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

Abstract

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.Understanding turbulent transport is crucial in numerous plasma physics frameworks, ranging from plasma laboratories such as nuclear fusion devices 1 to astrophysical systems such as the solar tachocline 2 or the atmospheres 3 . In this letter, we focus on the turbulent transport in toroidal nuclear fusion devices (tokamaks), where accurate predictions are essential on the route towards the steady-state production of energy. Together with turbulence, energetic particles (EPs) constitute a ubiquitous component of current and future tokamaks, due to both nuclear reactions and heating systems. EPs are characterized by energies larger than the thermal energy. Whereas the impact of turbulence on EP transport has been analyzed and found to be weak 4 , the effect of EPs on turbulence has not been much studied so far (see e.g. Ref. 5) and represents the aim of our study. This analysis is done via the excitation by EPs of a class of modes naturally existing in tokamaks: the geodesic acoustic modes (GAMs) 6 , which are the oscillatory component of large scale E × B zonal flows. The EP-driven GAMs are called EGAMs. These modes have been predicted theoretically 7,8 , observed experimentally 9,10 and reported very recently numerically in the absence of turbulence 11 in gyrokinetic simulations with the 5D Gysela code 12 . The motivation of the present work relies upon fluid simulations where the turbulence level was controlled by GAMs in the core/edge transitional regime 13 . In addition, experimental evidence of the role of GAMs in the edgeturbulence suppression has been reported for the first time during the analysis of the L-H transition in the AS-DEX Upgrade tokamak 14 . However, in the context of core-turbulence suppression, the role of GAMs is less evident for several reasons. First, these modes are Landau damped in the core plasma. Second, since they are nonlinearly generated by turbulence, their external control a) Electronic mail:david.zarzoso-fernandez@polytechnique.org; Current address: Max-Planck-Institut für Plasmaphysik, EU-RATOM Association, Boltzmannstr. 2, Garching D-85748, Germany has proven difficult. Last, their frequency ω GAM is close to the characteristic turbulence frequency ω turb , which means that the shearing rate provided by GAMs might be large compared to the autocorrelation time. In that respect, theoretical predictions of the shearing effect ...

Relation between energetic and standard geodesic acoustic modes

“…Due to their similarity with GAMs, the EGAMs are good candidates for reducing turbulent transport, thanks to vortex shearing. However, recent numerical simulations suggest that the impact of EGAMs on turbulence may not always be as positive as expected, and requires further investigation 16 . EGAMs can also be responsible for fast ion losses under certain circumstances 17 .…”

Effects of passing energetic particles on geodesic acoustic mode

“…14 A recent fluxdriven 5D gyrokinetic simulation provided the direct evidence of the impact on turbulent transport of EGAMs. 15 In view of that the radial inhomogeneity can lead to the continuous spectrum of GAMs, 16,17 Qiu et al developed the nonlocal theory of EGAMs by taking into account the coupling to the GAM continuous spectrum 18 and the nonuniformity of EPs radial density profile. 17 Recently, the GAM/EGAM instability was studied in a local approach and by means of the eigenvalue analysis.…”

Nonlinear velocity redistribution caused by energetic-particle-driven geodesic acoustic modes, mapped with the beam-plasma system

“…[16]). The role of EGAMs as possible mediators between EP and turbulence has also been emphasized [11]. One of the main effects of EGAMs in tokamak plasmas is the redistribution of the EP population (crf.…”