On zero frequency zonal flow and second harmonic generation by finite amplitude energetic particle induced geodesic acoustic mode

Abstract

Nonlinear self-interaction of finite amplitude energetic particle induced geodesic acoustic mode (EGAM) is investigated using nonlinear gyrokinetic theory. It is found that both zero frequency zonal flow(ZFZF) and second harmonic can be driven by finite amplitude EGAM, with energetic particles (EPs) playing a dominant role in the nonlinear couplings through finite orbit width effects. For ZFZF, the effects of EPs on EGAM nonlinear self-coupling dominate that of the thermal plasmas which are also present; while the second harmonic generation is only possible via finite amplitude coupling though EPs. Our findings may improve the understanding of stabilizing zonal modes, and consequently, drift wave turbulence.

Saturation of energetic-particle-driven geodesic acoustic modes due to wave–particle nonlinearity

“…More in general, the nonlinear interaction of the bulk ions with the EGAM is expected to be important: this has been studied in part analytically in Qiu et al. (2017), and its study with ORB5 is in progress. Different values of the bulk temperature, and the EP concentration, have been considered.…”

“…Therefore, even if the linear resonance velocity falls near the tail of the bulk ions (see figure 1), no nonlinear interaction of those bulk ions with the EGAM is considered in our model. More in general, the nonlinear interaction of the bulk ions with the EGAM is expected to be important: this has been studied in part analytically in Qiu et al (2017), and its study with ORB5 is in progress. Different values of the bulk temperature, and the EP concentration, have been considered.…”

“…The wave–wave nonlinear dynamics of EGAMs has been studied analytically in Qiu et al. (2017), focusing in particular on the EGAM self-coupling. Numerical investigations of the dynamics of EGAMs with wave–particle and wave–wave nonlinearities have been carried out with the hybrid MHD-GK code MEGA (Ido et al.…”

“…turbulence. The wave-wave nonlinear dynamics of EGAMs has been studied analytically in Qiu et al (2017), focusing in particular on the EGAM self-coupling. Numerical investigations of the dynamics of EGAMs with wave-particle and wave-wave nonlinearities have been carried out with the hybrid MHD-GK code MEGA (Ido et al 2016), with a reduced 1-D model solved with the code COBBLES (Lesur et al 2016) and with the GK semi-Lagrangian code GYSELA (Zarzoso et al 2012) in the absence of turbulence, and with GYSELA in the presence of turbulence (Zarzoso et al 2013).…”

Effect of elongation on energetic particle-induced geodesic acoustic mode

“…These modes are zonal flows (ZF) [16,17] with finite frequency oscillations and are usually damped by collisionless Landau damping in the plasma core or by collisions at the edge of the device [18]. On the other hand, zero frequency zonal flows (ZFZF) [19] are damped by collisional damping only. However, energetic particles might overcome the thermal damping and drive geodesic acoustic modes as stated above, which are called, in this case, energetic particle-induced GAM (EGAM) [20][21][22][23][24][25].…”