1. Institute of Fusion Science School of Physical Science and Technology Southwest Jiaotong University Chengdu 610041 P. R. China
2. Southwestern Institute of Physics Chengdu 610041 P. R. China
3. School of Physical Science and Technology Southwest Jiaotong University Chengdu 610031 P. R. China
4. School of Physical Science and Technology Sichuan University Chengdu 610041 P. R. China