1. Department of Mechanical and Aerospace Engineering and Center for Energy Research, University of California at San Diego, San Diego, California 92093, USA
2. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
3. Sandia National Laboratories, Albuquerque, New Mexico 87123, USA
4. General Atomics, La Jolla, California 92121, USA
5. Department of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA