18th 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

CentroRicercheENEA,00044FrascatiC.P.65,Rome,Italy*e-mail: franco.alladio@enea.it

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-31 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)