1. Department of Mechanical and Aerospace Engineering, University of California at San Diego, La Jolla, California 92093, USA
2. Center for Energy Research, University of California at San Diego, La Jolla, California 92093, USA
3. Institute for Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf e.V., 01328 Dresden, Germany
4. General Atomics, San Diego, California 92121, USA