Modelling and application of doppler reflectometry for advanced turbulence studies on the asdex upgrade tokamak and the TJ-II stellarator

Resumen

The most advanced approach to make nuclear fusion an energy source available on earth is the confinement of high temperature plasmas with strong magnetic fields. This method is currently investigated in experimental devices such as tokamaks and stellarators, which use toroidal magnetic configurations designed to confine fusion plasma efficiently. However, turbulence contributes significantly to the transport of energy and particles,which results in a degradation of the confinement quality. Therefore, the understanding of the transport arising from turbulence and its suppression mechanisms may contribute to the improvement of the confinement and thus to the efficiency in present experiments and future fusion reactors. To this end, a careful experimental characterisation of the turbulence in fusion plasmas and its comparison with physical models is required, especially with results from gyrokinetic simulations. They can predict turbulence characteristics in detail, such as spectral properties, shape of turbulent structures and driving micro-instabilities. Doppler reflectometry is one of the diagnostic techniques used for turbulence investigations in fusion plasmas. Based on the backscattering of a probing electromagnetic wave off density fluctuations, it provides local measurements of their velocity (u⊥) perpendicular to the magnetic field and of the perpendicular wavenumber (k⊥) spectrum of the density turbulence. In addition, the radial correlation technique uses two Doppler reflectometer channels for providing information on the radial structure of the turbulence, in particular an estimate of its radial correlation length (Lr ). However, the complexity of the wave propagation and of the scattering process involved induces a non-trivial diagnostic response, which has to be considered for a proper interpretation of experimental measurements. In this thesis the Doppler reflectometry technique is thoroughly investigated analytically, numerically and experimentally. The advanced understanding obtained is then applied to turbulence studies which have been performed in both the ASDEX Upgrade tokamak and the TJ-II stellarator. The wave propagation and scattering relevant for Doppler reflectometry are investigated using a linear analytical model (Born approximation), a physical optics model and two-dimensional full-wave simulations. This allows for a detailed study of both the linear and non-linear diagnostic responses. In particular, a new regime with an enhanced backscattering response to the fluctuation level is found and characterized for the first time. In connection, its impact on the k⊥ spectra measurement is examined. Moreover the diagnostic response to Gaussian, flat and Kolmogorov-type spectra is studied in a broad k⊥ and turbulence level range. The radial correlation technique is also investigated in detail. A new analysis method is developed, which gives a measurement of the mean tilt angle of the turbulent structures. This is a relevant quantity predicted by theories and gyrokinetic simulations, which can provide information on the turbulence interaction with plasma flows and the type of dominant micro-instabilities. The method is based on the analysis of the time delay of the cross-correlation function of the Doppler reflectometer signals. It is found that diagnostic effects may impact the measurements of Lr and the tilt angle. Corresponding correction factors are derived and applied. A detailed characterization of L-mode plasmas using Doppler reflectometry is performed in both the ASDEX Upgrade tokamak and the TJ-II stellarator. Wavenumber spectra, u⊥ profiles and Lr are measured simultaneously. The experimental data is analysed based on the modelling results, and the diagnostic effects on the measurements are investigated. Furthermore, the experimental observations are linked to the theory of turbulence regulation by sheared flows and to different types of micro-instabilities. The experimental time delays of the cross-correlation are studied using the developed analysis techniques. The mean tilt angle of the turbulent structures is measured in the confined region of the ASDEX Upgrade tokamak and the TJ-II stellarator for the first time. The effect of the temporal decorrelation of the turbulence on the tilt angle measurement is investigated experimentally. In the ASDEX Upgrade tokamak, a change of the tilt angle has been measured between phases with either dominant ion or dominant electron heating. This change is consistent with a transition from a dominant ion-temperature gradient to a dominant trapped-electron mode driven turbulence. The tilt angle measured in the TJ-II stellarator is in qualitative agreement with results from linear gyrokinetic simulations. Moreover, a radial variation of the tilt angle consistent with expectations of the u⊥ shear has been observed. The profound study of Doppler reflectometry performed in this thesis has highlighted relevant diagnostic properties and effects, allowing for a more accurate and complete characterization of the turbulence. In particular, the finding of the enhanced power response regime has contributed to a better understanding of the measured k⊥ spectra, and the tilt angle measurement method has provided a new element for experimental investigations and comparisons with simulations and theories. These innovative methods make advanced studies with Doppler reflectometry possible, which may contribute substantially to a better understanding of the turbulence in fusion plasmas.