Regulation of Alfvén Eigenmodes by Microturbulence in Fusion Plasmas

Global gyrokinetic simulations of mesoscale reversed shear Alfven eigenmodes (RSAE) excited by energetic particles (EP) in fusion plasmas find that RSAE amplitude and EP transport are much higher than experimental levels at nonlinear saturation, but quickly diminish to very low levels after the saturation if background microturbulence is artificially suppressed. In contrast, in simulations coupling micro-meso scales, the RSAE amplitude and EP transport decrease drastically at the initial saturation but later increases to the experimental levels in the quasisteady state with bursty dynamics due to regulation by thermal ion temperature gradient (ITG) microturbulence. The quasisteady state EP transport is larger for a stronger microturbulence. The RSAE amplitude in the quasisteady state ITG-RSAE turbulence from gyrokinetic simulations, for the first time, agrees very well with experimental measurements.

Figure 1

Time history of perturbed electrostatic potentials $e\delta \varphi /T_e$ for RSAE ($n=2-5$) from simulation of RSAE only [panel (a)] and for ITG ($n=14$,16) from simulation of ITG-RSAE [panel (c)]. Corresponding zonal flow shearing rate ${\omega }_E/{\gamma }_4$, electron density perturbation $\delta n_e/n_e$ (%), and effective EP diffusivity $D_f$ (${\mathrm{m}}^2/\mathrm{s}$) are shown in panel (b) and panel (d).

Figure 2

Poloidal contour plots of perturbed electrostatic potential $e\delta \varphi /T_e$ [panel (a)] and electron density perturbation $\delta n_e/n_e$ [panel (b)] at $t=0.78\mathrm{ms}$ from simulation coupling ITG-RSAE turbulence.

Figure 3

Spectrum $S$ of electron density perturbation $\delta n_e$ as a function of poloidal wave number $k_{\theta }{\rho }_i$ in two radial regions [panel (a)], and radial profiles of electron temperature perturbation $\delta T_e$ [panel (b)] from GTC simulations and ECE measurements in DIII-D.

Figure 4

Time history of perturbed electrostatic potentials $e\delta \varphi /T_e$ for RSAE $(n=4,5)$, and ITG $(n=14,16)$ from simulations with a weaker [panel (a) using $0.8\nabla T_i$] or stronger [panel (c) using $1.3\nabla T_i$] thermal ion temperature gradient. Corresponding zonal flow shearing rate ${\omega }_E/{\gamma }_4$, electron density perturbation $\delta n_e/n_e$ (%), and effective EP diffusivity $D_f$ (${\mathrm{m}}^2/\mathrm{s}$) are shown in panels (b) and (d).