1. Consorzio RFX, Corso Stati Uniti 4, I-35127 Padova, Italy
2. Padova University, Department of Physics and Astronomy, Via Marzolo 8, 35131 Padova, Italy
3. CNR, Corso Stati Uniti 4, 35127 Padova, Italy
4. Centro Ricerche Energia, Ass. EURATOM-ENEA-CNR, Roma, Italy