Radiation impedance of a finite circular piston on a viscoelastic half-space with application to medical diagnosis

In a recent study a new analytical solution was developed and validated experimentally for the problem of surface wave generation on a linear viscoelastic half-space by a rigid circular disk located on the surface and oscillating normal to it. The results of that study suggested that, for the low audible frequency range, some previously reported values of shear viscosity for soft biological tissues may be inaccurate. Those values were determined by matching radiation impedance measurements with theoretical calculations reported previously. In the current study, the sensitivity to shear viscoelastic material constants of theoretical solutions for radiation impedance and surface wave motion are compared. Theoretical solutions are also compared to experimental measurements and numerical results from finite-element analysis. It is found that, while prior theoretical solutions for radiation impedance are accurate, use of such measurements to estimate shear viscoelastic constants is not as precise as the use of surface wave measurements.     

气体辐射太赫兹波产额与驱动激光强度的依赖关系

利用光电流模型研究了气体辐射太赫兹波产额与驱动激光峰值强度的依赖关系.模拟表明,无论是单色激光脉冲还是双色激光场,当激光较弱时,太赫兹产额随着激光强度的增加而迅速增大;而当气体电离接近或达到饱和后,进一步增强激光场强度,太赫兹产额出现了振荡变化.与通常采用较弱激光强度相比,更高的激光强度(致使气体达到完全电离)会进一步增强太赫兹波的辐射强度.分析各个电离事件所产生电子的个数和运动轨迹的对称性,可以很好地解释这些结果.     

On the effect of residual gas on ionization waves in inert gases

The paper deals with the influence of concentration of residual gases in spectroscopically pure neon on parameters of ionization waves in a low-pressure plasma. The change in the concentration of residual gases was produced by a purification of neon by molecular sieves. The measurements concerned the dispersion and frequency dependences of the amplitude together with the self-excitation limit of p-type ionization waves in neon.     

Experimental observation of ionization and shock fronts in foam targets driven by thermal radiation

The behaviours of ionization and shock propagation in radiatively heated material is crucial for the understanding of indirect drive inertial confinement fusion as well as some astrophysics phenomena. In this work, radiation field with a peak temperature of up to 155 eV was generated in a gold cavity heated by four laser beams on the SG-II laser system and was used to irradiate a plastic foam cylinder at one end. The radiatively ablated foam cylinder was then backlighted side-on by x-ray from a laser-irradiated Ti disk. By observing the transmission decrease due to the shock compression of the foam cylinder, the trajectories of shock front were measured, and from the onset of the intense thermal emission from the side of the cylinder, the propagations of the ionization front were also observed on the same shot. The experimental measurements were compared to predictions of the radiation hydrodynamics code Multi-1D and reasonable agreements were found.     

The dependence of the elastic coefficients of ADP on the electric field

Some components of the polarization tensor of the elastic coefficients of ADP are determined. The components of this tensor describe generally the linear dependence of the elastic coefficients of the crystal on the electric field intensity. The necessary experimental data were obtained by measuring the influence of a d.c. electric field on the frequency of longitudinally oscillating resonators in the form of thin bars made from ADP.In conclusion, the authors would like to thank the computing centre of Khartoum University for assistance in processing the measurements.     

冲击压缩下甲烷-空气混合气体状态参数实验研究

 为研究强冲击状态下混入少量空气的甲烷气体的冲击状态参数,利用二级轻气炮加载技术,使加速到5 km/s的钨合金飞片撞击封装有常态下空气混入量依次为零（纯甲烷气体）、1%、5%、10%的甲烷-空气混合气体铝靶。采用六通道瞬态光学高温计记录冲击压缩气体的光辐射历程曲线,得到了相同初始条件下4种不同比例混合气体的冲击状态参数。结果表明,在强冲击压缩下,混合气体的冲击温度随着空气混入比例的增大而增高,冲击波后混合气体存在非平衡辐射过程。采用Saha电离平衡方程,对空气混入量为10%的混合气体的电离度进行了估算。结果表明,常态下空气含量C_air≤10%的甲烷 空气混合气体具有电探针保护能力。     

Operation of the EGA electron gun at high gas pressures

Summary This paper presents experimental results on the effects of increasing gas pressure on electron gun operations. The electron gun used is the gun EGA which has been developed for the TSS-1 mission. Various gases have been used with pressures in the range from 10⁻⁶ to a few times 10⁻³ mbar. The measurements, taken in a vacuum chamber, show a steady increase in the slope of theI–V characteristics of the gun when the pressure is increased, as a consequence of ionization phenomena induced by the electron beam. Further measurements have been taken of the overall current on a target at some distance from the gun itself. An attempt is also made at a theoretical calculation of the effects observed, based on simplified models.     

里德伯Cs原子与CCl4分子的碰撞电离研究

为描述里德伯Ｃｓ原子与ＣＣｌ４分子碰撞电离实验研究中所发现的若干特征,对里德伯原子与分子碰撞的理论模型－准自由电子模型提出两点修正：（１）考虑原子实在碰撞过程中的作用,并且计入量子亏损;（２）考虑碰撞后正负离子的分离几率。利用修正的模型进行了理论计算,得到了与实验测量较为吻合的结果,。同时与其它文献结果进行了比较。     

On parametrization and mixture laws for electron ionization coefficients

In this paper we discuss the methods of calculation of electron ionization coefficient data. First we give a summary of the data for analytic parametrization of ionization coefficients. Such data may be useful for particle and radiation detectors and for studies of breakdown voltages in gas dielectrics. We have applied extended Townsend formula to fit the experimental data for a number of gases and shown that it provides excellent fits for the entire range of E/N where data are available. We also tested the application of the common E/N and the common mean energy combination of data for pure gases to obtain ionization coefficients for mixtures. The standard combination procedure gives poor results in general but the common mean energy procedure provides an extended region of reasonable usefulness. Test calculations were made for Ar–CH₄ mixtures which were found to be the most difficult combination of the selected gases for the application of the mixture law.     

Influence of various gases on the emission of laser generated plume

We report the experimental results on the investigation of the emission charac teristics of the XeCl laser generated plume from the Copper target in variant gas ambient. The dependence of the emission intensities on the gas pressure are strikingly different among these gases. This behavior is attributed to the difference in ionization of the employed gases.     

振荡吸积盘的光变研究

理论研究指出随机振荡吸积盘可能引起活动天体的光变,然而观测数据分析表明光变中除了含有随机噪声外还存在混沌因素.将混沌因素引入到随机振荡吸积盘中,构成"混沌+随机"振荡吸积盘模型.通过分析扰动的相图,直观再现了混沌吸引子的状态.研究结果表明:在随机因素占主导时,光变混乱无序;随机因素与混沌因素相当时,光变上下起伏类似于心电图;混沌因素占主导时,光变具有一定有序性.模拟光变曲线的关联维与观测数据的关联维一致,表明模拟光变曲线与观测结果之间存在内在联系.     

稀有气体在电离区压缩特性研究

采用Saha方程加Debye-Hückel修正简单近似模型,给出了稀有气体He, Ne, Ar, Kr, Xe在电离区的物态方程以及离化度.计算结果与已有实验和理论计算进行比较,验证了模型的可靠性.通过对稀有气体等温和冲击压缩特性理论分析,讨论了其压缩和电离的规律性.论证了在稀有气体He, Ne, Ar, Kr, Xe中,气体Xe具有较好的抗压缩性.     

Townsend coefficient and efficiency of the formation of runaway electrons in neon

Basic ionization and drift properties are simulated for neon by the method of multiparticle dynamics. This calculation revealed that, in neon—in just the same way as in other gases that were studied previously—the Townsend ionization regime is realized even in strong fields if the distance between electrodes is rather large. The dependences of basic ionization and drift properties on the reduced electric-field strength are obtained. The results agree with available experimental data. The escape curve separating the region of efficient electron multiplication from the region in which electrons leave the discharge gap without undergoing multiplication is found for neon. The efficiency of the formation of a runaway-electron beam in helium and neon is simulated.     

Experimental determination of the impulse response of cascade arcsusing a correlation technique

Low-current cascade arcs are one of the simplest configurations for investigating the behaviour of low-temperature arcs. In cascade experiments, arc conditions may be strictly controlled, in which case the results become reproducible, allowing detailed measurements of high resolution to be made. The response of cascade arcs to a small change in the current gives insights into the physical processes taking place in an arc, such as ionization/recombination, diffusion and energy transfer between the electrons and the heavy particles. In this work, a new method of measuring time constants is described, in which correlation techniques utilising a pseudo-random binary sequence are employed to deduce the impulse response of the arc. This type of experiment provides a useful check on the validity of the collision models used for the reaction processes in the arc plasma and it is planned to investigate other gases in the future. In this paper, the experimental arrangement and the data analysis are discussed and experimental results are presented     

Modal Control of Cantilever Beam Using a Gyrostabilizer

In this paper, an experimental model of a horizontal cantilever beam with a rotating/oscillating attached to the shaker for harmonic excitation at the one end and a gyrostabilizer at the other end is built to verify the equations of the Lagrangian model. The primary focus of the study was to investigate the parameters of excitation amplitude, natural frequency, rotating mass (disk mass), and disk speed of gyro that would minimize the amplitude of the beam to identify these effects. Numerical and experimental results indicate that the angular momentum of the gyrostabilizer is the most effective parameter in the reduction of beam displacement.     

Experimental study of magnetic forces on normal and null flux coil arrangements in the inductive levitation system

We report on measurements of lift and drag forces exerted by various small-scale current-carrying superconducting coils, placed near a large rotating disk. In detail we have studied the influences of shape and arrangement of the coils, as well as the influences of plate thickness, velocity and suspension height. In some experiments the drag force was measured under constant load condition. Most of the experimental results are compared with exact calculations based upon the general theory recently given by Miericke and Urankar [1] for infinitely wide but arbitrarily thick sheet tracks. The agreement between experiment and theory was generally good, and we believe that predictions for large-scale systems can be made with confidence.     

Confined direct analysis in real time ion source and its applications in analysis of volatile organic compounds of Citrus limon (lemon) and Allium cepa (onion)

The DART (direct analysis in real time) ion source is a novel atmospheric pressure ionization technique that enables efficient ionization of gases, liquids and solids with high throughput. A major limit to its wider application in the analysis of gases is its poor detection sensitivity caused by open-air sampling. In this study, a confined interface between the DART ion source outlet and mass spectrometer sampling orifice was developed, where the plasma generated by the atmospheric pressure glow discharge collides and ionizes gas-phase molecules in a Tee-shaped flow tube instead of in open air. It leads to significant increase of collision reaction probability between high energy metastable molecules and analytes. The experimental results show that the ionization efficiency was increased at least by two orders of magnitude. This technique was then applied in the real time analysis of volatile organic compounds (VOCs) of Citrus Limon (lemon) and wounded Allium Cepa (onion). The confined DART ion source was proved to be a powerful tool for the studies of plant metabolomics.     

电负性气体的掺入对容性耦合Ar等离子体的影响

本文利用朗缪尔静电探针对掺入了电负性气体O₂, Cl₂, SF₆的由4068 MHz激发的单射频容性耦合Ar等离子体进行了诊断测量. 测量结果表明: 随着电负性气体流量的增加, 电子能量概率分布函数出现了高能峰, 高能峰且有向高能侧漂移的现象; 电负性气体掺入Ar等离子体后显著降低了等离子体的电子密度; 电子温度随着电负性气体流量比的增加而升高. 另外, 本文还测量了掺入三种电负性气体后在不同流量比下的混合气体等离子体的电负度α . 对实验现象进行了初步的解释. 关键词： 电负性等离子体 电子密度 电子温度 电负度     

"RLC串联电路暂态过程的研究"实验中电容系统误差的测量与修正

在"RLC串联电路暂态过程的研究"实验中,由于示波器两测量探头间电容和电容箱"零电容"的存在,导致阻尼振荡周期的理论计算值与实验值之间可能产生非常大的误差.本文通过测量上述两类电容,对周期的理论计算值进行了修正,使之与实验值的相对误差明显减小.     

Positron impact ionization of molecular oxygen

We have carried out distorted wave calculations of positron ionization of molecular oxygen in order to compare with experimental measurements. In this work the oxygen molecule was represented by a Gaussian wave function which includes contributions from alpha and beta electrons. We find that our CPE model produces results which are in good agreement with the combined measurements for total ionization and positronium formation.