Abstract Details
Abstracts
Author: Greg A Riggs
Requested Type: Poster
Submitted: 2023-03-30 14:13:24
Co-authors: M.Koepke, W.Heidbrink, M.VanZeeland, D.Spong
Contact Info:
West Virginia University
135 Willey St.
Morgantown, WV 26506
USA
Abstract Text:
Nonlinear coupling of toroidicity-induced Alfven eigenmodes (TAEs) has been observed in both experiment and simulation of toroidally-confined plasmas [e.g., Podesta 2011, Nucl. Fusion 51, 063035; Spong et al 2021, Nucl. Fusion 61, 116061]. As quadratic nonlinearities formed between TAEs are believed to mediate energy transfer from TAEs to low-frequency MHD modes and/or zonal flows, an enhanced understanding of these phenomena may elucidate the saturated amplitude of these instabilities.
We report the analysis of time-series from inductive coils, taken from 4 DIII-D shots heated by neutral beam injection (NBI). A three-point scan of beam power (3, 4, and 5MW) compares coupling in different regimes of fast-ion drive. Bispectral techniques are leveraged to assess the phase-coherency between multiple TAEs and nonlinearly-generated modes. Theoretically, the addition of a quadratic nonlinearity to a system of two independent, sinusoidal oscillators leads to two purely real bispectral features, which represent coherence with the sum and difference frequency. Whether the phase of the bispectrum (i.e., "biphase") is an even or odd multiple of pi indicates the sign of the coupling coefficient.
Facilitated by wavelet methods, highly time-resolved biphase dynamics are documented for prominent bispectral features. In particular, submillisecond (~750 𝝁s) quadratic coupling is consistently detected via stationary biphase and confirmed via band-pass filtering; duration of coupling is well-correlated with changes in TAE amplitude.
*Work supported by US DOE under DE-FC02-04ER54698. Partial financial support from NNSA-JPHEDP grant DE-NA0003874 and DOE-FES grant DE-SC0021404 is gratefully acknowledged. GR and MK express deep gratitude to the DIII-D team for necessary support and impactful collaboration during this project.
Comments:
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