Improvement on vertical stabilization control of KSTAR

By dedicated experimental measurements on the VS controllability done in 2012-2015, it was found that the maximum controllable vertical displacement ΔZ_MAX is 2.47±0.94 cm for the plasmas with elongation=1.75, γ_z ∼ 110 rad/s. In order to improve VS controllability for the extended axisymmetric magnetic controls, in 2013-2014 campaigns introduction of a dedicated speed estimator enabled to reduce the PD gains with same controllability. After that, a new approach using decoupling in the frequency domain was introduced in 2015, in order to aim more effective reduction of fast/slow control competitions. The application of the decoupling, combined with a relay-feedback tuning technique, demonstrated extension of the available vertical position up to -10 cm without manual compensations of the Z target and the shape target. (author)