光反射材料对飞行时间探测器性能的影响

在北京正负电子对撞机直线加速器试验束上测试TOF闪烁体在不同光反射材料下(tyvek, teflon, millipore, ESR及镀铝薄膜)的性能, 包括时间分辨率, 衰减长度及光在闪烁体中的有效传播速度. 就时间分辨率来说镀铝薄膜较好达到95.6±2ps, 就衰减长度来说ESR最好, 对于光在闪烁体中的有效传播速度几种光反射材料大致相同.     

ON THE ACCURATE MODELING OF MILLIMETER WAVE FIXED WIRELESS CHANNELS EXCEEDANCE PROBABILITY

The ever increasing demand for high date rate multimedia services has led to the deployment of Fixed Wireless Access (FWA) networks operating in frequencies above 10GHz. Propagation characteristics of such networks include line-of-sight (LOS) transmissions highly influenced by the presence of rain. In this paper a methodology for evaluating the outage probability of a FWA channel is introduced, making use of the forward scattering amplitude by distorted raindrops of transmitted signals. Expressions for the imaginary part of the scattering amplitude are derived through a regression fitting analysis on the results of the Method of Auxiliary Sources (MAS) to the problem of electromagnetic scattering from a Pruppacher-Pitter raindrop. These expressions are employed and an analytical method to evaluate the rain attenuation exceedance probability over a fixed wireless access link is presented. The derived exceedance probabilities are compared with experimental data from ITU-R databank with encouraging results.     

Influence of Raindrop-Size Distribution on the Differential Reflectivity up to Submillimeter Wavelength of 0.96 mm

The rain attenuation was calculated by using the Marshall and Palmer, the gamma and the Weibull raindrop-size distributions. The microwave experimental measurement data from 8.4 GHz (3.75 mm) to 312.5 GHz (0.96 mm) were compared with our calculations. The Weibull distribution has the best agreement with the measurements, especially above 100 GHz. The differential reflectivity Z _DR was also calculated using these three distributions. To this end, we used the horizontal and vertical radar cross sections which were computed from Waterman's T-matrix method. It is shown that Z _DR greatly depends on raindrop-size distributions and has potential for determing drop-size distributions with high accuracy.     

Effects of charged sand on electromagnetic wave propagation and its scattering field

Based on the Rayleigh’s scattering theory, the effects of sandstorms on the propagation of electromagnetic wave with different visibilities are presented by solving the scattering field of charged sand particles. Because of the electric charges on the sand surface, the theoretical attenuation will be large enough to match the measured value under certain conditions. And the results show that the effect of sand with electric charges all over its surface on electromagnetic wave attenuation is the same as that of sand without charge, which proves that electric charges distribute on partial surface of the sand in fact.     

塑料光纤的远场辐射图和微分模式损耗研究

通过观察塑料光纤远场辐射图,分析了阶跃型塑料光纤中的模式耦合效果.实验结果表明,阶跃型塑料光纤的模式耦合效果很明显,模式耦合从光纤传输5米左右开始发生,到传输20～30 m时,模式耦合完成,光纤末端能量分布达到稳态.采用剪断法测量了塑料光纤的微分模式损耗,发现微分模式损耗随着模阶的增加而增加,大多数模阶的微分模式损耗在150～180 dB/km范围内.     

超声分子束在HL-1M托卡马克等离子体中的消融和穿透

讨论并研究了超声分子束在托卡马克等离子体中的消融和穿透问题，其中包括了分子束的绝热膨胀，团簇的形成和解离，束的静电屏蔽效应和冷通道效应.研究结果表明，在考虑了这些效应以后数值得到的结果与实验结果相近.这些研究对于更好地理解超声分子束与等离子体的相互作用和优化设计加料系统有一定作用. 关键词： 超声分子束 加料 托卡马克等离子体 消融和穿透     

On Invariants of Virtual Links

We propose a new method of generalizing classical link invariants for the case of virtual links. In particular, we have generalized the knot quandle, the knot fundamental group, the Alexander module, and the coloring invariants. The virtual Alexander module leads to a definition of VA-polynomial that has no analogue in the classical case (i.e. vanishes on classical links).     

Robust Optimal Stopping-Time Control for Nonlinear Systems

   Abstract. We formulate a robust optimal stopping-time problem for a state-space system and give the connection between various notions of lower value function for the associated games (and storage function for the associated dissipative system) with solutions of the appropriate variational inequality (VI) (the analogue of the Hamilton—Jacobi—Bellman—Isaacs equation for this setting). We show that the stopping-time rule can be obtained by solving the VI in the viscosity sense and a positive definite supersolution of the VI can be used for stability analysis.     

The Effect of Attenuation Induced by Sand and Dust Storms on Ka-Band Electromagnetic Wave Propagation Along Earth-Space Paths

Attenuation of millimeter waves induced by sand and dust storms along horizontal paths is analyzed and its expressions applied to earth-space paths is presented. Graphs of the specific attenuation and attenuation versus height and visibility are gotten for two kinds of particle size distribution, namely exponential, lognormal. Results obtained are briefly discussed.     

Diagrams of divide links

Recently N. A'Campo suggested a construction of a link from a generic immersion of a curve into a 2-disk. It is tightly related to the singularity theory. In this paper, we give a simple procedure to draw a diagram of the link from a picture of the curve.