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.