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Comparison of Models to Experiment for the Purposes of Axisymmetric Control in Alcator C-Mod

Gerasimos Tinios, Steve F. Horne, Ian H. Hutchinson, Stephen M. Wolfe

Fusion Science and Technology

Volume 30 / Number 2 / November 1996 / Pages 201-218


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Linear control models are tested against experimental data from the Alcator C-Mod tokamak. A nonrigid, approximately flux-conserving, perturbed equilibrium plasma response model is used, together with a detailed toroidally symmetric model of the conducting vacuum vessel and the supporting superstructure, and experimentally determined power supply responses. Experiments are conducted with vertically unstable plasmas where the feedback is turned off and the plasma response is observed in an open-loop configuration. The agreement between theory and experiment is found to be very satisfactory, proving that the perturbed equilibrium plasma response model and a toroidally symmetric electromagnetic model of the vacuum vessel and the structure can be trusted for the purposes of calculations for control law design. The closed-loop behavior is also examined by injecting step perturbations into the desired vertical position of the plasma. The control hardware introduces nonlinearities that make it difficult to explain observed behavior with linear theory. Nonlinear simulation of the time evolution of the closed-loop experiments is able to account for the discrepancies between linear theory and experiment. Satisfactory agreement is then obtained between the model including the full multiple input/multiple output control system and the experimental observations.

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