[1] Dartmouth Coll, Dept Phys & Astron, Hanover, NH 03755 USA
[2] Ecole Polytech Fed Lausanne, Assoc Euratom Confederat Suisse, Ctr Rech Phys Plasmas, CH-1015 Lausanne, Switzerland
[3] Univ Texas Austin, Inst Fus Studies, Dept Phys, Austin, TX 78712 USA
[4] Univ New Hampshire, Dept Phys, Durham, NH 03824 USA