1. Research and Development Department, Electron Tube Division, Hamamatsu Photonics K.K., Iwata 438-0193, Japan
2. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
3. Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
4. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA