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Validation of full-f global gyrokinetic modeling results against the FT-2 tokamak Doppler reflectometry data using synthetic diagnostics

Abstract : Introduction The anomalous energy transport remains one of the main issues in magnetic confinement fusion research. According to the present day understanding [1, 2] the anomalous transport is determined by the multi-scale drift-wave turbulence and the nonlinear interaction of its components. The gyrokinetic (GK) simulations [3, 4] provide an efficient tool for studying the nonlinear turbulent plasma dynamics and in the recent papers [5, 6] such simulations of the electron and ion distribution functions from the first principles were performed for the small research limiter tokamak FT-2. The interplay of the multi-scale turbulence components was studied and successful quantitative comparison was presented with O-mode Doppler reflectometry (DR) and enhanced microwave scattering in the upper hybrid resonance [5, 6]. Unfortunately, the interpretation of DR data is complicated due to the contribution of the poorly localized small-angle scattering, which can lead to the overestimation of the turbulence radial correlation length in the linear scattering regime [7, 8], and to its underestimation in the strongly nonlinear regime [9]. In the latter case the turbulence wavenumber spectrum measurements are questionable, and only the plasma turbulence mean velocity could be determined [10]. Moreover the 2D propagation effects additionally complicate the experimental data interpretation and its comparison with the theory predictions. To overcome these issues, synthetic diagnostic can be used both for the interpretation of the experimental results and for the code benchmarking [11, 12]. In this paper two versions of the X-mode DR synthetic diagnostics are compared in the framework of the ELMFIRE global GK modeling of the FT-2 tokamak ohmic discharge [6]. The X-mode DR signal is computed both in the linear theory approximation using the reciprocity theorem [13] and by the full-wave code IPF-FD3D [14]. The temporal behavior of the DR signal frequency spectra and the dependence of its amplitude, frequency shift and shape on the probing antenna position are computed and compared to those measured in the experiment at the FT-2 tokamak. In the case of multi-frequency probing the radial correlation (RC) DR cross-correlation function (CCF) is also determined by both of the synthetic diagnostics and compared to that obtained in the experiment. The experimental approach The experiment was performed at the FT-2 tokamak (the major radius R = 0,55 m, the limiter radius a = 0,079 m) in the hydrogen ohmic discharge (with plasma current I p = 19 kA, central density n e (0) = 410 19 m-3 and electron temperature T e = 470 eV). The discharge is similar to that utilized for successful comprehensive benchmarking of the ELMFIRE GK code in [5, 6], expect for the toroidal magnetic field (B t (0) = 1.7 T instead of 2.1 T). The measured electron density and temperature profiles for this discharge used in the GK modelling, as well as the ion temperature profile are shown in Fig. 1. The vertically movable (by ±2 cm) X-mode double antenna set (shown in Fig.2) installed at high magnetic field side in the vicinity of equatorial plane allowed plasma probing at variable incidence angle with frequencies in the range f i = (50-75) GHz. It was used both in DR measurements utilizing the quadrature scheme and in the RCDR technique [15], as described in [16]. In the latter case the reference channel generator was tuned to the master frequency f 0 =70 GHz determining the measurement position in the vicinity of r = 5 cm, whereas another generator was used in the second, channel to determine the turbulence two-point CCF.
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Submitted on : Friday, October 9, 2020 - 5:13:44 PM
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E Gusakov, A Altukhov, A Gurchenko, Stéphane Heuraux, M Irzak, et al.. Validation of full-f global gyrokinetic modeling results against the FT-2 tokamak Doppler reflectometry data using synthetic diagnostics. IRW 14th Lausanne, Suisse 22-24th May 2019, May 2019, Laussane, Switzerland. ⟨hal-02963102⟩



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