Radio Frequency(RF) Diagnostic for Energetic Particle and Wave-particle Interaction Study in KSTAR

Abstract

Energetic particles are the essential component in the transport processes and stability of fusion plasmas [1], in particular of the burning plasmas that produce high-energy alpha particles. As a potential energetic particle diagnostic, the radio frequency (RF) spectroscopy is suggested as an effective method for its non-invasiveness and small footprint [2–5]. In KSTAR, we have developed a fast RF diagnostic system with the temporal and spectral resolutions of Δf_res~0.5 MHz and Δt_res~1 μs for the study of wave-particle interactions. The RF system implemented two different types of measurement. One is the direct measurement by spiral-shaped antennas (f=0.1–0.88 GHz) located in front of the KSTAR H-port aperture [4, 6], and the other is an indirect measurement of whistler-frequency range (~GHz) waves embedded in the electron cyclotron emissions which are measured by mm-wave mixer antennas [7]. One of the common wave phenomena that drew our attention is the edge deuterium ion cyclotron harmonic emissions (d-ICEs) and their dynamic changes in KSTAR H-mode discharges. Edge d-ICEs are routinely observed in both KSTAR ELMy and ELM-suppressed H-mode discharges. During the inter-ELM-crash periods, the d-ICE harmonics frequency range and intensity drastically change. Fast-ion Dα (FIDA) [8] measurement shows that the edge beam-ion fraction increases with the appearance of the ELM filament [9]. Full-orbit simulations with the Lorentz-orbit (LORBIT) code [10] suggest that the fast ion orbits move outward during the period of edge d-ICE enhancement. These observations are consistent with the previous study on the beam-driven ICE using 1D3V particle-in-cell simulations [11]. Our study shows that RF diagnostic can be a quantitative indicator of the presence of fast ions at the outer edge region.

This work was supported by the R&D programs of “KSTAR Experimental Collaboration and Fusion Plasma Research” (KFE Grant Code: EN2301-14) and “ITER Burning Plasma Research and Development of ITER Plasma Exploitation Plan” (KFE Grant Code: IN2304-9) through the Korea Institute of Fusion Energy (KFE) funded by the Government funds.

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