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Abstract

Experiments and axisymmetric MHD simulations on tokamak disruptions have explicated the
underlying mechanisms of Vertical Displacement Events (VDEs) and a diversity of disruption dynamics. First,
the neutral point, which is known as an advantageous vertical plasma position to avoiding VDEs during the
plasma current quench, is shown to be fairly insensitive to plasma shape and current profile parameters.
Secondly, a rapid flattening of the plasma current profile frequently seen at thermal quench is newly clarified to
play a substantial role in dragging a single null-diverted plasma vertically towards the divertor. As a
consequence, the occurrence of downward-going VDEs predominates over the upward-going ones in bottom-diverted
discharges. This dragging effect is absent in up-down symmetric limiter discharges. These simulation
results are consistent with experiments. Together with the attractive force that arises from passive shell currents
and essentially vanishes at the neutral point, the dragging effect explains many details of the VDE dynamics over
the whole period of the disruptive termination.