1. National Institute for Fusion Science, National Institutes of Natural Sciences 1 , Toki 509-5292, Japan
2. The Graduate University for Advanced Studies, SOKENDAI 2 , Toki 509-5292, Japan
3. University of Wisconsin-Madison 3 , Madison, Wisconsin 53706, USA
4. University of Texas 4 , Austin, Texas 78712, USA
5. General Atomics 5 , San Diego, California 92186, USA