1. Institute for Photonics and Nanotechnologies CNR-IFN
2. 20133 Milano
3. Italy
4. Centre for Plasma Physics
5. School of Maths and Physics
6. Queen's University Belfast
7. UK
8. Inst. for the Structure of Matter CNR-ISM
9. Monterotondo
10. Department of Physics
11. Politecnico di Milano
12. Center for Free-Electron Laser Science
13. DESY