射频超导腔加速性能的改进

 为提高超导加速腔的加速梯度和Q值，改进了薄膜型超导腔的加速性能。研究证明，对于铜铌溅射腔，在无氧铜衬底和铌膜之间加入NbN 层可以提高铌膜的超导转变温度，改善晶格结构；对纯铌超导腔提出了改进方法，在传统的纯铌超导腔表面制备多层的超导-绝缘-超导复合膜可以屏蔽Nb腔表面的界面场，提高超导腔的临界磁场，从而提高了铌腔的加速梯度。     

A low-noise SIS receiver measured from 480 GHz to 650 GHz using Nb junctions with integrated RF tuning circuits

A heterodyne receiver using an SIS waveguide mixer with two mechanical tuners has been characterized from 480 GHz to 650 GHz. The mixer uses either a single 0.5 × 0.5 µm² Nb/AlO_x/Nb SIS tunnel junction or a series array of two 1 µm² Nb tunnel junctions. These junctions have a high current density, in the range 8 – 13 kA/cm². Superconductive RF circuits are employed to tune the junction capacitance. DSB receiver noise temperatures as low as 200 ± 17 K at 540 GHz, 271 K ± 22 K at 572 GHz and 362 ± 33 K at 626 GHz have been obtained with the single SIS junctions. The series arrays gave DSB receiver noise temperatures as low as 328 ± 26 K at 490 GHz and 336 ± 25 K at 545 GHz. A comparison of the performances of series arrays and single junctions is presented. In addition, negative differential resistance has been observed in the DC I–V curve near 490, 545 and 570 GHz. Correlations between the frequencies for minimum noise temperature, negative differential resistance, and tuning circuit resonances are found. A detailed model to calculate the properties of the tuning circuits is discussed, and the junction capacitance as well as the London penetration depth of niobium are determined by fitting the model to the measured circuit resonances.     

A 275–370 GHz Receiver Employing Novel Probe Structure

We present the results of the development of a 275–370 GHz, fixed-tuned double sideband (DSB) receiver based on superconductor-insulator-superconductor (SIS) junction mixer. The mixer block uses a full height rectangular waveguide and employs a novel radial-like probe structure with integrated bias-T. The measured uncorrected receiver noise temperature is 30–50 K corresponding to about 2–3 quantum noise across the full frequency band with an IF from 3.8 to 7.6 GHz. The mixer is to be used on the Atacama Pathfinder EXperiment (APEX) submillimeter telescope in Chile.     

一类具有非线性发生率和恢复率函数的SIS传染病模型

在传染病模型建模中,采用合理的非线性发生率所得到的动力学性态与实际更加接近,并且在实际的疾病防治过程中,由于受到医院各种医疗资源的影响,染病类的恢复率也会有一定的限制.建立了具有非线性发生率和恢复率函数的SIS传染病模型并分析了其动力学性态,分析这个模型,得到了无病平衡点和地方病平衡点的存在性和稳定性的条件,以及出现Hopf分支的条件.通过数值模拟,给出系统随两个分支参数变化的分支曲线图及系统的相图.     

A 650-GHz Band SIS Receiver for Balloon-Borne Limb-Emission Sounder

A superconducting low-noise receiver has been developed for atmospheric observations in the 650-GHz band. A waveguide-type tunerless mixer mount was designed based on one for the 200-GHz band. Two niobium SIS (superconductor-insulator-superconductor) junctions were connected by a tuning inductance to cancel the junction capacitance. We designed the R_nC_j product to be 8 and the current density to be 5.5 kA/cm². The measured receiver noise temperature in DSB was 126-259 K in the frequency range of 618-660 GHz at an IF of 5.2 GHz, and that in the IF band (5-7 GHz) was 126-167 K at 621 GHz. Direct detection measurements using a Fourier transform spectrometer (FTS) showed the frequency response of the SIS mixer to be in the range of about 500-700 GHz. The fractional bandwidth was about 14%. The SIS receiver will be installed in a balloon-borne limb-emission sounder that will be launched from Sanriku Balloon Center in Japan.     

An SIS epidemic model with diffusion

The aim of this paper is to study the dynamics of an SIS epidemic model with diffusion. We first study the well-posedness of the model. And then, by using linearization method and constructing suitable Lyapunov function, we establish the local and global stability of the disease-free equilibrium and the endemic equilibrium, respectively. Furthermore, in view of Schauder fixed point theorem, we show that the model admits traveling wave solutions connecting the disease-free equilibrium and the endemic equilibrium when R_0 1 and c c~*. And also, by virtue of the two-sided Laplace transform, we prove that the model has no traveling wave solution connecting the two equilibria when R_0 1 and c ∈ [0, c~*).     

SIS reaction–diffusion model with risk-induced dispersal under free boundary

A spatial susceptible–infected–susceptible epidemic model with a free boundary, where infected individuals disperse non-uniformly, is investigated in this study. Spatial heterogeneity and movement of individuals are essential factors that affect pandemics and the eradication of infectious diseases. Our goal is to investigate the effect of a dispersal strategy for infected individuals, known as risk-induced dispersal (RID), which represents the motility of infected individuals induced by risk depending on whether they are in a high- or a low-risk region. We first construct the basic reproduction number and then understand the manner in which a nonuniform movement of infected individuals affects the spreading–vanishing dichotomy of a disease in a one-dimensional domain. We conclude that even though the infected individuals reside in a high-risk initial domain, the disease can be eradicated from the region if the infected individuals move with a high sensitivity of RID as they disperse. Finally, we demonstrate our results via simulations for a one-dimensional case.     

The explicit series solution of SIR and SIS epidemic models

In this paper the SIR and SIS epidemic models in biology are solved by means of an analytic technique for nonlinear problems, namely the homotopy analysis method (HAM). Both of the SIR and SIS models are described by coupled nonlinear differential equations. A one-parameter family of explicit series solutions are obtained for both models. This parameter has no physical meaning but provides us with a simple way to ensure convergent series solutions to the epidemic models. Our analytic results agree well with the numerical ones. This analytic approach is general and can be applied to get convergent series solutions of some other coupled nonlinear differential equations in biology.     

利用失效模式和影响分析评估SIS系统安全性

失效模式和影响分析是评估SIS控制系统、设备、部件和功能设计潜在风险的有效工具。本文从应用和设计的角度,通过FMEA识别系统中失效风险,采用FMEA指导修改设计,达到避免或降低SIS应用中风险的目的。