1. State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
2. Dipartimento di Fisica “G. Occhialini,” Università degli Studi di Milano-Bicocca, Milano 20126, Italy
3. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
4. Institute for Plasma Science and Technology, National Research Council, Milan 20125, Italy