Comparison of Models to Experiment for the Purposes of
Axisymmetric Control in the Alcator C-Mod Tokamak
Dr G. Tinios (MIT)
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 the 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 behaviour
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 behaviour 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 full multiple
input/multiple output model of the system and the experimantal observations.
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