Speaker
Description
Tungsten impurities in tokamak plasmas introduce significant challenges due to their high radiative efficiency. Following the recent update of the ITER baseline, the importance of understanding tungsten impurity generation and transport has increased significantly. In KSTAR, the installation of a tungsten monoblock cassette divertor in 2023 has opened a new operational regime for ITER-relevant plasma-surface interaction studies. This study provides a statistical assessment of impurity behavior with tungsten divertor in KSTAR. Changes in carbon and tungsten source levels are assessed across 2021-2024 KSTAR campaigns. The effective charge (Zeff) and total radiated fraction (frad) behavior are investigated in terms of carbon and tungsten impurity levels. Subsquently, the impact on plasma performance and confinement is also explored. A comprehensive set of diagnostics is employed for impurity measurement in this study. Filterscope are used to measure the line emissions of neutral tungsten (W I, 400.9 nm) and C2+ ions (C III, 464.7 nm) in the plasma boundary, and visible spectroscopy measures the spectral lines of other light impurities. Vacuum ultraviolet (VUV) spectroscopy and X-ray crytsal spectroscopy (XICS) are used to assess the highly charged tungsten ions in the plasma core. Infrared imaging video bolometer (IRVB) provides 2D radiation profiles and the total radiated power. In addition, this study provides the averaged Zeff, derived from the line-integrated visible bremsstrahlung emissivity, line-averaged electron density, and core electron temperature. General trends before and after tungsten divertor installation are firstly investigated, followed by comparison between plasma configurations such as lower and upper single null configurations. Impurity behaviors in L-mode and H-mode are analyzed with various NBI and ECH heating powers. A focused analysis is dedicated on H-mode discharges with the tungsten divertor, where ELMs are known to be a primary source of tungsten sputtering. The tungsten source levels in inter- and intra-ELM phases are compared. Statistical analysis of their behavior and tungsten core accumulation is performed with respect to ELM frequency. The discharges with ELM mitigation by RMP are also included in statistics. This 0-D parameter avoids detailed profile fitting and reconstruction, and subsquently allows robust statistics over a large number of discharges. This study possibly contributes to the control of tungsten impurity and radiated power in both present partial- and future full-tungsten wall environments in KSTAR.