Plasma Modeling Results and Shape Control Improvements for NSTX
Authors: E. Kolemen, D.A. Gates, S. Gerhardt, R. Kaita, H. Kugel, D. Mueller, C. Rowley and V. Soukhanovskii
Abstract:
New shape control implementations and dynamics studies on the National Spherical Torus
eXperiment (NSTX) [8] are summarized. In particular, strike point position, X-point height and squareness
control, and two new system-identification methods / control-tuning algorithms were put into operation.
The PID controller for the strike point was tuned by analyzing the step response of the strike point position
to the poloidal coil currents, employing the Ziegler-Nichols method. An offline system identification of the
plasma response to the control inputs based on ARMAX [5] input-output models was implemented. With
this tool, rough estimates of the improvements were realized and several control improvements were
identified. An online automatic relay-feedback PID tuning algorithm, which has the advantage of tuning the
controller in one shot, was implemented, thus optimizing the use of experimental time. Using these new
capabilities, all four upper/lower/inner/outer strike points were simultaneous controlled and a combined Xpoint
height, strike point radius control was implemented. The new and improved control with better
accuracy and robustness enabled successful plasma operations with the liquid lithium divertor. Additionally
this year, the first independent squareness control was developed. This will enable better optimization of
the NSTX shape for stability and high performance in the future.
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Submitted to: Nuclear Fusion, (October, 2011)
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Download PPPL-4695 (pdf 2.47 MB 16 pp)
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