1. College of Science, Donghua University, Shanghai 201620, China
2. Member of Magnetic Confinement Fusion Research Centre, Ministry of Education, Shanghai 201620, China
3. General Atomics, PO Box 85608, San Diego, California 92186-5608, USA
4. ITER Organization, Route de Vinon, CS 90 046, 13067 Saint Paul Lez Durance, France