1. National Fusion Research Institute, Daejeon 305-333, South Korea
2. Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea
3. Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA
4. School of Space Research, Kyung Hee University, Yongin, South Korea
5. Department of Astronomy and Space Science, College of Natural Sciences and Institute for Basic Science Research, Chungbuk National University, Cheongju 28644, South Korea