Design of a multifunctional Doppler backscattering diagnostic for the Pegasus-III experiment.

Authors

E Wikarta, U Kumar, V H Hall-Chen, S J Diem, X Li, K T E Chua, T S P See, A C Sontag, Z Wilderspin

Published in

The Review of scientific instruments. Volume 97. Issue 11. Nov 01, 2026.

Abstract

The Doppler backscattering (DBS) diagnostic measures flows and electron density fluctuations. Recent work indicates that DBS can also infer the magnetic pitch angle [Yeoh et al., Nucl. Fusion 66, 016052 (2026)]. We present the preliminary design of a DBS for the Pegasus-III experiment. This DBS will serve two objectives. First, it will advance diagnostic science by supporting understanding of the DBS instrumentation functions, using DBS to constrain the magnetic equilibrium, and data-driven inference of plasma properties from DBS signals. Second, it will support Pegasus-III's research directions, such as solenoid-free plasma initiation and O-X-B mode conversion for heating and current drive, since density fluctuations can affect O-X mode conversion efficiency, while magnetic pitch angle measurements can help constrain the optimum O-X launch angle window. This ex-vacuum DBS system uses a single channel, tunable Ka-band source, a corrugated horn antenna, and a homodyne I/Q receiver with baseband digitization. For greater flexibility, which is especially important for pitch angle measurements, the quasi-optical elements include a rotatable spinner for O- and X-mode selection and a mirror with 2D steering. Using the Scotty beam-tracing code [Hall-Chen et al., Plasma Phys. Controlled Fusion 64, 095002 (2022)], we find that, for a range of poloidal launch angles of 8°-18° and a corresponding toroidal launch angle between 0° and 3° for maximal backscattered DBS power, the DBS system is capable of measuring ion-scale density fluctuations 1 ≲ |k⊥,c| ≲ 8 cm-1 at a range of normalized radial coordinates from the outer core (ρ ∼ 0.65) to just beyond the last-closed flux surface (LCFS), where ρ = 0 corresponds to the magnetic axis and ρ = 1 corresponds to the LCFS. The system is also designed with additional toroidal steering capability, -4° to 8°, to resolve the toroidal response needed for magnetic pitch angle measurements.

PMID:
43018431
Bibliographic data and abstract were imported from PubMed on 12 Nov 2026.

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