Improved fusion plasma performance in fusion devices enabled by a new impurity powder injection system

Low-Z impurities have been injected into a number of devices to improve performance, enabled by a relatively new apparatus that is designed to inject a wide range of impurity powders and granules. This paper summarizes the technical aspects of the injector and onboard diagnostics, while focusing on new ELM control results from the EAST and KSTAR devices. The new impurity injector was developed over the last 4 years, and uses vertically-mounted piezoelectric crystals for a horizontal drive off the edge of a surface into a drop tube. This apparatus can inject many different impurity species and particulate sizes. The lateral back-and-forth motion of up to 10 mm allows reproducible and calibratable impurity injection rates. An accelerometer and an optical sensor measure the injection rates. Boron powder injected with the system described above eliminated ELMs in EAST, while also modestly increasing stored energy and confinement and destabilizing an edge-localized mode with multiple harmonics. To assess the applicability of this technique to future devices (e.g. material inventory control requirements), the dependence of the minimum impurity injection rate for ELM elimination was measured as a function of heating power in EAST. The minimum injection rate tripled as the heating power was doubled, which is qualitatively consistent with more efficient screening by the scrape-off layer with increasing edge temperature and density. In KSTAR, both boron and boron nitride powder injection reduced the ELM frequency and created periods of ELM quiescence, while maintaining plasma stored energy. There was no low frequency mode triggered via impurity injection, as was observed in EAST. Boron nitride powder reliably reduced divertor Da emission, while boron powder did not. Taken together, recycling reduction does not appear to be a requirement to trigger ELM quiescent phases in KSTAR.