[1] EURATOM, FOM, Inst Plasma Phys Rijnhuizen, NL-3430 BE Nieuwegein, Netherlands
[2] Eindhoven Univ Technol, Dept Appl Phys, NL-5600 MB Eindhoven, Netherlands
[3] Leiden Univ, Leiden Inst Chem, NL-2300 RA Leiden, Netherlands
[4] EURATOM, Forschungszentrum, Inst Plasmaphys, D-5245 Julich, Germany
[5] EURATOM, Ecole Royal Mil, Lab Phys Plasmas, B-1000 Brussels, Belgium