1. Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden
2. Centre for Plasma Physics, School of Mathematics and Physics, Queen's University of Belfast, Belfast BT7 1NN, United Kingdom
3. Univ Potsdam, Inst Phys & Astron, D-14476 Potsdam, Germany
4. DESY, D-15738 Zeuthen, Germany