7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023

Radial electric fields in core & edge flux-driven turbulence
G. Dif-Pradalier1, P. Donnel1, Y. Munschy1, Y. Sarazin1, R. Varennes1,2,L. Vermare3, X. Garbet1,2, Ph. Ghendrih1, V. Grandgirard1, O. Panico1
1CEA, IRFM, France,
2Nanyang Technological University, Singapore
3
LPP, Ecole Polytechnique, Francee-mail (speaker): guilhem.dif-pradalier@cea.fr
Microscale turbulence is known to nonlinearly organise
through generation of secondary or tertiary mesoscopicstructures such as zonal flows, fronts or staircases.
Radial electric field (Er) evolution underpins thesequestions and is central to understanding the onset andsustainment of transport barriers. From a modellingstandpoint, these questions are sensitive to assumptionsof scale separation and thus best understood in aflux-driven framework. This talk tries to articulate recent
experimental observations, in particular on the WEST
tokamak prior to the H-mode transition and recent results
provided by flux-driven turbulence simulations with thegyrokinetic GYSELA code.
Experiments in Tore Supra limited plasmas [1], WEST
[2] and MAST [3] diverted L-mode or H-mode
discharges have shown a clear role of magnetic ripple
and plasma current on radial electric field flow and shear
levels in the edge. In particular, the edge Er wellmeasured using Doppler Backscattering is found todeepen with increasing plasma current Ip (decreasingsafety factor q). The physics underpinning this Ip
dependence is not straightforward. Plasma current indeed
impacts transit times and orbit widths, which areinstrumental in a large variety of neoclassical andturbulent processes. Neoclassical transport regimes areaffected through modification of the bounce frequency oftrapped particles. Turbulence is directly affected throughenhancement with q of linear grow rates of electrostatic
instabilities. The plasma current may also influence thesynergy between neoclassical and turbulent dynamics via
flow
damping.
Dedicated
GYSELA
numerical
experiments point towards subtle trade-offs between
collisional flow damping and turbulent flow drive
through turbulent Reynolds stresses as Ip is changed [4].
Increasing the safety factor indeed increases turbulent
transport, consistently with global scaling laws butdecreases edge perpendicular flows. Surprisingly
however, neoclassical calculations show no significant
effect of the safety factor on flows, which leads to asomewhat counter-intuitive situation: turbulent stresses
are clearly responsible for flow drive, but generated
flows get weaker (in absolute value) as turbulence
intensity increases, therefore emphasising the fact thatcollisional flow damping wins over flow drive as Ip
increases. These observations are qualitatively consistentwith the experimental trends albeit not quantitatively. A

detailed analysis of these questions will be presented aswell as ongoing steps to improve upon them.
In particular, the incidence of realistic Scrape-Off Layerboundary conditions with fully kinetic ion and electron
responses is missing in the aforementioned study. This isa topic of great current interest and promise, especially inview of pedestal physics modelling. The important roleof a spreading-mediated interplay between core and edge,
originating near the magnetic separatrix has recently
been shown [5] in flux-driven regimes with a limiter
configuration and a simplified Boltzmann electron
response. Detailed vorticity balance shows that thediamagnetic component of the Reynolds stress plays acentral role in the build-up phase of the Er well [5] in theL-mode edge as well as in the core shear layers [6] of
tokamak plasmas. These findings resonate with recent
experimental observations that additional ingredients tothe conventional (non diamagnetic) Reynolds stresses arelikely required to account for measured flow generationlevels [7]. Through SOL—edge interplay, a stabletransport barrier self-consistently builds up in GYSELA
inside the separatrix. The causal chain that leads to its
onset and sustainment has been specifically investigated[5] and will be discussed. At last, current steps towards afully integrated core to SOL gyrokinetic flux-driven
framework will be discussed. In particular, we will
discuss strategies to incorporate key aspects of sheath
physics [8] in flux-driven gyrokinetics without resolving
the Debye sheath. This ongoing step is essential for acomprehensive modelling of the plasma edge.
References:[1] P. Hennequin et al, 37th EPS conference on Plasma
Phys. (2010)[2] L. Vermare et al, Nucl. Fusion 62:026002 (2022)[3] H. Meyer et al, J. Phys: Conf Ser. 123:012005 (2008)[4] R. Varennes, PhD thesis, Aix-Marseille University(2022)[5] G. Dif-Pradalier et al., Communications Physics
5:229 (2022)[6] Y. Sarazin et al., Plasma Phys. Control. Fusion
63:064007 (2021)[7] R. Gerrú et al., Nucl. Fusion 59 106054 (2019)[8] Y. Munschy et al., submitted to Nucl. Fusion (2023)