Impact of Energetic-Particle-Driven Geodesic Acoustic Modes on Turbulence

The impact on turbulent transport of geodesic acoustic modes excited by energetic particles is evidenced for the first time in flux-driven 5D gyrokinetic simulations using the Gysela code. Energetic geodesic acoustic modes (EGAMs) are excited in a regime with a transport barrier in the outer radial region. The interaction between EGAMs and turbulence is such that turbulent transport can be enhanced in the presence of EGAMs, with the subsequent destruction of the transport barrier. This scenario could be particularly critical in those plasmas, such as burning plasmas, exhibiting a rich population of suprathermal particles capable of exciting energetic modes.

Figure 1

Color map of the $E\times B$ diffusivity (${\chi }_{E\times B}$) with the three phases analyzed in this Letter.

Figure 2

Time evolution of the distribution function at $v_{\parallel }=v_0$ (top panel) and the derivative in velocity space at the EGAM resonant velocity (lower panel).

Figure 3

Time Fourier transform of the $\sin \theta$ component of ${\varphi }_{1,0}$ for both simulations with (solid red line) and without (dotted blue line) EPs. For the simulation with EPs, the imaginary part of ${\varphi }_{1,0}/|{\varphi }_{1,0}|$ is plotted (shadowed region).

Figure 4

Time evolution of the $E\times B$ diffusivity for both simulations. For the simulation with EPs, the respective contributions to ${\chi }_{E\times B}$ of axisymmetric modes (dotted black line) and of nonaxisymmetric modes (solid black line) are distinguished. For the simulation without EPs, no distinction is made (dashed blue line).

Figure 5

Oscillating part of $R_0/L_T$ in two time intervals of the simulation with EPs: end of phase $B$ (bottom) and during phase $C$ (top).