1. General Atomics, San Diego, California 92186-5608, USA
2. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
3. Center for Energy Research, University of California San Diego, La Jolla, California 92093-0417, USA
4. Department of Engineering Physics, University of Wisconsin Madison, Madison, Wisconsin 53706, USA
5. University of Toronto Institute for Aerospace Studies, Toronto, Ontario M3H 5T6, Canada