M. Sébastien HACQUIN
Soutiendra publiquement son Habilitation à Diriger des Recherches intitulée :
Study of plasma instabilities in fusion devices with reflectometry diagnostics
Soutenance prévue le 28 novembre 2025 à 10h00
Lieu : CEA – IRFM, salle René Gravier, Bâtiment 506
Composition du jury :
| Dr Rita LORENZINI | Consorzio RFX / ENEA – Padoue | Rapportrice |
| Pr Carlos SILVA | IPFN / IST – Lisbonne | Rapporteur |
| Pr Elisabeth WOLFRUM | Max Planck IPP – Garching | Rapportrice |
| Pr Peter BEYER | PIIM / AMU – Marseille | Examinateur |
| Dr Stefano CODA | SPC / EPFL – Lausanne | Examinateur |
| Dr Xavier LITAUDON | IRFM / CEA – Cadarache | Examinateur |
| Pr Stéphane HEURAUX | IJL / UL – Nancy | Invité |
Résumé :
Plasma instabilities are responsible for a loss of plasma confinement that limits the performance of fusion devices, then making their investigation a key topic in fusion research. The work presented here focuses on plasma instabilities observed in different machines, primarily the JET and Tore Supra tokamaks, and investigated using reflectometry diagnostics, which are based on radar-like techniques probing the plasma with millimetre waves and then measuring the fluctuations of the signal reflected by the plasma. The first instabilities studied are the so-called Alfvén Eigenmodes, which are MHD-like modes triggered by energetic ions in fusion plasmas. It was shown in JET experiments that reflectometry could be efficiently used to characterise the frequency evolution of these modes with high resolution. The second kind of instabilities which was studied deals with small-scale density fluctuations linked to turbulence. In the core region of tokamak plasmas, two mechanisms believed to generate turbulence are Ion Temperature Gradient (ITG) instabilities and Trapped Electron Modes (TEM). It was demonstrated from reflectometry measurements in Tore Supra, TEXTOR and JET, that the presence of quasi-coherent modes in the density fluctuation spectra was a signature of TEM-dominant turbulence regimes. This experimental finding was supported by gyro-kinetics computations and retrieved with synthetic reflectometry simulations. To allow for statistical studies of the turbulence characteristics, a database of fluctuation spectra measured with a fixed-frequency reflectometer in Tore Supra discharges was built. Using a multi-component model to fit the fluctuation spectra, a statistical study of the fluctuation characteristics and their dependence on different plasma parameters (collisionality, additional heating, etc.) was carried out. The changes of the fluctuation spectra according to the plasma confinement regime were thus characterised. Lastly, the potential of Ultra-Fast frequency-Swept Reflectometry (UFSR) to investigate the radial evolution of turbulence was evidenced. In particular, a radial reversal of the fluctuation frequency spectrum measured by UFSR was observed in the pedestal region of I-mode plasmas in ASDEX-Upgrade and M-mode plasmas in JET. A model of density fluctuations was proposed and tested with a synthetic reflectometry diagnostic to reproduce these experimental observations. UFSR is also a valuable tool to measure the radial wave-number spectrum of the density fluctuations. Synthetic reflectometry simulations were carried out to assess the effects of edge turbulence, the level of which is usually an order of magnitude larger than in the core region, on measurement of core turbulence. It appears that edge turbulence enhances the amplitude of the wave-number spectrum of core density fluctuations without significantly changing its shape. All these results highlight the ability of reflectometry diagnostics to study the various types of instabilities affecting the stability and the performance of fusion plasmas. Contributing to a better understanding of key physic processes, such studies are therefore essential to prepare the exploitation of the next generation of fusion machines, starting with ITER.