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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 . [ABSTRACT FROM AUTHOR]

The radial electric field is driven to positive value by off-axis second harmonic electron cyclotron heating (ECH) in the Compact Helical System [Plasma Physics and Controlled Nuclear Fusion Research 1988, Nice (International Atomic Energy Agency, Vienna, 1989), Vol. II, p. 411]. The observed positive electric field is associated with the outward particle flux enhanced with ECH. The enhanced particle flux triggered by the production of the electrons accelerated perpendicularly to the magnetic field with ECH results in the change of the electric field. [ABSTRACT FROM AUTHOR]