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Description
A suite of Langmuir probes (LPs) installed on the KSTAR lower tungsten divertor, consisting of 52 in the D-port and another 52 in the L-port, has been operated since 2023. The D-port Langmuir probes are battery-biased at -240 V to measure temporal behavior of the ion saturation currents. During the 2024 campaign, a single sweeping Langmuir probe driven by a bipolar supply (-100 to +100 V, 1 kHz, triangular waveform) was operated to acquire the IV-characteristics. To suppress effects of the edge-localized-mode (ELM) on the IV-characteristics, an ELM filter was first applied to the raw signals. The IV characteristics were further processed by applying either a 1 kHz comb filter or a 30 kHz low-pass filter to the ELM-filtered data. Plasma parameters, such as density and temperature, were extracted by fitting the filtered IV-characteristic curves within 50 ms time windows using the Weinlich-Carlson asymmetric double-probe (WC-ADP) model [1]. The resulting temporal trends show good agreement with mid-plane edge ECE (electron cyclotron emission) [2]. From the KSTAR 2025 campaign, nine additional power supplies and associated circuitries have been installed, enabling simultaneous sweeping of the bias voltage (-100 to +100 V) for a total of ten designated Langmuir probes. During the measurements, it has been observed that such a simultaneous sweeping introduces noise crosstalk which is not negligible. In particular, bias-voltages with the triangular waveform produce peaked noises near the local extrema that couple, i.e., crosstalk, into other Langmuir probes. Although triangular bias-voltage provides a uniform dV/dt, the sign reversals at the turning point drive large capacitive (displacement) and inductive currents, producing artificial ringing signals. We, therefore, investigate and compare the noise characteristics under triangular and sinusoidal bias-voltages, and estimate parasitic-R/C leakage and displacement currents.
[1] Rudischhauser, Lukas, et al. "The Langmuir probe system in the Wendelstein 7-X test divertor." Review of Scientific Instruments 91.6 (2020).
[2] Bong, Seungmin, et al. "Newly designed Langmuir probe system at the tungsten lower divertors in KSTAR." 9th Asia-Pacific Conference on Plasma Physics MF2-9-I2 (2025)
Acknowledgments
This work was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant Nos. RS-2022-00155917 and NRF2021R1A2C2005654). This work was also supported by the R&D Program of the “KSTAR Experimental Collaboration and Fusion Plasma Research (EN2503)” trough the Korea Institute of Fusion Energy (KFE) funded by the Korean Ministry of Science and ICT.