ELM suppression and recycling reduction by boron nitride and boron injection in KSTAR

Abstract

Periods of ELM quiescence up to 5 s were observed with boron nitride (BN) injection into KSTAR ELMy H-modes with an innovative Impurity Powder Dropper (IPD) [1]. In addition, divertor Da emission was substantially reduced with boron (B) injection into ELMy H-modes, indicating improved recycling control. In both cases, there was no adverse impact on plasma stored energy. While edge fluctuations decreased overall, it appears that a new mode at ~ 180 kHz frequency appeared in beam emission spectroscopy data, suggesting a possible modification to the edge transport with active impurity injection. The powders were dropped into 0.5 MA plasmas with 1.6 MW of neutral beam heating and pulse durations of 10 - 20 s. Photodiode signals and real-color fast camera images show the powders entering the plasma. A series of 2.5 mg doses of BN, delivered in 0.1 s bursts, was observed to eventually reduce the ELM amplitude and frequency without changing the stored energy or plasma density. Analysis of the BES data during the ELM-free phase showed increased coherent mode activity near 180 kHz, corroborated by magnetics data. A continuous BN dose of 2.5 mg/s for 10 s reduced the ELM amplitude and frequency, but did not suppress ELMs. In addition, several 2.5 mg doses of B during a single discharge reduced recycling as evidenced by the reduced baseline Da level during the following shot. A 10 mg dose of B resulted in a disruption.