[1] Princeton Univ, Princeton Plasma Phys Lab, Princeton, NJ 08544 USA
[2] MIT, Plasma Sci & Fus Ctr, Cambridge, MA 02139 USA
[3] Univ Calif San Diego, Energy Res Ctr, San Diego, CA 92103 USA
[4] Oak Ridge Natl Lab, Oak Ridge, TN USA
[5] Gen Atom Co, San Diego, CA USA