Description
The radial electric field (Er) is known to have a significant impact on turbulent transport reduction [1] and a key role in the transition toward the higher confinement regime. Experimentally, the radial shape of the perpendicular velocity of density fluctuations, dominated by the ExB velocity, appears significantly influenced by the magnetic topology [2-5], the plasma current [2] and the plasma shape [6]. Among the various possible mechanisms involved in the formation of the Er profile, turbulence related effects could explain some of these sensitivities [7,8] through the generation of zonal flow (ZF) via Reynold’s stress. Part of these turbulence-generated flows are Geodesic Acoustic modes (GAMs). GAMs appear in the form of a coupling between a ZF and an axisymmetric (m = 1) pressure sideband mode due to geodesic curvature, and may inform on the turbulence intensity in case of constant flow damping.
In this contribution, we investigate how GAMs evolve in relation to different Er profiles. In particular, the evolution of GAM amplitude between favourable (FAV) and unfavourable (UNFAV) drift configurations is addressed as well as the influence of the plasma current, based on Doppler Backscattering measurements from WEST [3] and TCV [6,9] experiments. Most of the time, the GAM activity appears stronger in UNFAV drift configuration as compared to its FAV counterpart. More precisely, GAMs systematically have a lower amplitude when the Er profile exhibits a deeper well. This observation motivates to compare GAMs between matched positive and negative triangularity plasmas in different heating schemes, known to exhibit different Er profiles at the edge [6]. Consistently with the influence of the drift configuration, GAMs are stronger in PT plasmas, which have a weaker Er well. Interpretation of such results are discussed in the light of experimental and numerical observations, as well as theoretical considerations.
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[3] L. Vermare et al., Nucl. Fusion 62.2 (2021).
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[5] S. Rienäcker et al., 5Oth EPS Conference on Plasma Physics. (2024)
[6] S. Rienäcker et al, Nucl. Fusion 66 014002 (2026)
[7] M. Peret et al., Physics of Plasmas (2022)
[8] R. Varennes et al., Plasma Phys. Contr. Fus. (2023).
[9] S. Rienäcker et al., Plasma Phys. Contr. Fus. 67 065003 (2025)
^ See author list of B.P. Duval et al 2024 Nucl. Fusion 64 112023
* See http://west.cea.fr/WESTteam
+ See author list of E. Joffrin et al 2024 Nucl. Fusion 64 112019