1. Institute of Plasma Physics; Chinese Academy of Sciences; P.O. Box 1126; Hefei 230031 P. R. China
2. Center of Medical Physics and Technology; Hefei Institutes of Physical Science; Chinese Academy of Sciences; Hefei 230031 P. R. China
3. Department of Thermal Science and Energy Engineering; University of Science and Technology of China; Hefei Anhui Province 230026 P. R. China
4. Department of Physics and Materials Science; City University of Hong Kong; Tat Chee Avenue; Kowloon Hong Kong China