Plasma modelling results and shape control improvements for NSTX

E. Kolemen, D. A. Gates, S. Gerhardt, R. Kaita, H. Kugel, D. Mueller, C. Rowley, and V. Soukhanovskii

Nuclear Fusion 51 : 113024, November 2011.

Abstract:

New shape control implementations and dynamics studies on the National Spherical Torus eXperiment (NSTX) 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 analysing 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 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 X-point 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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