1. Plasma Physics Laboratory, Kyungpook National University, 1370 Sankyuk-Dong, Puk-Gu, Daegu 702-701, Korea
2. Plasma Research Center, University of Tsukuba, Ibaraki 305-8577, Japan
3. Device Development Division, Korea Basic Science Institute, 52 Yeoeun-Dong, Yusung-Gu, Daejeon 305-333, Korea