Nonlinear excitation of geodesic acoustic mode by reversed shear Alfvén eigenmodes in non-uniform plasmas

Abstract

Effects of plasma non-uniformities and kinetic dispersiveness on the spontaneous excitation of geodesic acoustic mode (GAM) by reversed shear Alfvén eigenmode (RSAE) are investigated numerically. It is found that, due to the turning points induced by the shear Alfvén continuum structure, the nonlinear excitation of GAM is a quasiexponentially growing absolute instability. As the radial dependence of GAM frequency and pump RSAE mode structure are accounted for, the radially inward propagating GAM is preferentially excited, leading to core localized thermal plasma heating by GAM collisionless damping. Our work, thus, suggests that GAM excitation plays a crucial role in not only RSAE nonlinear saturation, but also anomalous fuel ion heating in future reactors.

Nonlinear dynamics of the reversed shear Alfvén eigenmode in burning plasmas

“…Thus, a thorough and quantitative investigation of the RSAE saturation and its impact on particle transport and fusion performance naturally requires taking both routes to the nonlinear physics into consideration on the same footing [2,58,68]. This is of particular importance but extremely channeling for either analytical theory [140] or global gyrokinetic simulations [144,145]. A viable route to tackle the difficulty of this multi-scale, multi-physics problem could be provided by the phase space zonal structure theoretical framework [2,68,69,108,119], which describes the nonlinear wave-particle interaction using coupled Dyson-Schrödinger model equations and could be naturally extended to nonlinear wave-wave couplings in the form of nonlinear radial envelope equations.…”

“…Another factor neglected in the present work is the interplay with universal microscopic turbulences, despite that the spontaneously generated ZFZSs are bound to have an intense interactions with them [53]. This is a broad and intense research field, which has recently drawn significant research attentions on the cross scale interactions between AEs and turbulences as either direct coupling [150,151], or indirect interaction via ZFZS [54,[152][153][154] or the EP phase space structures [54,145,152,153]. All these complications demonstrate that the reactor-grade burning plasma is inherently a complex multi-scale system [2,54], where the RSAE might play a central role.…”