New Doppler Backscattering Diagnostic Designed for Pegasus-III Fusion Experiment
Scientists have presented a preliminary design for a Doppler backscattering (DBS) diagnostic system for the Pegasus-III spherical tokamak experiment. DBS diagnostics measure plasma flows and electron density fluctuations, and recent work suggests they can also infer magnetic pitch angles. The system could advance both diagnostic science and Pegasus-III's core research goals, including solenoid-free plasma initiation and plasma heating via mode conversion.
A team of researchers has detailed the preliminary design of a Doppler backscattering diagnostic for the Pegasus-III Experiment, a spherical tokamak at the University of Wisconsin-Madison. The system is designed to serve dual purposes: advancing the scientific understanding of DBS instrumentation and supporting Pegasus-III's specific research directions, such as solenoid-free plasma startup and O-X-B mode conversion for heating and current drive. The ex-vacuum system employs a single-channel, tunable Ka-band microwave source, a corrugated horn antenna, 2D mirror steering, and a homodyne I/Q receiver. Using the Scotty beam-tracing code, the team found the system can measure ion-scale density fluctuations with perpendicular wavenumbers between 1 and 8 cm⁻¹, probing radial locations from the outer core out to just beyond the last-closed flux surface. Additional toroidal steering capability is incorporated specifically to enable magnetic pitch angle measurements, which can help locate the mode conversion window critical for plasma heating efficiency. The design builds on recent findings suggesting DBS can constrain magnetic equilibrium, adding a data-driven inference dimension to the diagnostic's capabilities.
What's missing
The paper is a preliminary design study and does not report experimental results from a built or operated system; performance figures are based on beam-tracing simulations rather than empirical validation. Key open questions include how well the system will perform under actual Pegasus-III plasma conditions and the sensitivity limits of the homodyne receiver in practice.
What different sources said
- arXiv physicsCenter
Design of a multifunctional Doppler backscattering diagnostic for the Pegasus-III Experiment
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