1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA
2. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA
3. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
4. Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627, USA
5. Physics Department, Auburn University, Auburn, Alabama 36849, USA