β_p-collapse-induced vertical displacement event in high β_p tokamak disruption

Extremely fast vertical displacement events (VDEs) induced by a strong β_p collapse were found in a vertically elongated (κ ≈ 1.5), high β_p (β_p ≈ 1.7) tokamak with a resistive shell through computer simulations using the tokamak simulation code. Although the plasma current quench which has been shown to be the prime cause of VDEs in a relatively low β_p tokamak (β_p ∼ 0.2) (Nakamura Y et al 1996 Nucl. Fusion 36 643), was not observed during the VDE evolution, the observed growth rate of VDEs was almost five times (γ ≈ 655 s^-1) faster than the growth rate of the usual positional instability (γ ≈ 149 s^-1). The essential mechanism of the β_p-collapse-induced VDE was clarified to be the intense enhancement of positional instability due to a large and sudden degradation of the magnetic field decay n-index in addition to the significant destabilization due to a reduction in the stability index n_s. The radial shift of the magnetic axis caused by the β_p collapse induces eddy currents on the resistive shell, and these eddy currents produce a large degradation of the n-index. It is pointed out that the shell geometry characterizes the VDE dynamics, and that the VDE rate depends strongly both on the magnitude of the β_p collapse and the n-index of the equilibria just before the β_p collapse occurs. The JT-60U vacuum vessel is shown to possess the capability of preventing β_p-collapse-induced VDEs.