1. Department of Mechanical and Aerospace Engineering, University of California San Diego, La Jolla, California 92093, USA
2. Center for Energy Research, University of California San Diego, La Jolla, California 92093, USA
3. Center for Astrophysics and Space Sciences, University of California San Diego, La Jolla, California 92093, USA
4. Department of Physics, University of California, San Diego, California 92093, USA
5. Center for Fusion Sciences, Southwestern Institute of Physics, Chengdu, Sichuan 610041, People's Republic of China