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Benchmarking of Full-F Global Gyrokinetic Modeling Results Against the FT-2 Tokamak Doppler Reflectometry Data Using Synthetic Diagnostics

Abstract : Two versions of the X-mode Doppler reflectometry (DR) synthetic diagnostics are developed in the framework of the ELMFIRE global GK modeling of the FT-2 tokamak ohmic discharge. In the "fast" version DR signal is computed in the linear theory approximation using the reciprocity theorem utilizing the probing wave field pattern provided by computation taking into account the 2D plasma inhomogeneity effects, whereas the alternative "slow" version of DR synthetic diagnostic is based on the full-wave code IPF-FD3D describing the probing and scattered wave propagation in turbulent plasma. The DR signal frequency spectra and the dependence of their frequency shift, width and shape on the probing antenna position are computed and shown to be similar to those measured in the high field side probing DR experiment at the FT-2 tokamak. The GAM characteristics provided by the measurements and by the synthetic DR are close within a 12% accuracy. However the variation of the DR signal at growing incidence angles in experiment is slower than predicted by both synthetic diagnostics. The substantial difference was found also in the decay of the DR signal CCFs with growing frequency shift in the probing wave channels. The quick decrease of the RCDR coherence observed in the experiment and full-wave synthetic diagnostic compared to the fast synthetic RCDR is attributed to the phase modulation of the probing wave in the former two cases due to the long-scale density fluctuations, which is shown to be close to π/4 both in the specially performed measurements and in the GK modelling. In spite of the fact that this value indicates only the beginning of the transition to the fluctuation reflectometry nonlinear regime, which therefore is not influencing the DR spectra, it already has a strong influence on the RCDR performance.
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A Altukhov, A Gurchenko, E Gusakov, M Irzak, O Krutkin, et al.. Benchmarking of Full-F Global Gyrokinetic Modeling Results Against the FT-2 Tokamak Doppler Reflectometry Data Using Synthetic Diagnostics. 27th IAEA Fusion Energy Conference - IAEA Gandhinagar India, Oct 2018, Gandhinagar, India. ⟨hal-02970365⟩

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