Remediation of Time-Delay Effects in Tokamak Axisymmetric Control Loops by Optimal Tuning and Robust Predictor Augmentation
D. Sondak, R. Arastoo, E. Schuster, M.L. Walker
Fusion Engineering and Design 86 (2011) 1112-1115
Abstract
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It is sometimes incorrectly assumed that, because superconducting tokamaks already have significant intrinsic or
imposed sources of control delay, introducing extra delays/lags into the axisymmetric control loops will have
negligible detrimental impact on the plasma control. This study exposes and quantifies the detrimental effects
imposed by time delays/lags in the control loop in superconducting tokamaks, using as an example the plasma current
control and radial position control in a vertically stable circular plasma in the KSTAR tokamak. Delays and lags
in the power supplies, data acquisition, and vessel structure are taken into account. Optimal tuning of PID
controllers in combination with an ohmic-flux control strategy is proposed as a possible method for remediating the
negative effects of time delays/lags. In addition, an augmentation of the control loop by the introduction of a
robust predictor has been proposed to improve the performance of the time-delayed closed-loop system when the amount
of delay/lag in the loop is unknown. The Nyquist dual locus technique based on the Argument Principle in complex
theory is employed to assess stability of the optimally tuned closed-loop system in the presence of time delays.