Speaker
Description
A Langmuir probe (LP) system, designed to measure basic plasma parameters and their poloidal profiles, was installed in the upgraded tungsten divertor of KSTAR [1]. Since the KSTAR 2023 campaign, temporal ion saturation currents have been measured in the lower divertor region by applying a negatively DC-bias of -240 V to the LP using batteries, with a DAQ sampling frequency of 200 kHz. As the ion saturation current alone is insufficient to determine plasma parameters such as electron density(ne) and electron temperature(Te), a voltage-sweeping method was adopted to obtain IV characteristics over time. The applied voltage waveform is generated by an SDG1032X function generator and subsequently amplified using a KEPCO BOP100-4D bipolar power supply. The corresponding current is measured across a 10 Ω resistor placed between the LP tip and the power supply. Single voltage-sweeping LP data were successfully obtained during the KSTAR 2024 campaign by applying a 1 kHz triangular waveform ranging from -100V to +100V.
From the KSTAR 2025 campaign, ten (LPs) were operated with voltage sweeping to measure of ne and Te at certain regions near the inner and outer strike point. The LP measurements at the scrape-off layer(SOL) are useful characterizing divertor detachment and underlying physics. When deuterium and nitrogen gases were injected into H-mode plasmas with the newly installed W divertor, deuterium fuelling increased the ELM frequency, thereby promoting tungsten flushing and reducing core radiation. In contrast, nitrogen seeding led to increased core radiation despite lower tungsten sputtering, indicating changes in transport. Further analysis of target electron density and temperature measured by a voltage sweeping LP will be performed [2]. The results will be compared and discussed with the tungsten optical emission spectroscopy (OES) diagnostics in the divertor region.
Acknowledgements
This work was supported by the R&D Program of the “KSTAR Experimental Collaboration and Fusion Plasma Research (EN2503)” and Grant Nos. RS-2022-00155917 and NRF2021R1A2C2005654, Grant Nos. RS-2022-00155960 trough the Korea Institute of Fusion Energy (KFE) funded by the Korean Ministry of Science and ICT.
References
[1] Yegeon Lim, et al., "Development of a fixed Langmuir probe system for newly installed tungsten monoblock lower divertors in KSTAR." Fusion Engineering and Design 205 (2024): 114552.
[2] Junghoo Hwang, et al., “Experiment investingation of deuterium and nitrogen-seeded H-mode plasma in KSTAR with new W divertor.”, FEC(2025), in progress.