beta(p)-collapse-induced vertical displacement event in high beta(p) tokamak disruption

Extremely fast vertical displacement events (VDEs) induced by a strong beta(p) collapse were found in a vertically elongated (kappa approximate to 1.5), high beta(p) (beta(p) approximate to 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 beta(p) tokamak (beta(p) similar to 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 (gamma approximate to 655 s(-1)) faster than the growth rate of the usual positional instability (gamma approximate to 149 s(-1)). The essential mechanism of the beta(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 beta(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 beta(p) collapse and the n-index of the equilibria just before the beta(p) collapse occurs. The JT-60U vacuum vessel is shown to possess the capability of preventing beta(p)-collapse-induced VDEs.