Surface heat flux control by divertor impurity seeding on the DIII-D tokamak*

Control of the intense heat flux to the divertor is one of the outstanding problems in tokamaks. One technique that has shown promise is impurity seeding, i.e., the injection of low-Z gaseous impurities to radiate and dissipate the power before it arrives to the divertor target plate via thermal conduction. As a first step in the development, the DIII-D team has implemented a feedback system to control the injection of seed gas based on real-time surface heat flux estimation, as injecting excess impurity gas is disadvantageous. A model-based approach, successfully benchmarked against off-line infrared camera measurements for H-mode plasma discharges, provides real-time estimation of the plasma heat flux on the divertor target. The real-time surface heat flux is estimated in the DIII-D plasma control system, which then uses a proportional–integral–derivative (PID) algorithm to control the injection of gas and mitigate divertor heat flux. This paper presents the design of the new feedback system, validation of the model-based estimation against IR camera measurements as well as the real-time regulation of the peak divertor heat flux using impurity seeding in H-mode plasma discharge.