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This paper presents the simulation results of a multistage depressed collector for a MW-class gyrotron. The energy distribution of spent electrons is obtained by a beam–wave interaction code. By increasing the number of depressed electrodes, the maximum collection efficiency can be increased to 81% with a low percentage of backstreaming 0.04% with regard to a four-stage depressed collector. By considering the secondary electron emission, the effects of different kinds of secondary electrons are simulated in the CST software, which is developed by Computer Simulation Technology Company in Germany. In order to reduce bad effects of secondary electrons, the structure of the fourth depressed electrode has been adjusted to form a slope. The axial length and slant angle of the slope have been optimized. The results indicate that the collection efficiency can exceed 70% and the percentage of backstreaming is less than 1%. Besides, in view of the limited power capacity of the collector wall, a magnetic field sweeping system has been introduced. The maximum power density is reduced to 392 W/cm2, and the deposited length of the electron beam trajectory is 470 mm.

Coaxial cavities are the main interaction structures of high-power high-frequency gyrotrons at present. With the increase of operating frequency and output power, ohmic loss becomes to be a vital issue needed to be analyzed compulsorily for coaxial-cavity gyrotron. It would lead to operating frequency shift and efficiency decline. In addition, the long pulse operation would be deteriorated to a large extent as a consequence of rather severe ohmic losses. Therefore, it is of great meaning to analyze the ohmic losses of coaxial cavity more accurately. In this paper, a new method is introduced to analyze the influence of the ohmic losses on coaxial cavities and TE34,11 mode operating at 170 GHz is taken as an example to compare this method with other two methods. The results show that this method can be used to analyze the coaxial cavity with higher accuracy and fewer restrictions.

Medium-power gyrotron operating in high frequency usually operates in harmonics to reduce the required guiding magnetic field strength. Low guiding magnetic field is in favor of compact gyrotron, which makes it easier to apply the gyrotron to large scale devices and to extend the applications of gyrotron to wider fields. However, high harmonic modes suffer from low intensity of cyclotron radiation and additional competition with neighboring fundamental modes. As we all know, single mode operation can be realized by using a special startup scenario and a positive magnetic field taper can increase output efficiency. The influence of magnetic field taper on gyrotron operation is studied by generalizing the well-known η⊥μ,F plot, which just discussed the transverse efficiency using Gaussian field profile and not took the self-consistent non-stationary codes or ohmic dissipation into consideration. In this paper, linear and self-consistent nonlinear codes are used to investigate influences of magnetic field taper on start current and output parameters of a 394 GHz gyrotron, which is designed for enhancement of Nuclear Magnetic Resonance spectroscopy through Dynamic Nuclear Polarization. Simulation results show that when the magnetic field taper is +0.04 T the maximum output power is obtained and the maximum value is 3% larger than that corresponding to uniform magnetic field. Appropriate startup scenario is designed for single mode operation and multimode simulation is carried out to demonstrate that mode TE261+ dose dominate in mode competition in the designed startup path.

Monolayer molybdenum disulfide (m-MoS2) has attracted significant interest owing to its unique electronic and optical properties. Chemical vapor deposition has been well recognized as one of the preferred methods to prepare MoS2 film. Little is known, however, about the effect of the temperature gradient from MoO3 powder to growth substrate on shape, size evolution and growth mechanism. Herein, by controlling the temperature gradient from the pre-vaporization temperature of MoO3 powder to the pre-nucleation temperature of growth substrate at nucleation stage, the inversive, equal, positive temperature gradient (ITG, ETG and PTG) are originally designed. Additionally, we investigate the morphology variation from monolayer to multilayer domains with the change in the temperature gradient on the basis of classical crystal growth theory and the Antonie equation. Also, the IGT could effectively suppress the nuclei density and facilitate the growth of m-MoS2 domains. The results suggest introducing temperature gradient is a significant way for tuning the morphological evolution and achieving the controllable shape of monolayer MoS2 film. And thus, the conclusion will further lay the foundation for expanding the process window of m-MoS2 growth.

We experimentally studied unidirectional perfect absorbers with asymmetric sandwich structures composed of two thick metallic films with different thicknesses and a truncated symmetric photonic crystal (PC). Single-channel and multichannel near-perfect absorbers were obtained by adjusting period number of the PC. According to electromagnetic field intensity distributions at absorption wavelengths, the physical mechanism of near-perfect absorption derived from the coupling effect between optical Tamm state and Fabry-Pérot resonance. The unidirectional absorption phenomena were attributed to weak electric field distributions at absorption wavelengths for opposing directions of light incidence. The experimental values showed good agreement with theoretical results.

We demonstrate theoretically and experimentally that complete light transmission can be realized using a photonic heterostructure containing a tunable dielectric layer inserted between two different truncated photonic crystals (PCs). A perfect tunneling state is produced within enlarged photonic band gap (PBG) of the heterostructure by varying the thickness of inserted dielectric layer and the transmittance of the tunneling state depends on the dielectric layer thickness. Additionally, the tunneling state frequency varies with inserted layer thickness but is always located within the small overlapped PBG of two PCs. Therefore, both a perfect tunneling state and an ultrawide PBG can be realized in these heterostructures. The experimental results showed good agreement with theoretical values.

We have theoretically investigated the electronic resonant tunneling effect in graphene superlattice heterostructures, where a tunable graphene layer is inserted between two different superlattices. It is found that a complete tunneling state appears inside the enlarged forbidden gap of the heterostructure by changing the thickness of the inserted graphene layer and the transmittance of the tunneling state depends on the thickness of the inserted layer. Furthermore, the frequency of the tunneling state changes with the thickness of the inserted graphene layer but it always located in the little overlapped forbidden gap of two graphene superlattices. Therefore, both a perfect tunneling state and an ultrawide forbidden gap are realized in such heterostrutures. Since maximum probability densities of the perfect tunneling state are highly localized near the interface between the inserted graphene layer and one graphene superlattice, it can be named as an interface-like state. Such structures are important to fabricate high-Q narrowband electron wave filters.