[1] Univ Milano Bicocca, CNISM, Dipartimento Fis, I-20100 Milan, Italy
[2] Assoc EURATOM ENEA CNR, Ist Fis Plasma, I-20100 Milan, Italy
[3] Univ Roma Tor Vergata, Dipartimento Fis, Ctr NAST, I-00133 Rome, Italy
[4] Univ Roma Tor Vergata, CNISM, I-00133 Rome, Italy
[5] Politecn Milan, Dept Energy, I-20133 Milan, Italy
[6] Rutherford Appleton Lab, ISIS Facil, STFC, Didcot OX11 0QX, Oxon, England
[7] PTB, D-38116 Braunschweig, Germany
[8] Univ Roma Tor Vergata, Dipartimento Ingn Meccan, I-00133 Rome, Italy