Excitation of toroidally localized harmonics of global Alfvén eigenmodes

Journal Article · Mon May 22 00:00:00 EDT 2023 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/acd204· OSTI ID:1974812

Fredrickson, E. D.

Abstract The spherical tokamak NSTX and the upgraded, higher toroidal field, NSTX-U, routinely create plasmas with large un-thermalized populations of super-Alfvénic fast ions. This population of non-thermal energetic ions excites a broad spectrum of Alfvénic waves, from the lower frequency (≈30 kHz–≈200 kHz) toroidal Alfvén eigenmodes (Cheng and Chance 1986 Phys. Fluids   29 3695), to the higher frequency (400 kHz–3 MHz) global Alfvén eigenmodes (GAEs) and compressional Alfvén eigenmodes (Goedbloed 1975 Phys. Fluids   18 1258). In this paper we present evidence that the GAE non-linearly excite modes, presumably GAE, at frequencies consistent with non-linear or 3-wave coupling. The observation of the excitation of 2 ^nd harmonic GAE through the intrinsic non-linearity of Global Alfvén modes demonstrates that the non-linear terms can act as an exciter-antenna inside the plasma, broadcasting at harmonics of the mode frequency and with concomitantly shorter wavelengths. As with experiments using external antenna to excite otherwise weakly stable Alfvénic modes, this data can provide information about the stability of modes at harmonics of the GAE. It may also provide information on the nature of the non-linearities in the wave dispersion equation or a direct measure of the mode amplitude (Smith et al 2006 Phys. Plasmas   13 042504). We also report that the short wavelength 2 ^nd harmonic GAE can be strongly toroidally localized. The observation that the shorter wavelength ( n ≈ 20) 2 ^nd harmonic modes can be toroidally localized potentially has implications for the impact of fast-ion driven instabilities on fast-ion confinement in ITER and future fusion reactors (Gorelenkov et al 2014 Nucl. Fusion   54 125001).

• Research Organization: Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC02-09CH11466
• OSTI ID: 1974812
• Alternate ID(s): OSTI ID: 1972381; OSTI ID: 1986324
• Journal Information: Nuclear Fusion, Journal Name: Nuclear Fusion Vol. 63 Journal Issue: 7; ISSN 0029-5515
• Publisher: IOP Publishing
• Country of Publication: IAEA
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
nonlinear waves
Alfvén waves
global Alfvén eigenmodes
spherical tokamaks