EXPERIMENTAL-STUDY OF THE VERTICAL STABILITY OF HIGH DECAY INDEX PLASMAS IN THE DIII-D TOKAMAK

Experiments on the stabilization of highly elongated, vertically unstable plasmas were carried out on the DIII-D tokamak. Identification of the closed-loop transfer function showed that vertical stability could be usefully modelled as a second order dynamical system. The effect of varying the controller gains and the vertical field decay index was studied and found to be qualitatively as predicted by a low order model proposed previously. The implementation of a new hybrid inboard/outboard coil positional control with differing controller dynamics allowed operation of DIII-D up to 92% of the limiting equilibrium field decay index of the vacuum vessel for the plasmas used. This improved control allowed operation at a plasma elongation κ of up to 2.5. © 1990 IOP Publishing Ltd.