Stabilization of plasma vertical position of elongated tokamak using upper and lower triangular coils

Improvement of the vertical stability of tokamak plasma with only local coils above and below the plasma was observed for the first time. The local coils in triangular shape, named upper and lower triangular (ULT) coils, were installed in TOKASTAR-2, a tokamak-stellarator hybrid device at Nagoya University. The ULT coils generate the effective radial magnetic field B R eff, which is expected to stabilize the plasma vertical position. The region of the plasma position where vertical displacement events (VDEs) did not occur was extended by applying the ULT magnetic field. Stabilization of the plasma vertical position was related to the ratio of the vertical-direction derivative of the effective radial field generated by the ULT coils, ∂ B R eff / ∂ Z , to that of the axisymmetric radial field, ∂ B R / ∂ Z. Moreover, even when VDEs occurred in discharges with the ULT magnetic field, the VDE speed was mitigated by the effect of the ULT magnetic field; the VDE speed increased with ∂ B R / ∂ Z, while decreased with ∂ B R eff / ∂ Z.