1. C.N.R. National Research Council, ISMAC-Institute for Macromolecular Studies, Corso G. Pella 16, 13900 Biella, Italy
2. C.N.R. National Research Council, ISMAC-Institute for Macromolecular Studies, Corso G. Pella 16, 13900 Biella, Italy,
3. C.N.R. National Research Council, IFP-Institute of Plasma Physics, via R. Cozzi 53, 20125 Milan, Italy