Dark photons have emerged as promising candidates for dark matter, and their search is a top priority in particle physics, astrophysics, and cosmology. We report the first use of a tunable niobium superconducting radio-frequency cavity for a scan search of dark photon dark matter with innovative data analysis techniques. We mechanically adjusted the resonant frequency of a cavity submerged in liquid helium at a temperature of 2 K, and scanned the dark photon mass over a frequency range of 1.37 MHz centered at 1.3 GHz. Our study leveraged the superconducting radio-frequency cavity’s remarkably high quality factors of approximately ${10}^{10}$, resulting in the most stringent constraints to date on a substantial portion of the exclusion parameter space on the kinetic mixing coefficient $\epsilon$ between dark photons and electromagnetic photons, yielding a value of $\epsilon <2.2\times {10}^{-16}$.

Recently, heat treatment in the range of 250 °C–500 °C has been attempted with the aim of improving the quality factor ( Q ) of superconducting radio-frequency (SRF) cavities worldwide. Herein, medium-temperature (mid-T) furnace baking experiments were conducted at the Institute of High Energy Physics to obtain higher Q values. First, 1.3 GHz 1-cell cavities received mid-T furnace baking at different temperatures, resulting in not only the improvement of Q but also an anti- Q -slope behavior. Furthermore, mid-T furnace baking was applied to six 1.3 GHz 9-cell cavities, all of which indicated higher Q and anti- Q -slope behavior. The average Q was 3.8 × 10 ¹⁰ at 16 MV m ⁻¹ . The maximum gradients of the 9-cell cavities were 22.7–26.5 MV m ⁻¹ . Finally, the mid-T furnace baked 1.3 GHz 9-cell cavities were welded with helium vessel, with no degradation in the vertical tests. In addition, the mid-T furnace baking process was simplified in comparison with European X-ray Free Electron Laser and Linear Coherent Light Source because of the cancellation of light electro-polishing. This simplification is beneficial in the mass production of SRF cavities with high Q , which are widely adopted in free electron lasers and high-energy colliders.

Bulk Nb superconducting radio-frequency (SRF) cavities are widely used in accelerators, and their accelerating gradient and general performance are limited by the superheating field ( B _sh ). To push the theoretical limit of the B _sh , new multilayer structures are required. We fabricated FeSe-coated Nb films using pulsed laser deposition, performed structural characterizations, and measured the transport and magnetic properties for this superconductor-superconductor bilayer structure with smooth surface. Additionally, the measured B _c1 of FeSe-coated Nb film is greatly enhanced, while the B _sh of the FeSe layer is expected to be higher than that of bulk Nb, yet the superconducting transition temperature ( T _c ) is less than 5 K. This work presents the first fabrication of a new coating layer: FeSe deposited on Nb, showing the possibility of using iron-based materials for multilayer structures in SRF cavities.

A circular electron positron collider (CEPC) will adopt hundreds of 650-MHz superconducting cavities with high quality factor (Q) and accelerating gradient (E_acc). Two 650-MHz single-cell cavities made of fine-grain niobium were first treated via buffered chemical polishing (BCP), which was easy and convenient. However, the vertical test results could not meet the specification of the CEPC (4 × 10¹⁰ at 22 MV/m). Therefore, electro-polishing (EP) of 650-MHz single-cell cavities was conducted, which was complicated but remarkably effective. Both 650-MHz single-cell cavities achieved state-of-the-art gradients of 35 MV/m after the EP process, which is extremely high for large elliptical cavities (frequency < 1 GHz). One cavity achieved an intrinsic quality factor (Q₀) of 4.5 × 10¹⁰ at 22.0 MV/m, which was higher than the CEPC spec. The other cavity obtained a lower Q₀ of 3.4 × 10¹⁰ at 22.0 MV/m, which may have resulted from the cancellation of high-temperature annealing.