Onset conditions for disruptions during VDEs

The Vertical Displacement Event, VDE, evolves on a slow timescale relative to the ideal MHD Alfvenic timescale. At some point the kink mode is strongly destabilized, leading to the final disruption. In most cases this occurs when q-edge is slightly less than two. Non-axisymmetric halo currents are often observed, well before the disruption. The evolution of the plasma during the VDE is modeled as a sequence of shrinking equilibria, where the core current profile remains constant so that the safety-factor at the axis, q-axis, remains fixed and the q-edge systematically decreases. Stability analysis shows that the plasma is indeed stable or has a small growth-rate until q- edge drops below 2, at which point the growth-rate rises rapidly, approaching $\gamma T_A =1$. The kink mode is characterized by ${m/n=2/1}$, where ${m}$ and $n$ are the poloidal and toroidal mode numbers. The surface currents associated with the MHD perturbation are computed. These may be related to the non-axisymmetric component of the halo currents and may be providing stability at modest growth-rates. This model is compared with observations on NSTX.