Development of the density profile and fluctuation reflectometry on EXL-50U spherical torus

The EXL-50U spherical torus (ST) is designed to investigate key technologies for hydrogen-boron fusion within a compact ST configuration. To enable accurate plasma density profile and fluctuation measurements, X-mode polarized Kα-band (26.5–40 GHz) and U-band (40–60 GHz) reflectometry systems have been designed and installed on the EXL-50U. Additionally, Ku-band (12–18 GHz) and K-band (18–26.5 GHz) reflectometry systems are developed to facilitate edge profile measurements under low toroidal magnetic field conditions. The microwave transmission and reception chains consist of rectangular waveguides, twisted waveguides, and pyramidal horn antennas. Electromagnetic simulations have been performed to calculate the S-parameters of the transmission line configurations, optimizing signal transmission efficiency. In the density profile measurement systems, a voltage-controlled oscillator (VCO) with rapid sweep capabilities—controlled by a custom-designed FPGA-based circuit is employed to achieve high temporal resolution. For density fluctuation measurements, a dielectric oscillator operating at fixed frequencies of 26, 32, 36, 40, 44, 48, 52 and 55 GHz is used to detect density perturbations at corresponding cutoff layers. The results show that the fluctuation reflectometry observed a valid signal. The density profile obtained from the profile reflectometry has been cross-validated against data from both Thomson Scattering (TS) and the Hydrogen Cyanide (HCN) interferometer, ensures reliability, and supports a comprehensive physical analysis of the experimental results.