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Pushing the physics of edge transport barriers towards the wall : how do boundary conditions impact confinement transitions in tokamaks?

Abstract : This work deals with the understanding of transport barrier establishment in the edge of magnetically confined fusion plasmas. To that end, two main axis were explored. First, an experimental characterisation of the rotation profiles by Doppler Back-Scattering reflectometry (DBS) have been performed in the WEST tokamak. On the other hand, a theoretical development of transport models implying a spectral description of the turbulence and its interplay with sheared flows have been developed. In fact, tokamak plasmas can be decomposed in three regions of interest : a confined core region where the fusion reactions take place, a plasma-wall interaction region where the plasma intercepts the wall leading to power and particle exhaust and a transition region between the two firsts called edge region. The establishment of a transport barrier in this latter is attributed to the generation of a strongly sheared flow leading to a mitigation of the turbulence. Experimentally, the build up of the barrier appeared very sensitive to edge plasma conditions such as the magnetic configuration, i.e. the existence and the position of an X-point (where the poloidal magnetic field is null) as well as edge density amplitude.An experimental characterisation of edge rotation profiles has been managed in various plasma conditions showing impacts of heating power injection, magnetic geometry and density on both profiles amplitude and shape. In particular, the magnetic topology of the plasma appeared strongly influencing the rotation behaviour. The X-point position, i. e. symmetrically in the top or the bottom part of the plasma corresponding to the so-called unfavourable and favourable configurations, induces the presence or the absence of rotation well at the confined plasma boundary. This discrepancy becomes blurred when the plasma current increases. Moreover, the first observations of an increased confinement regime in the WEST tokamak show edge velocity records. Interestingly, a deeper well in the rotation profile is observed in unfavourable configuration even if the density profile exhibits a sightly weaker gradient at the edge (or a weaker pedestal).Aiming at understanding these features, a theoretical development has been derived to describe both transport and sheared flow/turbulence interplay. A spectral approach of the edge turbulent equations led to a description of all the features of the interchange turbulence plunged into a background sheared flow. From this starting point, a reconstruction of the principal transport observables such as fluctuation levels, fluxes or flow generation through the Reynolds stress creation is driven. Applied to the plasma-wall interaction region, this model gives predictions for particle exhaust characteristic width. Interestingly, the model remains simple enough to include more complex geometric and collisional effects. Indeed, this turbulence description only depends on three control parameters : the curvature drive depending on the geometry, the parallel dynamics features depending on both geometry and density conditions and the structure tilt due to magnetic and background flow shear. These effects are investigated and discussed regarding the impacts of plasma shaping and edge density on these control parameters. Then, this model has been verified against a broad set of 2D flux-driven simulations with control parameters in the range of the experimental ones. Furthermore, a comparison of the model predictions with experimental data revealed three validation steps. First, the model recovers turbulent spectra measured in the TJ-K torsatron. Then, comparing the predictions with turbulent features and background density profile decay lengths measured with Langmuir probes in Tore Supra results in a sound quantitative agreement. Finally, the model of flow generation by the turbulence recovers the experimental observations mentioned above concerning the impact of the magnetic geometry and the plasma current.
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Submitted on : Monday, May 30, 2022 - 2:56:37 PM
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  • HAL Id : tel-03681704, version 1

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Mathieu Peret. Pushing the physics of edge transport barriers towards the wall : how do boundary conditions impact confinement transitions in tokamaks?. Plasma Physics [physics.plasm-ph]. Institut Polytechnique de Paris, 2022. English. ⟨NNT : 2022IPPAX001⟩. ⟨tel-03681704⟩

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