Magnetic triggering of ELMs in TCV

Experiments on the TCV tokamak have shown that rapid vertical movement of diverted ELMy H-mode plasmas can affect the time sequence of edge localized modes. The effect is attributed to the induction of an edge current during the movement of the plasma column in the spatially inhomogeneous vacuum field of a single-null configuration. In TCV the fast vertical movement is provoked by the positional control coils inside the vacuum vessel, however it is argued that a similar effect might be produced in larger devices only using poloidal field coils external to the vessel. A simple model, which includes plausible elements of the dynamical behaviour of the edge pressure gradient and edge current, which together dictate the MHD stability of the discharge against edge-localized, current-driven modes, is used to reproduce some of the features seen in these experiments.