Validating and optimising mismatch tolerance of Doppler backscattering measurements with the beam model

TL;DR

This paper uses a beam model to analyze and optimize mismatch attenuation in Doppler backscattering measurements across different tokamaks, improving the accuracy of density fluctuation amplitude determination.

Contribution

It quantitatively validates the beam model for mismatch attenuation correction in DIII-D and other tokamaks, and suggests optimization strategies for probe beam parameters.

Findings

Beam model accurately predicts mismatch attenuation in various tokamaks.

Optimizing probe beam width and curvature improves mismatch tolerance.

Model applicability extends across multiple plasma devices.

Abstract

We use the beam model of Doppler backscattering (DBS), which was previously derived from beam tracing and the reciprocity theorem, to shed light on mismatch attenuation. This attenuation of the backscattered signal occurs when the wavevector of the probe beam's electric field is not in the plane perpendicular to the magnetic field. Correcting for this effect is important for determining the amplitude of the actual density fluctuations. Previous preliminary comparisons between the model and Mega-Ampere Spherical Tokamak (MAST) plasmas were promising. In this work, we quantitatively account for this effect on DIII-D, a conventional tokamak. We compare the predicted and measured mismatch attenuation in various DIII-D, MAST, and MAST-U plasmas, showing that the beam model is applicable in a wide variety of situations. Finally, we performed a preliminary parameter sweep and found that the mismatch tolerance can be improved by optimising the probe beam's width and curvature at launch. This is potentially a design consideration for new DBS systems.