1. Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08540, USA
2. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
3. Departement de Physique, Université Paris XI-Ecole Polytechnique, LOA-ENSTA 9128 Palaiseau, France
4. Laboratoire Plasma et Conversion d'Energie, CNRS, INPT, UPS, 31062 Toulouse, France