Wall conditioning effects of boron powder injection in KSTAR with lower tungsten divertor*

Boron powder (<150 μm diameter, 99.9% purity) was injected into KSTAR L- and H-mode plasmas using the impurity powder dropper (IPD) to evaluate its wall conditioning effects. In the L-mode experiment, powder was injected at a rate of 8-17 mg/s in short 0.2 s bursts, for a total of 27 mg over eight discharges. In the H-mode experiment, powder was injected at a rate of 30-40 mg/s, in 0.5-1 s intervals, for a cumulative total of 132 mg of boron over two discharges. These experiments were performed in lower single null discharges with the new tungsten monoblock divertor. The filterscope system on KSTAR was used to measure the relative changes in the W-I (400.9 nm wavelength) line radiation. These data showed that, in the lower divertor, the line intensity is reduced after boron powder injection. The compact advanced extreme-UV spectrometer (CAES) system was used to measure W radiation (~5 nm wavelength) in the core. This measurement found little to no change in the W line intensity in the core after boron powder injection. Visible survey spectroscopy data show that impurity concentration of oxygen and nitrogen was reduced with boron injection, indicating a wall conditioning effect. In addition, boron powder injection resulted in decreased radiated power, increased core electron density, and decreased core electron temperature. These results further suggest that boron powder injection had a wall conditioning effect.