1. Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan
2. The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
3. Plasma Science and Fusion Center at Massachusetts Institute of Technology, 77 Massachusetts Avenue, NW16, Cambridge, Massachusetts 02139-4307, USA