1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China
2. Institut für Energie- und Klimaforschung–Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
3. Max-Planck-Institute for Plasma Physics, 17491 Greifswald, Germany