1. Plasma Research Center (PRC), University of Tsukuba, Tsukuba, Ibaraki, 305-8577, Japan
2. National Institute for Fusion Science (NIFS), Toki, Gifu, 509-5292, Japan
3. Toshiba Electron Tubes & Devices Co., Ltd. (TETD), Otawara, Tochigi, 324-8550, Japan
4. Japan Atomic Energy Agency (JAEA), Naka, Ibaraki, 311-0193, Japan