Suppression of stimulated Brillouin and Raman scatterings using an alternating frequency laser and transverse magnetic fields

A novel scheme to suppress both stimulated Brillouin scattering(SBS) and stimulated Raman scattering(SRS) by combining an alternating frequency(AF) laser and a transverse magnetic field is proposed. The AF laser allows the laser frequency to change discretely and alternately over time. The suppression of SBS is significant as long as the AF difference is greater than the linear growth rate of SBS or the alternating time of the laser frequency is shorter than the linear growth time of SBS. However, the AF laser proves ineffective in suppressing SRS, which usually has a much higher linear growth rate than SBS. To remedy that, a transverse magnetic field is included to suppress the SRS instability. The electrons trapped in the electron plasma waves(EPWs) of SRS can be accelerated by the surfatron mechanism in a transverse magnetic field and eventually detrapped. While continuously extracting energy from EPWs, the EPWs are dissipated and the kinetic inflation of SRS is suppressed. The one-dimensional particle-in-cell simulation results show that both SBS and SRS can be effectively suppressed by combining the AF laser with a transverse magnetic field with tens of Tesla. The total reflectivity can be dramatically reduced by more than one order of magnitude. These results provide a potential reference for controlling SBS and SRS under the related parameters of inertial confinement fusion.

The Dynamic Volt-Ampere Characteristics of a Vacuum Arc at Intermediate-Frequency Under a Transverse Magnetic Field

In this study, the changes of a vacuum arc＇s appearance were observed and the volt-ampere characteristics of the vacuum arc at intermediate frequency were analyzed under a transverse magnetic field （TMF）. The TMF and phase shift time were calculated by using the TMF contact model and the large phase shift of the magnetic field at a higher frequency was conductive to the dispersion process of residual plasma. The arc velocity was higher at 800 Hz than at 400 Hz. It can be inferred that TMF will encourage arc movement at 800 Hz. Moreover, the arc movement has an impact on the arc voltage. Because of the increasing length of the arc column with a high arc velocity, the elongated arc causes the arc voltage to increase. Specifically, the volt-ampere characteristics of the vacuum arc are divided into three stages in this paper. The higher the frequency, the greater the initial rate of rise in the arc voltage and the larger the area surrounded by arc volt-ampere characteristics. The correlations between the arc voltage and the amplitude and frequency of the current are also presented.

Motion and Splitting of Vacuum Arc Column in Transverse Magnetic Field Contacts at Intermediate—Frequency

Arc motion and splitting of vacuum arc at intermediate frequency（400-800 Hz） were investigated under transverse magnetic field（TMF）.The experiment was performed on cup-type TMF contacts with contact diameter of 40 mm and a contact gap of 4 mm in a single-frequency circuit.With high-speed photography we characterized the arc appearance at different arc currents from 3.3 kA-rms to 10 kA-rms at intermediate frequencies.As arc current increases from3.3 kA-rms to 10 kA-rms the arc appearance changes obviously.When current value is 3.3 kArms（current frequency 400-800 Hz）,there is almost no splitting arc;when the current exceeds5 kA-rms（current frequency 400-800 Hz）,the arc rotates at a speed above 20 m/s,accompanied by an observable splitting arc.The splitting arc could be observed at different frequencies and the arc-voltage had no noises when splitting occurred.The motion direction and the velocity of arc column were studied.Finally,the formation of a split arc was discussed.

AN APPROXIMATE SOLUTION OF EIGEN－FREQUENCIES OF TRANSVERSE VIBRATION OF RECTANGULAR PLATES WITH ELASTICAL RESTRAINTS

This poper presents an approximate solution for calculating eigen-frequencies of transverse vibration of rectangular plates elastically restrained against rotation along edges. The formulae are not only very simple and easily programmed but also have high accuracy. Finally, some numerical results are given and compared with other results obtained.

Plane Vibrations in a Transversely Isotropic Infinite Hollow Cylinder Under Effect of the Rotation and Magnetic Field

The aim of this paper is to study the effects of rotation and magnetic field on the plane vibrations in a transversely isotropic material of an infinite hollow cylinder.The natural frequency of the plane vibrations in the case of harmonic vibrations has been obtained.The natural frequencies are calculated numerically and the effects of rotation and magnetic field are discussed.The numerical results obtained have been illustrated graphically to understand the behavior of frequency equation with different values of frequencyωunder effects the rotation and magnetic field.Comparison was made with the results obtained in the presence and absence of the rotation and magnetic field.The results indicate that the effect of rotation and magnetic field are very pronounced.

Nonlinear free vibration of reticulated shallow spherical shells taking into account transverse shear deformation

This paper deals with nonlinear free vibration of reticulated shallow spherical shells taking into account the effect of transverse shear deformation. The shell is formed by beam members placed in two orthogonal directions. The nondimensional fundamental governing equations in terms of the deflection, rotational angle, and force function are presented, and the solution for the nonlinear free frequency is derived by using the asymptotic iteration method. The asymptotic solution can be used readily to perform the parameter analysis of such space structures with numerous geometrical and material parameters. Numerical examples are given to illustrate the characteristic amplitudefrequency relation and softening and hardening nonlinear behaviors as well as the effect of transverse shear on the linear and nonlinear frequencies of reticulated shells and plates.

基于分数阶Zener模型的VTI黏弹性介质频变Q效应数值模拟

黏弹性是地球介质的基本属性.地震波在黏弹性介质中传播会发生振幅衰减和速度频散,导致能量和走时出现变化,影响地震资料的准确成像及解释.因此,研究地震波在黏弹性介质中的传播规律具有重要意义.当前,主要利用常Q(不随频率变化)模型(CQM)表征地震波在地球介质中的传播规律,与高温、高压或含流体介质中观测到的Q随频率变化规律不符.基于分数阶Zener模型(FZM),通过改变分数阶大小,本文研究了Q的频变特性随分数阶的变化规律.考虑速度和Q各向异性,推导了VTI介质的FZM黏弹性波方程,提出了VTI介质的频变Q效应数值模拟方法.数值算例表明:当Q随频率变化较小时,FZM与CQM方法的结果相似;当Q随频率变化剧烈时,FZM与CQM方法的模拟结果差异较大.

低频小型化兰姆波谐振器的研究

针对低频声学器件的小型化、大带宽问题,该文基于Y切-铌酸锂压电薄膜激发了反对称A0兰姆波模式,并采用有限元法对A0模式进行了结构设计与理论分析。研究了铌酸锂切型、电极材料、金属化比及膜层厚度变化对声速和机电耦合系数等性能的影响。结果表明,特定结构下,与常规声表面波相比,A0模式兰姆波声速较小,机电耦合系数较大,有利于低频声学滤波器小型化设计。通过研究不同结构对谐振器A0模式兰姆波的横向模态的影响,实现了杂波抑制。

浅议核磁共振测井仪器理论及应用

核磁共振测井仪器可以实现自旋核的极化、把体磁化强度M_(0)扳倒在与B_(0)垂直的平面上、自旋回波探测;回波串的衰减率反映了地层的孔径尺寸信息和流体类型信息,核磁共振技术在油田测井中得到了广泛应用,在油田储层评价中具有重要价值。

单轴压缩下不同饱和度红砂岩横波特征研究

为了研究饱和度对岩石超声波横波传播规律影响,以红砂岩为研究对象,开展不同饱和度下入射频率分别为50、100、200 kHz的超声波横波传播实验,分析饱和度及入射频率对横波声学参数变化规律。结果表明:干燥至饱和状态,横波波速随饱和度增大呈非线性下降趋势,幅值衰减系数随饱和度增大呈非线性递增趋势,均满足三次函数关系,属于高度相关;整体分析发现横波波速受饱和度变化影响大于幅值衰减系数,幅值衰减系数拟合参数能较好反映岩样物理量的变化;基于敏感性及决定系数的综合考虑,建议今后采用100 kHz作为含水砂岩超声波横波测试优势频率。研究成果可为不同饱和度下岩石超声波测试技术发展提供参考。

一种高频宽带横向叠堆型换能器的研制

针对声纳系统对换能器提出的高频宽带要求,研制了高频宽带横向叠堆型换能器。该换能器通过降低品质因数和多模耦合等方式,有效地拓展了换能器的工作带宽。利用有限元分析软件对单层1-3-2型复合材料及横向叠堆型复合材料进行仿真,得到单层1-3-2型复合材料的谐振频率和带宽的变化规律,并根据仿真结果优化横向叠堆型复合材料的尺寸,使得该横向叠堆型复合材料的工作频率处于高频范围。通过特殊的制备工艺法,制作了横向叠堆型换能器。经测试,该换能器的发送电压响应最大可达158 dB,-3 dB带宽达到70 kHz,谐振频率处的-3 dB波束开角为11°。该横向叠堆型换能器实现了高频宽带的目标。

基于FDTD的局部放电射频传感器特性研究

射频(RF)检测局部放电(PD)可以实现非接触式测量,因而具有较好的应用前景。以小环天线为例,对磁场天线作为局部放电射频传感器时其灵敏度的确定方法和影响灵敏度的因素开展了研究。首先,基于时域有限差分(FDTD)法建立了吉赫兹横电磁波(GTEM)小室的仿真模型,通过实验和仿真对比表明了仿真模型的有效性。在此基础上,考虑了天线结构、尺寸等因素对灵敏度的影响,确定了灵敏度随天线直径、垂直导体、线圈匝数的变化规律。本文的研究为进一步考察不同磁场天线传感器的特性,并对传感器特性进行优化,设计实现能够满足具体电力设备局部放电测试需求的新型射频磁场传感器奠定了技术基础。

C7横突入路星状神经节阻滞的超声解剖研究及临床应用

目的研究C7横突毗邻高频超声解剖特征及其在星状神经节阻滞(SGB)中的临床应用价值。方法应用高频超声获得52例(共104侧)健康成人双侧C7横突平面超声解剖声像图特征后存储,供术者学习并正确标记各组织结构。选取50例接受超声引导下SGB治疗的患者,将其分为BC7组(学习前25例)和AC7组(学习后25例)。记录2组患者手术时间、SGB成功率、调整针尖次数、麻醉药物剂量及不良反应情况。结果C7平面中观察到的主要肌肉是颈长肌和前斜角肌,描述了迷走神经位于颈动脉鞘内、胸膜位于锁骨下动脉后方、喉返神经位于甲状腺下动脉分支附近的超声解剖关系,星状神经节在椎前筋膜深面,颈长肌外侧、椎动静脉及前斜角肌内侧区域可见扁平状低回声结构,并发出数根低回声神经束声像图特征。2组患者均顺利完成超声引导下SGB,并出现霍纳综合征,SGB成功率为100%。BC7组患者手术时间[(5.36±1.11)min]长于AC7组[(3.08±0.86)min],BC7组患者调整针尖次数[(4.20±1.00)次]多于AC7组[(2.24±0.87)次],BC7组患者麻醉药物剂量[(1.82±0.28)mL]大于AC7组[(1.64±0.22)mL],差异均有统计学意义(P<0.05)。2组患者不良反应发生率比较,差异无统计学意义(P>0.05)。结论经C7横突入路毗邻结构超声学习后SGB穿刺安全性高,简单易操作。

Low frequency ultrasonic multi-mode Lamb wave method for characterizing the ultra-thin transversely isotropic laminate composite: Theory and experiment

A low-frequency multi-mode ultrasonic Lamb wave method suitable for character- izing the thickness, the density and the elastic constants of the ultra-thin transversely isotropic laminate composite is presented. The 'ultra-thin' here means that the thickness of the plate is much less than the wavelength of the ultrasonic wave so that the echoes from the front and back faces of the plate can't be separated in the time domain. The dispersion equations for the low frequency ultrasonic Lamb waves with the propagation directions parallel and vertical to the fiber direction are derived. In conjunction with the least square algorithm method, the secant algorithm is used to estimate the parameters of the ultra-thin fiber-reinforced composite layer. The evaluation errors and the sensitivity of the method to different paramters of the thin composite are analyzed. The technique has been used to characterize the ultra-thin grass fiber reinforced PES composite with thickness down to ten percents of the ultrasonic wavelength. It is observed that the agreement between the nominal and the estimation values is reasonably good.

Frequency difference lock-in phenomenon's weakening by transverse magnetic field in Y-shaped cavity dual-frequency laser

We report experimental progress in weakening the frequency difference lock-in phenomenon in a Y-shaped cavity dual-frequency laser. A cube coil pair is chosen to provide a uniform magnetic field for tunability and uniformity of magnetic field strength. When the transverse magnetic field intensity is 9 m T, the frequency difference lock-in phenomenon is evidently weakened and the frequency difference can be continuously tuned in the range of0.12 MHz to 1.15 GHz. Moreover, the relationship between the minimal frequency difference and magnetic field intensity are investigated and discussed. Then a Y-shaped cavity dual-frequency laser is expected to be utilized as an optimum light source for heterodyne interferometric sensing and precise laser measurement.

磁悬浮轴承横向磁通传感器设计与分析

横向磁通传感器因结构紧凑和高检测精度在磁悬浮轴承系统中具有广阔的应用前景。随磁悬浮轴承技术的发展,对横向磁通传感器的检测性能提出了更高要求。为进一步提升横向磁通传感器的性能,满足磁悬浮转子高精度位移监测需求,本文针对灵敏度指标对传感器进行设计与分析。通过建立传感器的数学模型和电磁场有限元分析,研究了激励频率和线圈参数之间的关系。对传感器线圈匝数与灵敏度的相关性进行了数值研究,从检测转子的角度分析了趋肤效应对传感器灵敏度的影响。设计了传感器信号处理电路实现由位移信号到电压信号的转换,并搭建实验平台对传感器的输出特性进行测量。实验结果表明,当灵敏度为20 mV/μm、检测范围为±500μm时,传感器的线性度为0.69%,且具有良好的动态特性,适用于磁悬浮轴承系统的转子高精度径向位置检测。

核电厂地震加速度计现场标定振动台的研制

针对现有的核电厂地震加速度计的标定方法无法同时涵盖规范规定参数检定要求,也无法实现核电现场就地标定检测的问题,设计了一种基于柔性金属弹片悬挂结构的便携式振动台,分析了振动台的机械结构,给出了振动台动力学方程;依照实际工程应用需求,选取了合适的参数和材料,进行了振动台的三维建模和有限元分析,制造了振动台样机,并对样机进行了测试,测试结果表明,振动台固有频率在1Hz以下,量程可达到1g,横轴灵敏度小于1%,净重在40kg以内,具备便携式特征,满足核电厂地震加速度计的现场标定参数检测的要求。

小鼠主动脉弓狭窄术左室肥厚模型的评价研究

目的探讨高频超声多普勒检测主动脉弓狭窄术(TAC)主动脉弓血流速度时间积分,早期评估左室肥厚(LVH)模型建立的可靠性。方法采用TAC小鼠LVH模型,多普勒超声检查主动脉的血液流速度时间积分,结合心动超声影像,左室质量指数,病理组织学方法检查,评价心脏功能。结果小鼠行TAC术后3 d,TAC组主动脉弓处血流速度峰值较假手术(Sham)组明显加快(P<0.01),结扎处血管直径TAC组较Sham组狭窄,具有统计学意义(P<0.01),提示TAC手术早期构建LVH模型成功。小鼠行TAC术后2周主动脉结扎处血流速度积分与左心室射血分数(LVEF)成负相关(r=0.901,P<0.01)。小鼠LVH模型TAC术后2周、4周后,心脏超声检查显示,收缩末期左室室间隔厚度,LVEF,短轴缩短率与Sham组有明显差异(P<0.01)。TAC术后2周、4周后小鼠心脏体积明显较Sham组增大,病理组织学检查,心肌组织发生不同程度的变化。结论应用高频多普勒超声检测TAC术后小鼠主动脉弓狭窄处的血流速度时间积分,可以评估LVH模型建立的可靠性。

基于连续横向枝节的波束扫描天线研究进展

近年来空天地技术飞速发展,大量不同轨道卫星与地面各种移动载体的实时高质量无线通信要求天线具有波束扫描性能,且易共形、重量轻、成本低、功耗小。与反射面和相控阵天线相比,基于平行板波导(Parallel Plate Waveguide,PPW)的连续横向枝节(Continuous Transverse Stub,CTS)天线较易于满足诸多要求。文章首先阐述了CTS天线及其扫描工作原理;然后对机扫和频扫天线的研究进展分别做了分析和总结,实现俯仰面机扫的方法有三种:移动馈源、可变倾角CTS天线和旋转加载在CTS天线上方的相移超表面。同时,整体旋转天线阵列还可进行方位面扫描,即实现二维机械扫描。最后指出CTS波束扫描天线所面临的问题、挑战和下一步研究方向。

基于横向扼流法的宽带带状线小室设计

为扩展带状线小室工作带宽,基于标准IEC61967-8对EM601-6型带状线小室进行改进,提出芯板开缝的方法扼止横向电流,在三维电磁仿真软件CST中建立了带状线小室的三维仿真模型,并计算出改进后带状线小室的S参数和电压驻波比(voltage standing wave ratio, VSWR).计算结果表明,芯板开缝的带状线小室的上限可用频率由6 GHz提高到了7 GHz.根据仿真结果加工制作的实物,虽然S21和S11比仿真结果差,但是在标准要求的范围内工作频段可由0~6 GHz扩展到0~7 GHz.