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 $\epsilon <2.2\times {10}^{-16}$.

We propose a novel scheme to generate attosecond soft x rays in a self-seeded free-electron laser (FEL) suitable for enabling attosecond spectroscopic investigations. A time-energy chirped electron bunch with additional sinusoidal energy modulation is adopted to produce a short seed pulse through a self-seeding monochromator. This short seed pulse, together with high electron current spikes and a cascaded delay setup, enables a high-efficiency FEL with a fresh bunch scheme. Simulations show that using the Linac Coherent Light Source (LCLS) parameters, soft x-ray pulses with a FWHM of 260 attoseconds and a peak power of 0.5 TW can be obtained. This scheme also has the feature of providing a stable central wavelength determined by the self-seeding monochromator.

Nb₃Sn-coating experiments on samples and single-cell cavities were conducted at Peking University (PKU) to understand the Nb₃Sn growth process using the vapor diffusion method. The evaporation of tin and tin chloride used in the vapor diffusion process was simulated and experimentally analyzed. The results show that the nucleation process is generally uniform, and the atomic ratios of Nb/O and Sn/O were found to be 1:2 within the 10 nm surface of the nucleated samples. Three tin sources were distributed along the cavity axis to obtain a uniform grain size on the cavity surface, and a surface tin content of 25~26% was achieved. The tin segregation effect was found in long-time coatings or coatings with insufficient tin, resulting in a low Sn% and bad cavity performance. By overcoming the tin segregation problem, a Nb₃Sn cavity with a 750 nm grain size was produced by 1200 °C coating for 80 min and 1150 °C annealing for 60 min. The acceleration gradient reached 17.3 MV/m without quenching and an obvious Q-slope at 4.2 K. The relationship between coating recipes and vertical test results is discussed and conclusive advice is provided in this paper.

High-brightness megahertz-rate electron beams are highly desired for cutting-edge applications in many areas of science. Photocathode electron guns capable of generating such beams with low dark current remain to be a challenging field. In this paper, we report breakthroughs with a hybrid gun combining a direct-current gap and a superconducting radio-frequency (SRF) cavity. The gun, employing K 2 CsSb photocathodes driven by a green laser, delivers a few MeV electron beam at 1 and 81.25 MHz rates with an average current up to 3 mA and a dark current several orders lower than current normal conducting continuous-wave guns. Emittance compensation and multipole field corrections have been applied to achieve a high-quality beam. The achievement will inspire the development of high-brightness SRF guns and promote megahertz-rate beam applications including coherent linac light sources and ultrafast electron diffraction/microscopy. Published by the American Physical Society 2024