1. Department of Electrical and Electronic Engineering, Toyohashi University of Technology, 1–1 Hibarigaoka, Tempaku, Toyohashi 441‐8580, Japan
2. Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki 370‐1292, Japan
3. NGK Insulators, LTD., 2‐56 Suda‐cho, Mizuho, Nagoya 467‐8530, Japan