7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Feasibility study on using Doppler backscattering measurements to infer themagnetic pitch angle
Valerian H. Hall-Chen1, Q.T. Pratt2, A.K. Yeoh1, J. Damba2, T.L. Rhodes2, N.A. Crocker2, J.C.
Hillesheim3, F.I. Parra4, J. Ruiz Ruiz5
1

Institute of High Performance Computing, Agency for Science, Technology, and Research(A*STAR)2
Department of Physics and Astronomy, University of California, Los Angeles
3
Commonwealth Fusion Systems
4
Princeton Plasma Physics Laboratory
5
Rudolf Peierls Centre for Theoretical Physics, University of Oxford
e-mail (speaker): valerian_hall-chen@ihpc.a-star.edu.sg

The Doppler backscattering (DBS) diagnostic is
typically used to measure flows and turbulent density
fluctuations in magnetic confinement fusion plasmas. In
this talk, we will show how DBS can be used to measure
the magnetic pitch angle in both the core and edge oftokamak plasmas. This is achieved by changing thetoroidal launch angle, such that the probe beam reaches
the cut-off at the approximately same poloidal location,
but at different toroidal angles. As the toroidal injection
angle varies, the returned DBS signal also varies. Since
the flows and turbulent density fluctuations are expected
to be similar for different toroidal injection angles, thedominant variation of the backscattered signal comesfrom the probe beams' wavevectors matching to themagnetic pitch angle instead. By measuring thesevariations and modelling them with Scotty [Hall-Chen,Parra, Hillesheim. PPCF 2022], the local magnetic pitch
angle can be extracted. We show an example using DBS
measurements of DIII-D. Since DBS is a microwave
diagnostic, this technique can be applied to burning
plasmas of future fusion reactors where optical
approaches like the motional Stark effect diagnostic
might be unsuitable. Such DBS measurements of thecore magnetic pitch angle could be important forreconstructing the equilibrium, especially in devices
without neutral beams, and for quantifying fast ioninstabilities. The latter then paves the way for better
understanding of fast ion transport.
Acknowledgements: This work was funded by theUrban and Green Tech Office, A*STAR, Green Seed
Fund C231718014. This material is also based uponwork supported by the U.S. Department of Energy,
Office of Science, Office of Fusion Energy Sciences,using the DIII-D National Fusion Facility, a DOE Officeof Science user facility, under Awards
DEFC02-04ER54698 and DE-SC0019352. This work
was also in part supported by a grant from theEngineering and Physical Sciences Research Council(EPSRC) [EP/R034737/1].
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