Effects of boron powder injection on wall recycling, impurity content, and ELMs in KSTAR

Boron and boron nitride powders were injected into KSTAR plasmas to evaluate their effects on impurity content, wall recycling, and ELMs. Analysis of spectroscopic measurements showed that O-II (441.5 nm) and W-I (400.9 nm) emission from the lower divertor region of KSTAR is reduced with increasing cumulative amounts of injected B. P_RAD and Z_eff also decreased with B powder injection, suggesting that the impurity content was reduced. A dynamic particle balance calculation was performed to infer the wall pumping rate. This analysis found that during the steady-state portion of the discharge, the wall pumping rate was virtually unchanged by previous B injection. In the start-up portion of the discharges, on the other hand, the wall fueling rate decreased with increasing amounts of cumulative B injected. B powder also decreased the edge collisionality and increased the ELM height. This effect persisted after the impurity injection ceased. In a reduced collisionality regime following B powder wall conditioning, BN powder was injected, which resulted in periods of ELM mitigation and suppression. The ELM suppression coincided with the onset of edge harmonic oscillations (n=1, 3 kHz, ρ~0.9). A similar result had been previously observed in KSTAR, which this experiment reproduced with the new W divertor.