Bonjour,
Nous vous prions de bien vouloir trouver ci-dessous l’annonce de la prochaine soutenance de thèse d’Olivier Panico :
Avis de Soutenance
Monsieur Olivier PANICO
Soutiendra publiquement ses travaux de thèse intitulés :
«Edge turbulence self-organization in fusion plasmas»
Soutenance prévue le mercredi 18 décembre 2024 à 10h
Lieu : CEA – IRFM, salle René Gravier, Bâtiment 506
Et par Zoom : https://cnrs.zoom.us/j/94199862105?pwd=MkOyfZotMWB9CdtR1yE3fJmPfhX8Qq.1
Composition du jury :
| Yann CAMENEN | PIIM, CNRS | Rapporteur |
| Benoit LABIT | SPC, EPFL | Rapporteur |
| Özgür GÜRCAN | LPP, CNRS | Examinateur |
| Peter MANZ | University of Greifswald, IPP | Examinateur |
| Steven TOBIAS | University of Leeds | Président du jury |
| Pascale HENNEQUIN | LPP, CNRS | Directrice de thèse |
| Yanick SARAZIN | IRFM, CEA | Directeur de thèse |
| Xavier GARBET | IRFM, CEA & NTU Singapore | Membre invité |
Abstract
This PhD work is a step forward in the characterization of turbulence self-organization in edge tokamak plasmas, key player in transport and confinement. Three main results are obtained: the plasma parameter regimes prone to turbulence self-organization are identified, some of the underlying physical mechanisms at work are unravelled, and some experimental evidence of self-organization is obtained by means of Doppler Back-scattering (DBS) measurements in tokamak plasmas.
Tokamaks aim at confining hot plasmas by means of large magnetic fields. The last closed flux surface separates the confined inner region from the scrape-off layer where the plasma interacts with materials. In the tokamak operational regime, cross-field transport – hence confinement – is governed by micro-scale turbulence. Understanding the mechanisms of its saturation would open the route towards its possible control. Plasma conditions at the edge transition region are key. In this region, turbulence generates avalanche transport events, which deteriorates the confinement, and zonal flows that efficiently contribute to turbulence saturation. Understanding and predicting turbulence self-organization – i.e. the self-consistent interplay between potentially radially structured ZFs, flux-surface averaged profiles and turbulent transport – in the various parameter regimes of edge tokamak plasmas constitutes the backbone of this work.
To this aim, the reduced nonlinear model Tokam1D, developed on purpose, evolves the mean profiles and the fluctuations in a self-consistent manner in the flux-driven regime. Importantly, in view of studying turbulence self-organization at mesoscales, no scale separation is assumed. The model features two instabilities thought to be dominant at the edge, namely collisional drift waves CDW, originating from a finite phase shift between density and electric potential fluctuations, and interchange due to the magnetic field inhomogeneity (curvature). Their control parameters exhibit different dependencies with respect to plasma parameters, so that different regimes can be expected in edge tokamak plasmas. The model is reduced to 1-dimension by retaining a single parallel and poloidal mode for the fluctuations, in the spirit of a generalized quasilinear approach.
A large scan of the instability control parameters, both at fixed source and fixed distance-to-threshold, allows one to pave the edge plasma parameter space. Large flow-to-turbulence energy ratios are predicted at low collisionality or large magnetic curvature. The latter favours interchange turbulence characterized by avalanche-like transport events and radially-structured ZFs leading to corrugated pressure profiles known as staircases. ZFs are driven by the electric and diamagnetic components of the Reynolds stress, whose phase alignment and relative amplitude vary with the turbulence regime. The system’s freedom to store energy both in the pressure profile and mean flows is shown crucial to the staircase existence. Overall, the confinement is improved in regimes featuring staircases.
Experimentally measurable signatures of avalanches are found in the form of a two slope radial correlation function, the smallest slope attributed to small-scale turbulent eddies, the largest capturing the avalanche typical propagation length. Experimental studies have been carried out on the Tore Supra (CEA Cadarache) and TCV (EPFL) tokamaks using a two-channel DBS for correlations. In Tore Supra long range correlations are observed when using poloidally and toroidally separated channels. When filtering out dominating geodesic acoustic modes, signatures of the elusive low frequency ZFs are found. In TCV, measurements using two co-located channels exhibit radial correlation functions with two slopes in certain regimes similarly to simulations. This result constitutes an additional indirect proof of the existence of avalanche events in tokamak plasmas, notoriously difficult to diagnose.
Keywords
Tokamak, Turbulence, Zonal flows, Avalanches, Staircases