1. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
2. Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA
3. Laboratoire de Physique des Plasmas, École Polytechnique, Palaiseau 91128, France
4. Space Science Center, University of New Hampshire, Durham, New Hampshire 03824, USA
5. Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623, USA