Real-time Feedback Control of the Impurity Powder Dropper in KSTAR

Our work enables feedback control of the KSTAR Impurity Powder Dropper (IPD) allowing us to precisely adjust IPD actuation for specific and reproducible powder flow rates. The IPD uses a piezoelectric vibratory feeder to move Boron (B) and Boron Nitride (BN) powders. The powder flow rate is monitored by an optical flowmeter, which measures light attenuation that is related to the powder flow rate, and the ablation monitor measures light originating from the plasma as it interacts with the injected impurity powder. Previously, feedforward control with user-inputted piezo actuation voltages regulated the powder injection. The magnitude of the piezo actuation voltage does not directly relate to a reproducible flow rate, as IPD reservoir or feeder fill levels can cause variations. To minimize and potentially eliminate flow variations, we implement an IPD flow rate feedback control scheme using a target drop rate and the flowmeter. Upgraded IPD connections enable the communication between the KSTAR Plasma Control System (PCS) and IPD, incorporating real time signals for the flowmeter and ablation monitor. The flow feedback control scheme will use a PID (Proportional-Integral-Derivative) controller to minimize error between the desired and the measured flow rate. The IPD injects powder through a 4 m drop tube, causing a latency between the powder passing the flowmeter and reaching the plasma. To use the IPD as an actuator in plasma detachment feedback control, this latency time must be considered. By accounting for these latency times, which average 2.5 s for B due to center funnel misalignment and 0.7 s for BN, timescales for detachment control can be properly adjusted to avoid oscillatory control behaviors.