Support of the model for ‘‘Vertical displacement episodes’’ from numerical simulation of episodes observed in the DIII-D tokamak

The plasma of a tokamak with a vertically elongated cross section is normally kept in its central position by the action of a feedback system that controls currents in external coils. If this control becomes unsuccessful, a vertical displacement episode develops. During such an episode, the vacuum vessel may be subject to large vertical forces. Since these forces are large enough to impact the design of future devices, there is a need to understand them. In this paper a comparison is described between data from such vertical displacement episodes observed in the DIII-D tokamak [Bull. Am. Phys. Soc. 31, 1533 (1986)] to those from a numerical simulation code designed to realize a previously suggested conceptual model for the plasma physics part of a displacement episode. The comparison suggests validity of the model and that the code provides a realization of the model. Other code runs suggest that before reasonable predictions can be made on vertical forces in future devices, specific scenarios for the time development of the currents of external coils during an episode are needed.