1. Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
2. Osaka University, 2-6 Yamada-Oka, Suita, Osaka 565-0871, Japan
3. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
4. The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
5. Diamond Research Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan