Effect of mismatch on Doppler backscattering in MAST and MAST-U plasmas

TL;DR

This paper validates a model for mismatch attenuation in Doppler backscattering diagnostics on MAST and MAST-U plasmas, improving measurement accuracy and diagnostic commissioning through detailed comparison with experimental data.

Contribution

It applies and validates a quantitative mismatch attenuation model across multiple cases and modes, including new comparisons and error identification in the DBS system.

Findings

Identified a ~3° error in MAST Q-band quasioptics.

Validated the model against multiple time points and modes.

Assessed other contributions to backscattered signals.

Abstract

The Doppler backscattering (DBS) diagnostic, also referred to as Doppler reflectometry, measures turbulent density fluctuations of intermediate length scales. However, when the beam's wavevector is not properly aligned perpendicular to the magnetic field, the backscattered power is attenuated. In previous work, we used beam tracing and reciprocity to derive this mismatch attenuation quantitatively. In this paper, we applied our model, in the small but finite mismatch limit, to a several new cases. We compared our predictions with multiple O-mode channels for the first time. We then identified a $\sim 3^{\circ}$ error in the MAST Q-band's quasioptics, showing that our model is useful for commissioning DBS diagnostics. For both O- and X-mode, we compared experimental data with our model's predictions at multiple times during the shots, unlike our previous work, where only a single time was analysed. Finally, we analysed other contributions to the backscattered signal, evaluating how much they affect our measurements of mismatch attenuation, giving comparisons with data from both MAST and MAST-U. This paper's detailed study systematically validates and demonstrates the usefulness of our model for quantitatively interpreting DBS data from spherical tokamaks.