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Doppler reflectometry is an important microwave diagnostic for turbulent fusion plasma. A low power probing beam between 40 and 100 GHz in O or X mode polarisation is scattered at certain density fluctuation wavenumbers, depending on frequency and angle between beam and density gradient. This way, a radially localised wavenumber resolved density fluctuation spectrum can be measured. However, the correspondence between fluctuation power and scattered microwave power is strongly non-linear and is investigated using fullwave simulations, with density fluctuations supplied by a plasma turbulence code. Earlier results for X mode polarisation have shown saturation of the spectral power to occur at high fluctuation amplitudes, an enhancement at intermediate amplitudes, and linear behaviour at low amplitudes. Together, these effects strongly modify the shape of the wavenumber spectrum [1]. Experimental data in X and O mode polarisation and accompanying plasma fluctuation data from the GENE code have been used to simulate the Doppler reflectometer response with the fullwave code IPFFD3D [2]. The gyrokinetic plasma turbulence code GENE is a physically comprehensive, hyperscalable Vlasov code that is used with the same ASDEX Upgrade equilibrium and plasma profiles as the experiment and the fullwave simulations. A flux tube surrounding the radial position of the microwave cutoff was simulated. The input parameters were adjusted slightly until the resultant turbulent fluxes matched the ones observed in experiment. The output density fluctuation fields are used as inputs in the fullwave simulations. IPF-FD3D is a finite difference time domain code for cold plasmas. Simulations were done in 2D for both polarisations in the perpendicular plane for 512 different turbulence time slices. The turbulence is considered frozen in time while the wave is propagating. The antenna angles and frequencies were adjusted to probe the same pol as the experiment. The setup is shown in Fig. 1. Comparison with experimental data showed a good agreement [2], with the O mode spectrum corresponding more closely to the GENE density spectrum. Furthermore, the fullwave simulation data in Fig. 2 shows the big difference between O and X mode polarisation, which were both probing the same fluctuations. The spectrum derived from O mode scattering looks similar to the real spectrum of the density fluctuations, albeit with power law exponent -8.7 instead of -6.7. The X mode spectrum is much shallower at the nominal fluctuation amplitude taken directly from GENE. In addition, the kink or knee of the spectrum is moved to larger wavenumbers compared to O mode and GENE. The knee should be the same, as it is determined by the instability that is driving the turbulence.

Using Fullwave Simulations to Understand the Turbulent Wavenumber Spectrum Measured by Doppler Reflectometry