Time-dependent SOLPS-ITER simulation for KSTAR target flux bifurcation analysis

Extensive tokamak boundary plasma studies have been conducted using the SOLPS-ITER code package including 2D fluid plasma and a kinetic neutral solver, and have generally focused on time-independent solutions. SOLPS-ITER code is inherently capable of time-dependent simulations. In addition, with recently developed features such as time-varying gas throughput, it can be applied to feedforward simulation, and feedback control through the IPS integrated modeling environment with system identification. A feedforward SOLPS-ITER simulation was performed for the target flux bifurcation observed in the KSTAR density ramp experiment, which is a dynamic problem. The observed bifurcation time scale was in good agreement with the value obtained from simulation within a factor of 2, and the core penetration of X-point radiation observed through the bolometer was also reproduced. Accordingly, the cause of bifurcation was explained through the two-point formatting equation. With data-driven system identification via DMD and SINDy, feedback control of upstream density using IPS was demonstrated and the validity of the model subject to bifurcation was tested.