3D full-wave modelling of microwave interactions with plasma density fluctuations

The scattering of microwaves by density fluctuations in magnetised plasmas where the inhomogeneity scale length is comparable to the wavelength is not fully understood. Yet microwaves are used extensively in magnetically confined fusion plasmas not only to provide a wealth of information through diagnostics but for heating and current drive. To this end a 3D full-wave finite difference time domain code (EMIT-3D) has been designed to model the quasi-3D Doppler reflectometry data from a novel synthetic aperture microwave imaging diagnostic (SAMI) and to understand the scattering ef- fects of turbulence on heating and current drive beams. SAMI captures a 2D view of the plasma in a ±40 ◦ illumination from the mid-plane. A vast spatial grid is required to capture the inhomogeneous, curved plasma and magnetic geometry whilst considerable acquisition time is required for Doppler resolution. For this reason EMIT-3D has been parallelised in 3D which is shown to scale well to large machines. EMIT-3D is shown to agree with the extensive benchmarking tests and demonstrates stability to large time iterations. EMIT-3D has been applied to electron cyclotron resonance heating (ECRH) deposition broadening in the DIII-D tokamak. Significant ECRH deposition broadening was measured in three different operating scenarios: L-mode, H-mode and negative triangularity. Each scenario corresponds to distinct turbulence characteristics in the edge region through which the beam must propagate. The turbulence is gen- erated through the Hermes model in the BOUT++ framework which takes as input the measured time averaged electron density, temperature and magnetic field profiles for the specific shot in question. The simulated turbulence is constrained to match the experimentally measured correlation length and normalised fluctuation levels. The predictions of the beam broadening from the simulations are found to agree very well with the experimentally-observed broadening in all cases.