Enhancement of detachment control with simplified real-time modelling on the KSTAR tokamak

Detachment control based on ion saturation current I_sat measurements from Langmuir probes (LPs) is implemented in the KSTAR tokamak and shown to be capable of following dynamic and constant target trajectories with good accuracy, in H-mode, by moderating the flow rate of nitrogen or deuterium. I_sat controllers normalize I_sat in order to form attachment fraction (A_frac) as their control parameter. The KSTAR implementation of A_frac control differs from previous work in that it continuously calculates a model for attached I_sat and uses that as the denominator in A_frac, whereas prior implementations either record peak I_sat at rollover as they pass it or take estimated I_sat,rollover as a manual input prior to the shot. The KSTAR controller therefore does not need to keep track of rollover status and keep separate targets for pre- and post-rollover states, and it can automatically adapt to changes in scenario at any time. It is also less vulnerable to noise as it will not lock in an outlier as a rollover point.