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Investigation of absolute instability (AI) and convective instability (CI) of the electron cyclotron maser has independently led to the birth of gyrotron oscillators and amplifiers. Here, it is demonstrated that these instabilities can form a cooperative relationship owing to the nonlinear behavior of the stimulated electron beam. The CI can be induced in a zero‐drive system with the assistance of AI, and the ohmic losses of all the excited waves inside the system are greatly reduced, which is called "instability entanglement" here. According to the theoretical and experimental study of a 167/330 GHz gyrotron, when instability entanglement occurs, the ohmic dissipation decreases to one‐ninth of the AI‐only condition, and the output power is enhanced by 20%. This discovery is promising for surpassing the ceiling of output power and frequency of gyrodevices placed by ohmic losses. [ABSTRACT FROM AUTHOR]

Free‐electron radiations prompt numerous advanced applications such as particle detectors, biological imaging, and light sources from microwave to X‐ray. While their polarization, directionality, and phase could be shaped with prosperous metamaterials, their natural broad spectra have not been tailored to yield frequency combs, which are state‐of‐the‐art technology in metrology, spectroscopy, and precision frequency synthesis. Here, the frequency comb directly emitted from the free‐electron radiation is demonstrated by simultaneously exciting a series of modes with equidistant spectral lines. Both the offset and repetition frequencies are readily adapted by structural parameters, and relative spectrum intensities between the comb teeth are customizable based on selective coupling, thus permitting flexible tunability over broadband. Moreover, the repetition rate is experimentally verified at the microwave regime. The proposed methodology implies that swift electrons present a natural and versatile platform for generating frequency combs, facilitating the development of metrology and spectroscopy in the terahertz band. [ABSTRACT FROM AUTHOR]

Recent advances in vortex Smith–Purcell radiation (V‐SPR) have spurred many breakthroughs in the generation of vortex beams for their potential applications in chiral detection, particle acceleration, communication, and imaging. However, advanced multi‐mode applications of the V‐SPR suffer from the incoherent spectrum and poor purity in producing high‐order topological charges. Here, by exploiting the rotational symmetry and Brillouin‐folding phenomenon, it is proved that the coherent V‐SPR with independently steerable high‐order topological charge is directly emitted from chiral spoof surface plasmons. Moreover, both topological charges and operation frequencies can be readily tuned, thus enabling flexible adaptability over a broad bandwidth. A proof‐of‐concept prototype is designed and fabricated in the microwave regime, in which the method is demonstrated from the perspective of symmetry and tunability. The proposed methodology can be exploited as a platform to investigate the interaction between chiral plasmons and swift electrons, promising a feasible scheme for generating coherent and tunable vortex beams from the microwave to the terahertz band. [ABSTRACT FROM AUTHOR]

In this study, high‐speed ballistic impact tests were conducted on GH4169 alloy samples with the aeroengine compressor blade leading‐edge feature to simulate the notch‐type foreign object damages (FOD). Macroscopic and microscopic characterization of FOD and high cycle fatigue tests were performed to investigate the effect of FOD depth on GH4169 alloy fatigue strength along with numerical analysis using the Kitagawa–Takahashi diagram. Results show that the incident side of notch‐type FODs is relatively smooth, whereas the exit side is rugged. The FOD depth ranges from 0.18 to 1.33 mm, and the fatigue strength of damaged samples is 37.93% – 97.04% of the undamaged samples. As the FOD depth increases, damage length, material losses, and the stress concentration coefficient of the FOD increase significantly and accompanied by the increasing of adiabatic shear bands, micro voids, and cracks, resulting in fatigue strength reduction. Numerical analysis indicates that the Kitagawa–Takahashi diagram can provide a basic model for the design of FOD tolerance. Highlights: The macro and micro features of FOD notches with varying damage depths were characterized.Grooves at FOD notch bottom may lead to premature initiation of high cycle fatigue cracks.Fatigue strength of FOD samples decreases with the increase of the damage depth.The K‐T diagram can provide a basis for the design of foreign object damage tolerance. [ABSTRACT FROM AUTHOR]

The dispersion characteristics of electromagnetic waves have led to a wide range of applications, such as pulse compression, superluminal propagation, wave tunneling effect, and electron–wave interaction. The near‐cutoff wave inside a guiding system is strong dispersive. When the wave frequency reaches the terahertz band, this strong dispersive property is extremely sensitive to the losses induced by the limited conductivity of the component. This phenomenon is traditionally investigated through perturbation of boundary conditions in the frequency domain, which does not deal well with broad‐spectrum signals. Two simple and efficient time‐domain schemes to study the strong dispersive propagation of terahertz waves are proposed. One is based on relatively concise principles, called the constant frequency method. The second is the recursive convolution method, which is suitable for dealing with a wave with wider spectrum range. The reliability of both approaches is verified by the comparison of the dispersion curves. Using the proposed methods, the pulse compression of terahertz wave inside a strong dispersive waveguide is verified in principle. [ABSTRACT FROM AUTHOR]