1. Istituto di Fisica del Plasma “P. Caldirola,” CNR, Milano, Italy
2. Dipartimento di Fisica “G. Occhialini,” Università degli Studi di Milano-Bicocca, Milano, Italy
3. School of Physics, State Key Lab of Nuclear Physics and Technology, Peking University, Beijing, China
4. Departments of Physics and Astronomy, Uppsala University, Uppsala, Sweden