1. Ultrashort Electron Beam Science RIKEN Hakubi research team, RIKEN Cluster for Pioneering Research (CPR), RIKEN Center for Advanced Photonics (RAP) , 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
2. Department of Nuclear Engineering and Management, Graduate School of Engineering, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan