[1] Technion Israel Inst Technol, Dept Phys, IL-3200003 Haifa, Israel
[2] Royal Inst Technol, KTH, Engn Mech Dept, S-11428 Stockholm, Sweden
[3] Imperial Coll London, Plasma Phys Grp, London SW7 2BW, England
[4] European Synchrotron Radiat Facil, CS40220, F-38043 Grenoble 9, France