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Description
In the advanced scenarios of future reactor burning plasmas, with a large fraction of non-inductive current maintained off-axis and the magnetic shear reversed in the plasma core region, multiple reversed shear Alfvén eigenmodes (RSAEs) could be excited by fusion alpha particles. Since RSAEs generally localize together around the SAW continuum extremum induced by the local minimum of the safety factor to minimize the continuum damping, the nonlinear mode coupling between RSAEs could be significant and could potentially contribute to their nonlinear saturation [1]. Recent simulation research found that RSAE could be saturated due to the nonlinear generation of high frequency harmonics [2]. Specifically, this harmonic could be a quasi-mode with double or triple toroidal mode numbers of the primary linearly unstable RSAE. With several RSAEs possessing several artificial toroidal mode numbers set in the simulation scenario, the high frequency quasi-mode was generated due to the “magnetic fluttering” nonlinearity contributed by kinetic electrons. Meanwhile, this quasi-mode suffers significant damping effect by thermal electrons and contributes to the saturation of the primary RSAE.
Motived by this simulation observation, in this work, we present a potential nonlinear saturation mechanism for RSAE via nonlinear quasi-mode generation. This nonlinear mode coupling is achieved through the non-adiabatic responses to electrons, corresponding to the magnetic fluttering nonlinearity as addressed in reference [2]. Meanwhile, the mode coupling is generalized from RSAE self-coupling in the simulation to include also the interaction between two RSAEs with different toroidal mode numbers, for which the nonlinear coupling could be much stronger. Generally, this quasi-mode could experience significant continuum or radiative damping, and provide a channel for primary RSAE energy dissipation. Using nonlinear gyrokinetic theory, the parametric dispersion relation of this nonlinear mode coupling process is derived. Focusing on the continuum damping of the quasi-mode, the resultant nonlinear damping to RSAE is then analyzed and estimated.
References:
[1] Tao W, Shizhao W, Briguglio S, et al. Nonlinear dynamics of the reversed shear Alfvén eigenmode in burning plasmas[J]. Plasma Science and Technology, 2024, 26(5): 053001.
[2] Ye L, Chen Y, Fu G. Nonlinear saturation of reverse shear Alfvén eigenmodes induced by kinetic electrons[J]. Nuclear Fusion, 2022, 63(2): 026004.
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