Evaluating Doppler backscattering for inferring internal magnetic pitch angle of Mega Ampere Spherical Tokamak-Upgrade plasmas

Spherical tokamaks (STs), such as the Mega Ampere Spherical Tokamak-Upgrade (MAST-U), have large magnetic pitch angles that vary significantly in space and time. Measuring this pitch angle in the core, with high temporal and spatial resolution, is key for equilibrium reconstruction and controlling magnetohydrodynamic stability. Existing techniques, such as the motional Stark effect diagnostic, might not survive the harsh conditions of burning plasmas in future devices like STEP. In this work, we assess the viability of using a Doppler backscattering (DBS) system to infer the magnetic pitch angle in MAST-U plasmas. DBS is a robust microwave diagnostic typically used for measuring turbulent density fluctuations and flows. Through toroidal steering, DBS operators aim to match the beam wavevector perpendicular to the magnetic field, as mismatch leads to a lower signal and is thus generally undesirable. Conversely, this variation of power on matching has enabled DBS to infer the magnetic pitch angle in DIII-D, a conventional tokamak. We find that while this technique is more challenging to implement in STs, it is nonetheless possible to infer the magnetic pitch angle in the plasma core of up to a normalized poloidal flux of 0.2 in MAST-U. Finally, we show how to optimize a DBS system specifically for measuring the pitch angle.