A new brain stimulation method: Noninvasive transcranial magneto–acoustical stimulation

We investigate transcranial magneto–acoustical stimulation(TMAS) for noninvasive brain neuromodulation in vivo.TMAS as a novel technique uses an ultrasound wave to induce an electric current in the brain tissue in the static magnetic field. It has the advantage of high spatial resolution and penetration depth. The mechanism of TMAS onto a neuron is analyzed by combining the TMAS principle and Hodgkin–Huxley neuron model. The anesthetized rats are stimulated by TMAS, resulting in the local field potentials which are recorded and analyzed. The simulation results show that TMAS can induce neuronal action potential. The experimental results indicate that TMAS can not only increase the amplitude of local field potentials but also enhance the effect of focused ultrasound stimulation on the neuromodulation. In summary, TMAS can accomplish brain neuromodulation, suggesting a potentially powerful noninvasive stimulation method to interfere with brain rhythms for diagnostic and therapeutic purposes.     

W波段同轴静磁波荡器自由电子激光优化设计

基于同轴静磁波荡器的自由电子激光主要利用环形电子束与同轴TE模式的相互作用产生电磁辐射,是一种重要的毫米波辐射源。分析了同轴结构作用区内外半径、工作模式、电子束电压、波荡器周期等参数对辐射频率的影响规律,研究了电子束平均半径的选取原则和束波互作用腔的设计方法,设计了辐射频率在W波段的基于同轴静磁波荡器的自由电子激光,并进行了粒子模拟,在电子束电压为720 kV,电子束流为1 kA的情况下,获得了频率107 GHz、辐射功率35 MW的TE01模,束波转换效率为4.9%,束波作用腔总长度小于200 mm,同轴静磁波荡器的磁场幅度0.34 T。     

由已知相应的静电静磁问题求电动力学问题的解

一.引言 在电磁现象问题中,比拟的方法是最常用的方法之一。在这方面,新提出的一个问题是从已知的静电或静磁问题出发,即可求相应的电动力学问题;应用这个方法,可以解决一些较复杂的问题。但是,现有的处理这个问题的方法还欠缺普遍性。本文的目的就是要使这个方法更具普遍性,就是说要使这个方法包括更大的范围。     

时间反演聚焦经颅磁声电刺激仿真与实验研究*

无创脑神经调控技术是生物医学领域的研究热点,经颅磁声电刺激是利用静磁场和声场的耦合而产生的感应电场作用于神经组织,对大脑的目标位置进行刺激和调控的一项技术。颅骨的存在使超声在传播过程中发生相位畸变和幅值衰减,聚焦区域偏离,难以实现精准聚焦。该文基于时间反演法,模拟颅内点声源发射脉冲以及超声传播过程,计算各个阵元接收到的时间差,按照后到先发的原则发射脉冲进行聚焦刺激。与传统相控阵聚焦相比,焦点偏移现象基本得到解决,焦域横向、纵向分辨率均有所提高,提高了声束聚焦精度和感应电场峰值。通过搭建实验平台,将两种聚焦方法所测得的声场归一化处理,验证了时间反演法能补偿焦点偏移,并通过实验证实了超声换能器声场和产生感应电场分布存在较高的一致性。基于真实颅脑结构的虚拟点源时间反演聚焦可以实现无创、精准、灵活的经颅磁声电刺激,有助于推动精准神经调控技术的发展。     

一种油介质开关振荡器研究

为了产生较高中心频率宽带高功率微波, 对一种填充变压器油作为绝缘介质的1/4波长开关振荡器进行了研究。首先对这种开关振荡器特性阻抗分布进行了分析;然后利用静态电场仿真结果和变压器油击穿实验数据, 分析了该振荡器的耐压能力;在此基础上, 利用CST软件对其瞬态工作特性进行了仿真, 考察了开关间隙击穿位置和间隙电压下降时间对产生阻尼正弦信号的峰值和频率的影响;最后介绍了该振荡器以一种方向系数为3的短螺旋天线作为辐射天线的实验测试结果, 结果表明, 该开关振荡器充电电压上升时间为15 ns时, 耐压达到-322 kV, 产生宽带高功率微波中心频率约360 MHz, 3 dB带宽约22%, 辐射因子170 kV。     

一种脉宽可调的陡化后沿高压脉冲发生器

磁开关因其独特的饱和导通机制而在脉冲功率技术领域应用广泛,利用磁开关饱和前后电感差异大的特点,可以将其用作撬断开关来陡化脉冲后沿。利用金属氧化物压敏电阻的稳压特性,能在负载上得到具有一定平顶的脉冲准方波,进而可通过改变磁开关的伏秒积来进行脉宽调整。提出一种脉宽可调的高压脉冲发生器技术方案,利用压敏电阻产生高压脉冲准方波,用磁开关作为撬断开关来陡化脉冲后沿,并通过改变磁开关复位电流的大小来控制磁开关的复位深度,进而实现脉宽可调。首先进行了理论分析及软件模拟研究,然后基于模拟结果开展了初步实验研究。初步实验得到的负载电压波形后沿小于30 ns,脉宽可调范围大约30%,验证了磁开关的后沿陡化效果以及用于脉宽调整的可行性。     

Binary black hole in a double magnetic monopole field

Ambient magnetic fields are thought to play a critical role in black hole jet formation. Furthermore, dual electromagnetic signals could be produced during the inspiral and merger of binary black hole systems. In this paper, we derive the exact solution for the electromagnetic field occurring when a static, axisymmetric binary black hole system is placed in the field of two magnetic or electric monopoles. As a by-product of this derivation, we also find the exact solution of the binary black hole configuration in a magnetic or electric dipole field. The presence of conical singularities in the static black hole binaries represent the gravitational attraction between the black holes that also drag the external two monopole field. We show that these off-balance configurations generate no energy outflows.     

Motion of an electron from a point source in parallel electric and magnetic fields

Negative ions undergoing near-threshold photodetachment in a weak laser field provide an almost pointlike, isotropic source of low-energy electrons. External fields exert forces on the emitted coherent electron wave and direct its motion. Here, we examine the spatial distribution of photodetached electrons in uniform, parallel electric and magnetic fields. The interplay of the electric and magnetic forces leads to a surprising intricate shape of the refracted electron wave, and multiple interfering trajectories generate complex fringe patterns in the matter wave. The exact quantum solution is best understood in terms of the classical electron motion.     

A very simple explanation of the effects of a.c. fields on the critical currents in type II superconductors

The electric field induced in a type II superconductor foil by an a.c. magnetic field is rectified by the non-linear static voltage current characteristic of the superconductor when there is a static current, so that the static critical current seems to decrease.     

Hot carrier drift induced EM wave instability in a magnetoactive solid state plasma

The effect of a strong longitudinal static electric field on the propagation and instability of transverse circularly polarised EM waves (left and right handed) in the presence of a static magnetic field along the direction of propagation in an InSb plasma has been studied under hot carrier conditions by a phenomenological approach. The results show the possibility of existence of wave instabilities for a wide range of system parameters. The growth rate decreases with the heating d.c. electric field and increases slightly with the static magnetic field.     

强电磁场对某型无线电引信安全性的影响

 为了研究强雷电电磁场（LEMP）对没有保护状态下无线电引信性能的影响，对LEMP进行了模拟。利用MARX发生器向宽带横电磁波（GTEM）室注入雷电电压波，在GTEM室内产生模拟的LEMP电场；利用浪涌发生器向亥姆霍兹线圈注入雷电电流波，在线圈内产生模拟的LEMP磁场。根据某型无线电引信可能遭遇的雷电电磁环境，对处于勤务处理状态的该型无线电引信进行了辐照效应实验，测试辐照前后引信的性能指标，对其差异进行比较，分析了LEMP对该型无线电引信性能指标的影响。结果表明，在强的雷电电磁场环境下LEMP电场会损坏该型无线电引信的高频组件，使其不能产生探测信号，导致引信不能正常发火；LEMP电场还会损坏检波电路，使检波直流电压不能达到正常指标，从而导致引信也不能正常发火；LEMP磁场对该型引信性能基本没有影响。     

经颅超声刺激人脑海马的数值仿真研究

颞叶癫痫、阿尔兹海默症等神经系统疾病与大脑海马的异常放电相关。近年兴起的经颅超声刺激治疗脑疾病具有无创、能深入脑组织和可调控海马神经元放电的特点。该文基于128阵元相控阵换能器、人头颅CT数据、水体建立超声经颅刺激海马数值仿真模型,数值仿真优选适应于海马刺激的换能器结构参数,并结合电容模型,探究超声声学参数对海马神经元放电的影响。结果表明曲率半径90 mm、开口半径56 mm、阵元半径2.0 mm、频率0.9 MHz的128阵元相控超声换能器可形成适应于刺激人脑海马大小的焦域;且在超声频率为0.9 MHz时对海马神经元放电有较好的抑制作用;在较小的占空比和较低的空间峰值时间平均声强条件下对神经元放电有较好的抑制作用。     

Study of superradiance based on a low-voltagebackward wave oscillator

According to the small size requirement for wide-band high-power microwave radiation, a superradiance backward wave oscillator (BWO) is proposed to generate such high-power microwave radiation with a low voltage (～20 kV) pulse power supply and low guiding magnet field (～0.1 T). In order to get a high-efficiency C-band superradiance BWO with a low beam voltage and a low guiding magnet field, the mechanism of superradiance in a BWO is explored in particle-in-cell simulation. With the oversized structure, the simulation shows that a microwave power of 405 kW with a frequency of 5.6 GHz and a spectrum width of 500 MHz can be obtained with a voltage of 23 kV and magnetic field of 0.1 T.     

Calculation of electric fields induced by body and head motion in high-field MRI

In modern magnetic resonance imaging (MRI), patients are exposed to strong, nonuniform static magnetic fields outside the central imaging region, in which the movement of the body may be able to induce electric currents in tissues which could be possibly harmful. This paper presents theoretical investigations into the spatial distribution of induced electric fields and currents in the patient when moving into the MRI scanner and also for head motion at various positions in the magnet. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic, full-body, male model. 3D field profiles from an actively shielded 4T magnet system are used and the body model projected through the field profile with a range of velocities. The simulation shows that it possible to induce electric fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The results are extrapolated to very high field strengths and tabulated data shows the expected induced currents and fields with both movement velocity and field strength.     

Semi-analytical study of meniscus oscillation in an electrospray under AC voltage

In this paper, the oscillation of the meniscus in an electrospray with AC voltage is semi-analytically investigated. The electric field is achieved using the charge simulation method and the mutual interaction between the electric field and hydrodynamics is iteratively explored to investigate the oscillation. After determining the static meniscus shape on a nozzle, the meniscus oscillation is simulated under sinusoidal AC voltage. The semi-analytic simulation can offer the oscillating meniscus shapes and contours of the electric field outside the meniscus according to the applied frequency. Furthermore, the velocity profile of the liquid inside the meniscus is achieved.     

A new analytical model for the surface electric field distribution and breakdown voltage of the SOI trench LDMOS

A new analytical model for the surface electric field distribution and breakdown voltage of the silicon on insulator (SOI) trench lateral double-diffused metal-oxide-semiconductor (LDMOS) is presented. Based on the two-dimensional Laplace solution and Poisson solution, the model considers the influence of structure parameters such as the doping concentration of the drift region, and the depth and width of the trench on the surface electric field. Further, a simple analytical expression of the breakdown voltage is obtained, which offers an effective way to gain an optimal high voltage. All the analytical results are in good agreement with the simulation results.     

同轴结构中压力波法测量空间电荷分布的物理模型研究

从压力波法测量平板样本中空间电荷分布的理论分析出发,提出了对于同轴结构的固体电介质中空间电荷分布的压力波法测量的物理模型,并得出了测量电流的解析的数学表达式.依据泊松方程,考虑样品内的电场强度、介电常数和空间电荷密度随外界声波扰动而发生变化,将圆柱试样受到的压力波的影响分解成样品形变和质点位移两部分,进而得到了以同轴结构中空间电荷分布的压力波法测量的电压与电流表达式.测量电流的数学表达式表明,对于同轴结构,压力波法的测量电流信号不像平板结构中那样基本正比于空间电荷分布,而是应该做进一步的修正处理.     

基于麦克斯韦方程的交变电流长螺线管磁场

为了分析方波驱动长螺线管内磁场畸变机理,利用麦克斯韦方程研究了螺线管内外的磁场分布情况。首先,利用麦克斯韦方程,分别建立了正弦波驱动螺线管内外电场、磁场模型,并结合安培环路定律和电磁感应定律选取了合适的边界条件,得到了正弦波驱动长螺线管的磁场分布;其次,通过傅里叶变换将方波信号变换为多个正弦信号叠加的形式,从而得到了方波驱动长螺线管磁场分布;最后,通过仿真试验重点分析了方波驱动信号频率对磁场的影响,并得出结论:方波驱动长螺线管磁场波形会失真、畸变,驱动信号频率较低、距离螺线管轴线距离较近处,磁场的方波特性较好。     

Magnetoelectric properties in A-type antiferromagnet in an electric field

Mean-field theory based on Heisenberg model is used to investigate the magnetic and magnetoelectric properties at a finite temperature in the A-type antiferromagnetic lattice, in which we consider that an applied electric field can generate a bulk magnetic moment. We have calculated the magnetic moment, magnetic susceptibility and magnetoelectric susceptibility as a function of temperature for A-type antiferromagnetic system. It is demonstrated that an applied electric field together with the coupling parameter has an effect on the magnetic ordering behavior. Our results are almost consistent with those of spin-wave theory within the range of low temperature.     

Calculation of electric and magnetic induced fields in humans subjected to electric power lines

In this work, analysis of the human body exposed to high voltage electric and magnetic fields is presented. The distribution of the electric field is obtained by using Laplace's equation. This relates the surface charge induced on the body to the potential in a reciprocal Laplace problem, which is then calculated by charge simulation method coupled with genetic algorithms to determine the appropriate arrangement of simulating charges inside the human body. The magnetic field intensity along the vertical center line of the human is calculated. Exposure to external electric and magnetic fields at power frequency induces electric field, magnetic field and currents inside the human body. The presented model for simulating electric and magnetic fields are a three dimensional field problem and introduced different types of charges to simulate the different elementary geometrical shapes of human body. The particular strength of the charge simulation method in this application is its ability to allow a detailed representation of the shape and posture of the human body. The results have been assessed through comparison induced current, electric field, magnetic field and there distribution over the body surface, as estimated in other experimental and computational work.