1. Southwestern Institute of Physics, P.O. Box 432, Chengdu, Sichuan 610041, China
2. Xia Men Tungsten Company, Ltd., Xiamen, Fujian 361010, China
3. ITER Organization, Route de Vinon-sur-Verdon, Saint-Paul-lez-Durance Cedex F-13067, France
4. CEA, IRFM, Saint-Paul-lez-Durance F-13108, France