Experimental and numerical study of turbulence in fusion plasmas using reflectometry synthetic diagnostics

Abstract

Anomalous energy and particle transport is closely related to micro-turbulence. Therefore plasma turbulence studies are essential for successful operation of magnetic confinement fusion devices. This thesis deals with the development of interpretative models for Ultra-Fast Swept Reflectometry (USFR), a diagnostic used for the measurement of turbulence radial wave-number spectra in fusion devices. While the interpretation of reflectometry data is quite straightforward for small levels of turbulence, it becomes much trickier for larger levels as the reflectometer answer is no longer linear with the turbulence level. It has been shown for instance that resonances due to probing field trapping can appear in turbulent plasma and produce jumps of the signal phase. In the plasma edge region the turbulence level is usually high and can lead to a non-linear regime of the reflectometer response. The loss of probing beam coherency and beam widening when the probing beam crosses the edge turbulence layer can affect USFR core measurements. Edge turbulence with a long correlation length leads to small beam widening and strong distortion of the probing wave phase. However backscattering effects from turbulence with short correlation lengths are also able to cause reflectometer signal change. To study turbulence wave-number spectra as well as reflectometer signal phase variations, signal amplitude variations can be analized. Unlike signal phase variation, amplitude does not suffer from resonant jumps, and can give more clear qualitative evaluation of turbulence structure. In the case when the turbulence amplitude peaked in the edge region, it can be detected as spectral peak near local Bragg resonance wave-number. USFR with a set of receiving antennas arranged poloidally was proposed to obtain more information on the edge turbulence properties. A displacement of the spectral peak appears when the receiving antenna is misaligned with the emitting one. Peak displacement measurements could provide additional information on probing beam shaping and turbulence properties and help in coherent mode observation as well. A 2D full wave code was applied as a synthetic diagnostic to Gysela gyro-kinetic code data to study Tore-Supra tokamak core turbulence. Radial correlation lengths computed from the amplitude of multi-channel fixed frequency reflectometry signals 5have shown good agreement with the turbulence correlation length directly computed from the simulation. The synthetic diagnostic was then applied to analyse the correlation length and wave-number spectra obtained by USFR in the ASDEX-Upgrade tokamak. A comparison between 1D and 2D results have shown different behaviour. However correlation lengths measured with UFSR signals are in the same order with turbulence ones.