1. Department of Aerospace Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
2. Lawrence Livermore National Laboratory, Livermore, California 94550, USA
3. Department of Mathematical Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
4. Centre de Recherches en Physique des Plasmas, Association Euratom-Confederation Suisse, Ecole Polytechnique Fédérale de Lausanne, CRPP-PPB, CH-1015 Lausanne, Switzerland