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

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