Conceptual study on using Doppler backscattering to measure magnetic pitch angle in tokamak plasma

We present a new method to measure the magnetic pitch angle profile in tokamak plasmas using Doppler backscattering (DBS), traditionally used for flow and turbulence studies. The DBS signal strength peaks when the beam wavevector is perpendicular to the magnetic field at the cutoff, independent of fluctuation amplitude[1]. By varying the toroidal launch angle, the angle at which the beam intersects the magnetic field changes, allowing inference of the pitch angle from the maximum in backscattered power. We performed toroidal angle scans on repeated DIII-D discharges and analysed the results using the Scotty Gaussian beam-tracing code[2]. DBS-inferred pitch angles agree with magnetics-only and MSE-constrained equilibrium reconstructions in the edge; in the core, DBS and magnetics-only differ by 1–2°, with sub-degree uncertainties. We attribute these uncertainties to toroidal steering error, number of angular samples, and shot-to-shot variation. Beam optimisation can further improve precision. Given its high spatial and temporal resolution, non-perturbative nature, and resilience to neutron damage, DBS is a strong candidate for pitch-angle measurements in future fusion devices.

[1] Hillesheim et al, Nucl. Fusion 55, 073024 (2022)

[2] Hall-Chen et al, Plasma Phys. Control. Fusion 64, 095002 (2022)