Liang, Y. H. M., Hall-Chen, V., Rhodes, T., Wang, Y., & Zhao, Y. (2026). A Doppler backscattering diagnostic for the EXL-50U spherical tokamak: Plasma considerations and preliminary quasioptical design. Fusion Engineering and Design, 228, 115770. https://doi.org/10.1016/j.fusengdes.2026.115770
Abstract:
The EXL-50U spherical tokamak was built by Energy iNNovation to develop technologies for proton–boron fusion (Liu et al., Phys. Plasmas 2024). In tokamaks, turbulence is the dominant mechanism of heat and particle transport. In this work, we designed a Doppler backscattering (DBS) diagnostic to measure plasma flows and turbulent electron-density fluctuations in the EXL-50U. Using the SCOTTY beam-tracing code (Hall-Chen et al., PPCF 2022), we found that operation across the U-band frequency range (40–60 GHz), together with poloidal steering, is capable of accessing a suitable range of fluctuation locations and wavenumbers. A quasioptical system was developed to meet these requirements under physical constraints, including port window availability and in-vessel space. Using the quasioptical system’s predicted beam properties, we calculated the attenuation due to the component of the probe-beam wavevector parallel to the magnetic field (mismatch) and determined the degree of mismatch that can be tolerated. We found that toroidal steering is required to minimise mismatch at the relevant cutoff locations, thereby maximising the backscattered signal. This alignment is particularly important due to the high magnetic pitch angle of the EXL-50U, ~35° at the outboard midplane. The beam properties were also used to predict the spatial resolution of high-
measurements, which we found to be satisfactory. Hence, the DBS system is capable of measuring scattering locations of 0.15 < rho < 1.0, with corresponding turbulent wavenumbers of 0.24 mm-1 < kperp < 0.95 mm−1. Here, rho is the normalised radial coordinate of the scattering location, and kperp is the measured fluctuation wavenumber.
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Funding Info:
This research is supported by core funding from: Future Energy Acceleration & Translation (FEAT) Strategic Research and Translational Thrusts (SRTT)
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This research / project is supported by the A*STAR, Science and Engineering Research Council - Central Research Fund
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