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Poster De Conférence Année : 2023

Transport and zonal flows dynamics in flux-driven interchange and drift waves turbulence

Résumé

The saturation of heat and particle turbulent transport in tokamak plasmas is efficiently controlled by large scale sheared flows. While collisions govern the linear damping of these flows, nonlinear couplings of turbulent fluctuations provide source terms via Reynolds' forces [1]. Numerical simulations [2] later confirmed by experimental measurements [3] have shown that self-generated zonal flows (ZF) can structure in so-called staircases. The mechanisms of their generation, their impact on turbulent transport and their robustness with respect to the various types of turbulence remain active research topics. In the present work, these issues are addressed by means of the reduced nonlinear model Tokam1D that features interchange and drift-wave turbulences, both suspected to be active at the edge of tokamak plasmas [4]. The model derives from the continuity and charge balance equations, where single poloidal and parallel wave numbers are retained and constant ion and electron temperatures are assumed. A generalized Ohm's law closes the system, linking the parallel current to the electric field and the electron pressure gradient. One of the strengths of this 1-dimensional model is to be flux driven: it evolves self-consistently the equilibrium and fluctuations of density and electric potential. It allows one to study the generation and structuration of large scale flows as well as their impact on turbulent transport. The linear properties of both instabilities are controlled by two plasma parameters, the mean curvature g of the magnetic field and the adiabaticity parameter C that scales like the square of the parallel wave vector divided by the electron-ion collision frequency. They exhibit rich characteristics in the parameter space. Consistently with previous findings, all the three control plasma parameters – g, C and the ion to electron temperature ratio =Ti/Te – are found to have a dual role, either stabilizing or destabilizing depending on the parameter regime. Also, they govern the phase shift between the density and electric potential fluctuations, hence the efficiency of the quasi-linear transport at prescribed fluctuation magnitude. The generation and structuration of ZFs and their interplay with turbulence and transport are analyzed in nonlinear simulations on confinement timescales. Whatever the values of the scanned parameters g, C and , ZFs are always active. They are driven by both components of the Reynolds stress, electric and diamagnetic [5], the contribution of the former being dominant when interchange dominates (large g). Two regimes are observed: ZFs are either structured in staircases or not. Staircases are found to emerge as a result of an anti-diffusive process. While maxima of the density gradient and of the shear of ZFs coincide, the ZF curvature governs the cross phase between density and electric potential fluctuations. These results help to characterize the large scale flow dynamics and their efficiency in regulating turbulent transport, and to discriminate plasma regimes where staircases are likely to be observed experimentally.
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Dates et versions

cea-04344751 , version 1 (14-12-2023)

Identifiants

  • HAL Id : cea-04344751 , version 1

Citer

O Panico, Y Sarazin, P Hennequin, Ö Gürcan, R Bigué, et al.. Transport and zonal flows dynamics in flux-driven interchange and drift waves turbulence. TTF 2023 - 27th Joint EU-US Transport Task Force Meeting, Sep 2023, Nancy, France. 2023. ⟨cea-04344751⟩
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