetic Confinement Fusion Research Centre, Ministry of Education, 201620 Shanghai, People's Republic of China