Plasma and Fusion Research

Volume 20, 1203048 (2025)

Rapid Communications

Extraction of Nonlinear Waveforms in Geodesic Acoustic Modes by using Conditional Average
Kazutaka OZAWA1), Makoto SASAKI1), Yohei HARA1), Tatsuya KOBAYASHI2)
1)
Collage of Industrial Technology, Nihon University, Narashino 275-8575, Japan
2)
National Institute for Fusion Science, National Institutes of Natural Sciences, Toki 509-5292, Japan
(Received 27 May 2025 / Accepted 29 July 2025 / Published 17 October 2025)

Abstract

In L-mode plasmas of the JFT-2M tokamak, we applied conditional averaging to time-series data of electrostatic potential fluctuations measured by the Heavy Ion Beam Probe (HIBP). The analyzed phase was dominated by Geodesic Acoustic Modes (GAMs), and the extracted nonlinear waveforms exhibited periodic harmonic structures. Fourier analysis revealed a dominant second harmonic, while the third harmonic was unexpectedly weaker than the fourth. This nontrivial trend deviates from standard gyrokinetic predictions with a simple configuration (circular cross-section with periodic boundary condition). The results implies that symmetry breaking due to the boundary and/or plasma shape could play key roles in GAM saturation and offer guidance for refining theoretical models.

Keywords

conditional average, geodesic acoustic mode, higher harmonics, nonlinear process

DOI: 10.1585/pfr.20.1203048

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