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18 th International Congress on Plasma Physics (ICPP 2016) The plasma centerpost obtained in the PROTO-SPHERA experiment F.Alladio , L.Boncagni, F.Causa, F.Crescenzi, E.Giovannozzi, A.Grosso, A.Lampasi, G.Maffia, A.Mancuso, P.Micozzi, V.Piergotti, G.Rocchi, A.Sibio, B.Tilia, V.Zanza Centro Ricerche ENEA, 00044 Frascati C.P. 65, Rome, Italy *e-mail: [email protected] The PROTO-SPHERA experiment explores a spherical tokamak where a plasma centerpost driven by electrodes replaces the metal centerpost. The spherical torus will be formed destabilizing the plasma centerpost, as in TS-3 1 experiments. PROTO-SPHERA uses the cylindrical (ø 2m) START vacuum vessel, extended to 2.5m heigth to host an annular gas- puffed anode and an annular 54 W-filaments heated cathode. PROTO-SPHERA must obtain in Phase-1 a stable plasma centerpost carrying 8.5kA, shaped by 8 PF-coils whose casings are electrically floating inside the vessel at ground potential. The worry was that localized anode arc-attachment could spoil the centerpost shape: the obtained plasmas, up to 3.4kA, 300ms, don't show any anode arc-attachment. The plasma itself charges the PF-coils casings: E-field and B-field, parallel near the cathode, are almost perpendicular at the anode, upon whose surface the ensuing E∧B drift spreads evenly the centerpost plasma. Boundary conditions are controlled by additional PF-coils outside the vessel, by biasing with resistors the internal PF- coils casings and by rebuilding in insulating material the flanges of the plasma current feed- throughs on both machine lids. After obtaining –8.5kA, 1s– centerpost plasma, Phase-2 of the experiment will try and produce a 240kA spherical torus around a 60kA plasma centerpost. 1. N. Amemiya et al., J. Phys. Soc. Jpn., 63, 1552 (1993)
