MECHANISM OF VERTICAL DISPLACEMENT EVENTS IN JT-60U DISRUPTIVE DISCHARGES

Enhanced vertical displacement events (VDEs), which are frequently obs erved in JT-60U disruptive discharges, are investigated using the Toka mak Simulation Code (TSC). The rapid plasma current quench can acceler ate the vertical displacement, owing to both the up/down asymmetry of the eddy current distribution arising from the asymmetric geometry of the JT-60U vacuum vessel and the degradation of magnetic field decay i ndex n, leading to high growth rates of positional instability. For a slightly elongated configuration (n = -0.9), the asymmetry of attracti ve forces on the toroidal plasma plays a dominant role in the VDE mech anism. For a more elongated configuration (n = -1.7), the degradation of Geld decay index n plays an important role on VDEs, in addition to the effect of asymmetric attractive forces. It is shown that the VDE c haracteristics of a highly elongated configuration with a rapid plasma current quench can be dominated by the held decay index degradation. It is also pointed out that both the softening of current quenches as was experimentally developed in the JT-60U tokamak, and the optimizati on of the allowable elongation of the plasma cross-section are critica l issues in the development of a general control strategy of discharge termination.