超强激光与等离子体平面靶相互作用中的自生磁场

为了解释超强激光与等离子体相互作用时产生的自生磁场及其产生机制,从动力论出发,用理论分析和数值模拟法研究了强激光打平面薄靶时,由温度梯度和密度梯度的非共线性所决定的自生磁场,得到了自生磁场空间分布的时间演化关系。研究结果表明,当激光入射等离子体时,由于不平行的密度和温度梯度,在等离子体表面会出现自生磁场。这种磁场明显地影响激光吸收和各种输送过程。     

强场物理中自生磁场的实验方法研究

激光和固体靶之间的相互作用会产生较强的自生磁场,该磁场对等离子体尾流加速、动力学行为、能量吸收等产生了重要的影响。为了得到强场物理中自生磁场的大小和空间分布,通过OMA光学多道分析谱仪和CCD摄像机进行测量,将相机镜头当作空间分辨,用于测量谐波的光谱,并且对散射光测量系统进行研究。文章首先介绍了自生磁场的产生机制,然后说明了实验内容及方法,最后给出实验结果及分析过程,产生的自生磁场可达到60～70T量级,且在一维空间上分布不均匀,接近于靶法线方向的磁场较强,呈现环形分布状态。该结果为强场物理中自生磁场的研究提供了一定的参考价值。     

激光核聚变中自生磁场与热输运的粒子模拟

利用相对论电磁粒子模拟程序研究了超强激光与等离子体相互作用过程中产生的自生磁场和电子热输运特性。讨论了自生磁场产生机制和非线性饱和过程。给出了自生磁场的线性增长率和各向异性参数之间的函数关系,用Spitzer-Harm理论分析了电子热传导中能量的运输情况,观察到由激光的非等方加热引起的电子纵向加热现象。细致研究这些过程对更好的理解快点火物理中自生磁场的产生、超热电子热输运等过程有重要意义。     

激光等离子体中密度孤波和自生磁场的数值模拟

为了深入理解激光与等离子体相互作用时产生的密度孤波和自生磁场的形成机制,从动力论出发,数值模拟了从波-波、波-粒相互作用出发的轴对称柱坐标下的密度扰动非线性控制方程,得到了密度孤波和自生磁场的形成和演化过程。数值结果表明,强度为4×1014W/cm2的激光打靶时形成的孤波最大密度扰动率达到82%,并产生30T的自生磁场,与实验测量结果相符合,为密度孤波和自生磁场的形成提供了理论依据。     

超强激光与等离子体作用产生自生磁场的新机制

应用多光子非线性Compton散射模型和粒子模拟程序,研究了Compton散射对等离子体中自生磁场的影响,提出了将超强入射激光脉冲和Compton散射光形成的耦合光作为形成自生磁场的新热电机制,给出了自生磁场的修正方程,并进行了仿真实验验证。结果表明:入射激光的ω0t350时(ω0和t分别是入射激光的圆频率和脉冲宽度),电子密度发生明显变化,这是由于散射使质子与电子碰撞频率增大,能量交换加快,超热电子获得更高能量,从而带动更多质子加速的缘故。自生磁场空间分布范围和强度增大,这是由于散射使等离子体碰撞频率及电子的温度和密度梯度增大的缘故。自生磁场先较快增加后指数增长,到达正饱和后缓慢减小,最后达到负饱和状态,形成对称结构。自生磁场最大值为5.3×103T,比散射前强。这是由于散射使更多粒子发生了二、三阶电离,更多超热电子形成了更强的电流,最后趋于稳定的缘故。强自生磁场处温度梯度最大,且沿y方向,电子有效温度为0.61 MeV,比散射前更接近理论结果0.63 MeV,这是散射对电子温度贡献的结果。     

相对论性激光等离子体中调制不稳定性

为了研究相对论性强激光与等离子体相互作用时自生磁场的调制不稳定性,从一组考虑了横等离激元波-波、波-粒相互作用和电子相对论效应的非线性动力学控制方程出发,通过线性分析得到了横扰动的色散方程。结果表明,自生磁场由于调制不稳定性,将会坍塌形成小尺度的局域结构。选取适当的参量,计算得到的自生磁场的特征尺度与已有理论模型的结果一致。     

超强激光与等离子体相互作用中的自生磁场及三维模拟

用3维粒子模拟程序对超热电子在等离子体靶表面中向前传输时所激发的电流密度,电场和磁场的发展过程进行模拟研究。数值模拟表明,在线性强激光作用下,由于电子初始时刻的无规则热运动,在等离子体临界表面上激发不稳定性,而不稳定性随时间发展和激光功率的进一步深入到等离子体内部,最终使等离子体表面处激发饱和自生磁场,饱和自生磁场对激光有质动力推开电子时所形成的高能电子运动产生抑制作用.通过该研究寻找相对论效应条件下的自生磁场,并为进一步开拓其在高能离子束的应用提供理论和技术上的指导。     

激光功率密度对自生磁场和电子热传导的影响

为了理解超强激光与等离子体相互作用中产生的自生磁场形成机制和电子热传导特性,采用相对论电磁粒子模拟程序,估算了不同激光功率密度下,在等离子体表面所形成的电磁不稳定性产生的自生磁场大小和空间分布,得到了超热电子和经典Spitzer-Harm理论描述的电子热流随激光功率密度的演化情形.结果表明,非Maxwell速度分布的等离子体,由于电子初始时刻的无规则热运动,在等离子体上激发电磁不稳定性,而不稳定性激发的强电磁场使电子束在非常短的距离内沉积能量,同时对在激光有质动力推开电子时形成的超热电子能量输运产生抑制作用.这一研究结果对更好理解惯性约束核聚变快点火过程中自生磁场的产生、电子热传导等方面有帮助的.     

飞秒激光等离子体中自生磁场实验研究

皮秒和飞秒激光技术的迅速发展,为激光等离子体物理开辟了新的理论和实验研究领域．自生磁场是激光与等离子体相互作用中很多物理过程的结果,反过来,自生磁场又对等离子体的运动产生直接或间接的影响．到目前为止,对环形磁场的理论和实验研究比较多．相比之下,对轴向磁场的探索则很不充分,尤其在超短激光脉冲与等离子体作用中,对轴向磁场的研究尚属空白．本文利用法拉第效应,对超短激光脉冲同等离子体相互作用中产生的轴向磁场进行了首次测量．实验中通过引入延迟光路的方法,大大降低了打靶激光发与发之间的涨落带来的误差．使用飞…     

柱面非均匀弱磁场对光纤陀螺磁敏感性的影响

以光纤陀螺(FOG)某电路板为例,扫描分析了其磁场分布规律,并利用矩阵光学和光传输理论,建立了柱面非均匀磁场中Faraday非互易相位差理论模型,进而仿真分析了实际电路板的辐射磁场对光纤陀螺的影响.分析结果表明:1)柱面非均匀磁场与光纤环距离越小,FOG磁敏感相位误差越大;2)柱面非均匀磁场对FOG所产生的磁敏感误差与角度之间呈倾斜正弦曲线,且倾斜度随磁场与光纤环间距减小而加剧;3)当R＜5r(磁场源位于光纤环外部,r为光纤环半径,R为磁场源与光纤环左侧边缘的距离)或R＜0.5r(磁场源位于光纤环内部)时,非均匀磁场均大于同量级均匀磁场对FOG的影响;4)非均匀场的存在影响FOG偏离固有磁轴方向;5)对于距离FOG较近的一些辐射强度非常小的普通电路板,也会导致FOG输出的不稳定和方向相关性.上述结果对于理解和分析实际磁场对FOG的影响,具有实际指导意义.     

西江牵引变电所电磁辐射研究

通过测量、分析和研究西江牵引变电所的电磁辐射,得到工频场强值、无线电干扰和场强距离衰减特性.在峰值检波模式下对9 kHz~1 GHz范围进行频率扫描,并对特定频点进行准峰值测量.为减小高压传输线对测量的影响,提出"沿对角线延长线"进行测量的新方案并用于测试.测量结果表明:周围环境中的电、磁场值小于变电所的发射限值,并且随距离增加呈现衰减趋势;工频场强值远小于国家标准中所规定的限值,符合人体健康标准.牵引变电所的电磁辐射低于发射限值,对人体及周边弱电设施的干扰很小.     

Approximate Crank-Nicolson schemes for the 2-D finite-difference time-domain method for TE/sub z/ waves

Two implicit finite-difference time-domain (FDTD) methods are presented in this paper for a two-dimensional TE/sub z/ wave, which are based on the unconditionally-stable Crank-Nicolson scheme. To treat PEC boundaries efficiently, the methods deal with the electric field components rather than the magnetic field. The "approximate-decoupling method" solves two tridiagonal matrices and computes only one explicit equation for a full update cycle. It has the same numerical dispersion relation as the ADI-FDTD method. The "cycle-sweep method" solves two tridiagonal matrices, and computes two equations explicitly for a full update cycle. It has the same numerical dispersion relation as the previously-reported Crank-Nicolson-Douglas-Gunn algorithm, which solves for the magnetic field. The cycle-sweep method has much smaller numerical anisotropy than the approximate-decoupling method, though the dispersion error is the same along the axes as, and larger along the 45/spl deg/ diagonal than ADI-FDTD. With different formulations, two algorithms for the approximate-decoupling method and four algorithms for the cycle-sweep method are presented. All the six algorithms are strictly nondissipative, unconditionally stable, and are tested by numerical computation in this paper. The numerical dispersion relations are validated by numerical experiments, and very good agreement between the experiments and the theoretical predication is obtained.     

A silicon MOS magnetic field transducer of high sensitivity

A structure has been devised which converts magnetic flux density change to a change in output current. The structure is essentially a P-channel MOST with the drain diffusion split into two halves. A magnetic field normal to the silicon surface deflects device current towards one half-drain. By operating the MOST in the "pinched-off" mode (VDS> VGS-VT) the output impedance is made high, so that large output voltage swings may be obtained. A theoretical study of the voltage and current distributions in the MOST channel has given data on the influence of device geometry on sensitivity. Experimental results indicate a linear relationship between output current and magnetic flux density, and an unexplained nonlinear variation of output with device current. Comparison of experimental results with theory indicates a carrier Hall mobility in the channel of 116 cm²/V.s.     

Measurement of an Electron Density Disturbance in a Positive Column with Magnetic Field

It is found that two columns of different luminosity which are separated by an intervened disc appear along a magnetic field, when a certain disc is inserted in a positive column with an axial magnetic field. These are a luminous column and a dark column which are extended from the disc toward the cathode and the anode side respectively. These deviations in luminosity decay from it in the direction of each side. Change in luminosity along the axis is confirmed by measurements of electron density distribution with movable probes.

An attempt to attribute this recovery toward the normal distribution to ambipolar diffusion of charged particles across the magnetic field is given.     

Dynamic cross correlation studies of wave particle interactions in ULF phenomena

Magnetic field observations made by satellites in the earth’s magnetic field reveal a wide variety of Ulf waves. These waves interact with the ambient particle populations in complex ways, causing modulation of the observed particle fluxes. This modulation is found to be a function of species, pitch angle, energy and time. The characteristics of this modulation provide information concerning the wave mode and interaction process. One important characteristic of wave-particle interactions is the phase of the particle flux modulation relative to the magnetic field variations. To display this phase as a function of time we have developed a dynamic cross spectrum program. The program produces contour maps in the frequency time plane of the cross correlation coefficient between any particle flux time series and the magnetic field vector. This program has been utilized in several studies of Ulf wave-particle interactions at synchronous orbit. Results obtained in these studies will be used to illustrate the operation of these programs and the type of information which can be obtained with this procedure.     

Plane waves in FDTD simulations and a nearly perfect total-field/scattered-field boundary

The total-field/scattered-field (TFSF) boundary has been successfully used for a number of years to introduce energy into finite-difference time-domain (FDTD) grids. If the propagation of the incident field is grid-aligned, a perfect TFSF implementation can be realized by using an auxiliary one-dimensional FDTD simulation which models propagation of the incident field. Here "perfect" implies the incident field propagation exactly matches the way in which the field propagates in the FDTD grid. However, for propagation which is not grid-aligned, no similarly perfect implementation has previously been presented. This work provides a framework for a perfect TFSF boundary for pulsed plane waves which do not propagate in a grid-aligned fashion. To achieve this, homogeneous plane-wave propagation is rigorously quantified. Using this knowledge and a specification of the desired incident field, the dispersion relation is used to ascertain the incident field at any point in the grid. It is required to account for, unlike in the continuous world, the electric field, the magnetic field, and the wavenumber vector not forming a mutually orthogonal set. Group velocity is also considered because of its relevance to the implementation.     

Wire-reference configurations in vehicle lateral control

Lateral control is an essential function for all forms of individual automated ground transport. Here two aspects of such control--the magnetic field distributions associated with a guideway-based, wire-reference configuration, and the associated vehicle-based sensors--are reviewed. Two wire-reference schemes have thus far been suggested. In the first, the amplitude characteristics of the magnetic field are employed to obtain the lateral control signal. In this paper, a theoretical analysis and detailed field measurements are used to define both "ideal" characteristics and those which would be encountered in a realistic operating environment. The resulting problems, which involve amplitude distortions of the field due to the proximity of steel-reinforcing materials, are defined, and their effects on vehicle control--poor tracking and passenger discomfort--are discussed. In the second approach, the lateral control signal is primarily dependent on the phase characteristics of the magnetic field. A theoretical analysis and a corresponding experimental field study, which are discussed herein, indicated that the problems associated with the first approach were largely overcome, as evidenced by successful full-scale tests of an automatically steered vehicle.     

E‐Field Control of Exchange Bias and Deterministic Magnetization Switching in AFM/FM/FE Multiferroic Heterostructures

The coexistence of electrical polarization and magnetization in multiferroic materials provides great opportunities for novel information storage systems. In particular, magnetoelectric (ME) effect can be realized in multi­ferroic composites consisting of both ferromagnetic and ferroelectric phases through a strain mediated interaction, which offers the possibility of electric field (E‐field) manipulation of magnetic properties or vice versa, and enables novel multiferroic devices such as magnetoelectric random access memories (MERAMs). These MERAMs combine the advantages of FeRAMs (ferroelectric random access memories) and MRAMs (magnetic random access memories), which are non‐volatile magnetic bits switchable by electric field (E‐field). However, it has been challenging to realize 180° deterministic switching of magnetization by E‐field, on which most magnetic memories are based. Here we show E‐field modulating exchange bias and for the first time realization of near 180° dynamic magnetization switching at room temperature in novel AFM (antiferromagnetic)/FM (ferromagnetic)/FE (ferroelectric) multiferroic heterostructures of FeMn/Ni₈₀Fe₂₀/FeGaB/PZN‐PT (lead zinc niobate–lead titanate). Through competition between the E‐field induced uniaxial anisotropy and unidirectional anisotropy, large E‐field‐induced exchange bias field‐shift up to $ {{{\Delta H_{ex}}}\over{{H_{ex}}}} = 218\%$ and near 180° deterministic magnetization switching were demonstrated in the exchange‐coupled multiferroic system of FeMn/Ni₈₀Fe₂₀/FeGaB/PZN‐PT. This E‐field tunable exchange bias and near 180° deterministic magnetization switching at room temperature in AFM/FM/FE multiferroic heterostructures paves a new way for MERAMs and other memory technologies.     

集体区自由电子激光器增益特性的研究

工作在超辐射模式下的集体区自由电子激光器的增益特性研究可以通过改变互作用区长度及泵浦强度来进行。我们利用偏转磁场可以获得任意的相互作用区长度。实验结果证明,工作在集体区的自由电子激光的辐射增益随泵浦磁场近似呈1.7次方关系,实验测得的增益系数范围为0.4dB/cm到1.38dB/cm。     

Propagation of Waves in a Plasma in a Magnetic Field

The propagation of electromagnetic waves in a plasma in a magnetic field as given by the Appleton-Hartree theory is discussed in terms of the wave normal surfaces instead of the more conventional propagation vector plots, and the "ordinary" and "extraordinary" waves are defined in terms of their polarizations instead of using a continuity argument. This gives a different picture of "a wave" which has some advantages. In particular, "whistlers" become obvious, as are regions of high reflection and high absorption. The Appleton-Hartree theory is then extended to include the effect of electron temperature, and this results in a third wave whose velocity is of the order of electron thermal motions.