SCIenTIfIC REPoRTS | 7: 16767 | DOI:10.1038/s41598-017-17011-y
Enhancement and suppression of
turbulence by energetic-particle-
driven geodesic acoustic modes
, K. Itoh
, K. Hallatschek
, N. Kasuya
, M. Lesur
, Y. Kosuga
& S.-I. Itoh
We propose a novel mechanism of enhancement of turbulence by energetic-particle-driven geodesic
acoustic modes (EGAMs). The dynamics of drift-wave-type turbulence in the phase space is investigated
by wave-kinetic equation. Spatially inhomogeneous turbulence in the presence of a transport barrier
is considered. We discovered that trapping of turbulence clumps by the EGAMs is the key parameter
that determines either suppress or enhance turbulence. In regions where turbulence is unstable,
EGAM suppresses the turbulence. In contrast, in the stable region, EGAM traps clumps of turbulence
and carries them across the transport barrier, so that the turbulence can be enhanced. The turbulence
trapped by EGAMs can propagate independent of the gradients of density and temperature, which
leads to non-Fickian transport. Hence, there appear a new global characteristic velocity, the phase
velocity of GAMs, for turbulence dynamics, in addition to the local group velocity and that of the
turbulence spreading. With these eect, EGAMs can deteriorate transport barriers and aect turbulence
substantially. This manuscript provides a basis to consider whether a coherent wave breaks or
strengthen transport barriers.
Problems including interactions between micro-turbulence and coherent waves are ubiquitous in a variety of sys-
. e coexistence of global-coherent Alfven wave and micro-turbulence has been observed at the surface
of the sun, which could link with the coronal heating problem
. Turbulence in planetary atmospheres generates
, such as jet stream on the earth, stripes of Jupiter, super rotation on Venus, and tachocline on the
. In magnetically conned plasmas, geodesic acoustic modes (GAMs), which are oscillatory zonal ows, have
attracted much attention as coherent waves, because GAMs are expected to suppress turbulence by their velocity
. Actually, the suppression of turbulence and transport have been reported in turbulence simulations
Experimental study has shown that the transition to high connement state is accompanied by GAMs
recently, the enhancement of turbulence by GAMs has been observed in rst principle simulations, with the sub-
sequent destruction of a transport barrier
. In this way, GAMs can either mitigate or enhance the turbulence.
is dual eect of the GAMs on turbulence requires theoretical investigation.
We investigate the phase-space dynamics of spatially inhomogeneous turbulence in the presence of GAMs.
GAMs are driven not only by turbulence
but also by energetic particles (EPs), which are called EGAM
Turbulence driven GAMs suppress turbulence, which can be understood in terms of energy conservation (the
total energy of GAMs and turbulence is conserved). In contrast, the impacts of EGAMs on turbulence is not
clear, in particular because EGAMs obtain their energy from EPs, not from turbulence. Actually, EGAMs have
been reported to enhance turbulence in spite of the fact that EGAMs have velocity shears
. In order to clarify
the impact of nite-frequency zonal ows on turbulence, we focus on EGAMs. e phase-space dynamics results
in trapping of turbulence wave-packets by EGAMs
. We found that the trapped turbulence wave-packets leak
across the transport barrier. As a result, turbulence is enhanced by EGAMs in the stable region, while turbulence
suppression is obtained in the unstable region. We discovered that trapping of turbulence clumps by the EGAMs
is the key parameter that determines either suppress or enhance turbulence. e propagation of the turbulence
is ballistic, with the phase velocity of the EGAM. us, the turbulence propagation is in some sense independent
from the background proles such as the gradients of density and temperature. us, turbulence carried by the
Research Institute for Applied Mechanics, Kyushu University, Kasuga, 816-8580, Japan.
Research Center for
Plasma Turbulence, Kyushu University, Kasuga, 816-8580, Japan.
Institute of Science and Technology Research,
Chubu University, Kasugai, 487-8501, Japan.
Max-Planck-Institute for Plasma Physics, 85748, Garching, Germany.
Lorraine University, Institut Jean Lamour, Nancy, 54-506, France. Correspondence and requests for materials should
be addressed to M.S. (email: email@example.com)
Received: 17 August 2017
Accepted: 20 November 2017
Published: xx xx xxxx