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Article Dans Une Revue Plasma Physics and Controlled Fusion Année : 2015

Investigating profile stiffness and critical gradients in shaped TCV ă discharges using local gyrokinetic simulations of turbulent transport

Résumé

The experimental observation made on the TCV tokamak of a significant ă confinement improvement in plasmas with negative triangularity (delta < ă 0) compared to those with standard positive triangularity has been ă interpreted in terms of different degrees of profile stiffness (Sauter ă et al 2014 Phys. Plasmas 21 055906) and/or different critical gradients. ă Employing the Eulerian gyrokinetic code GENE (Jenko et al 2000 Phys. ă Plasmas 7 1904), profile stiffness and critical gradients are studied ă under TCV relevant conditions. For the considered experimental ă discharges, trapped electron modes (TEMs) and electron temperature ă gradient (ETG) modes are the dominant microinstabilities, with the ă latter providing a significant contribution to the non-linear electron ă heat fluxes near the plasma edge. Two series of simulations with ă different levels of realism are performed, addressing the question of ă profile stiffness at various radial locations. Retaining finite ă collisionality, impurities and electromagnetic effects, as well as the ă physical electron-to-ion mass ratio are all necessary in order to ă approach the experimental flux measurements. However, flux-tube ă simulations are unable to fully reproduce the TCV results, pointing ă towards the need to carry out radially nonlocal (global) simulations, ă i.e. retaining finite machine size effects, in a future study. Some ă conclusions about the effect of triangularity can nevertheless be drawn ă based on the flux-tube results. In particular, the importance of ă considering the sensitivity to both temperature and density gradient is ă shown. The flux tube results show an increase of the critical gradients ă towards the edge, further enhanced when d < 0, and they also appear to ă indicate a reduction of profile stiffness towards plasma edge.

Dates et versions

hal-01465035 , version 1 (10-02-2017)

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G. Merlo, Stephan Brunner, Olivier Sauter, Y. Camenen, T. Ă Goerler, et al.. Investigating profile stiffness and critical gradients in shaped TCV ă discharges using local gyrokinetic simulations of turbulent transport. Plasma Physics and Controlled Fusion, 2015, 57 (5), ⟨10.1088/0741-3335/57/5/054010⟩. ⟨hal-01465035⟩
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