1. Institute for Electrical Engineering and Plasma Technology, Ruhr-University Bochum, D-44780 Bochum, Germany
2. Institute for Electrical Engineering and Plasma Technology, Ruhr-University Bochum, D-44801 Bochum, Germany
3. Elementary Processes in Gas Discharges, Faculteit Technische Natuurkunde, Technische Universiteit Eindhoven, 5600 MB Eindhoven, The Netherlands
4. Department of Physics, West Virginia University, Morgantown, West Virginia 26506-6315, USA
5. Department of Nuclear Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA