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.     

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。     

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。     

强激光驱动锥口多层靶的超热电子输运特性

为了提升超热电子的准直输运效率,提出了锥口多层靶模型,并通过二维PIC(particle-in-cell)模拟手段研究了强激光与锥口多层靶相互作用过程中超热电子的产生和输运特性。研究结果表明,相比于无锥结构的多层靶,锥口多层靶中输运的超热电子数目更多、能量更高且空间分布也更加集中,发散角被控制在-38°～38°之间的超热电子能量约增加了0.6倍,锥口多层靶能够提升超热电子的准直输运效率。锥口多层靶中超热电子准直输运效率提升的原因主要有三个方面:激光从锥壁上拉出了大量超热电子、锥壁对激光的聚焦作用增强了激光有质动力以及锥顶后方产生了较强的自生磁场分布。本文建立的模型对于提升"快点火"中超热电子的束品质具有重要意义。     

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。     

激光等离子体相互作用中的自生磁场和超热电子热输运

应用相对论电磁粒子模拟程序,研究了线性极化强激光入射到无碰撞密度均匀等离子体时被加速的超热电子及电磁不稳定性机制。讨论了电磁不稳定性激发的自生磁场和超热电子热传导特性。 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。结果表明,不稳定性激发的强电磁场使电子束在1 μm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电子热流产生抑制作用。     

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。     

三束飞秒激光辐照下铜膜内电子非平衡热输运

首先采用有限元法数值计算了铜膜内的电子温度和晶格温度分布变化,揭示了铜膜内电子非平衡热输运时间随飞秒激光光束参量的变化情况。仿真结果表明,铜膜内的电子非平衡热输运时间会随着泵浦光束数量及脉冲能量密度的增加而增加,并且使用三束飞秒泵浦激光作用时,电子非平衡热输运时间比单脉冲作用时的电子非平衡热输运时间增加了3倍。其次使用三束飞秒激光泵浦的泵浦-探测实验系统进行验证。实验结果表明:通过用具有一定延时的三束飞秒泵浦激光作用铜膜时,铜膜表面的瞬态反射率出现三次突变,使电子非平衡热输运时间得到极大延长,从而大幅度消除激光加工热障,并提高加工的质量、精度和效率。     

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。     

外加电场和磁场对太赫兹辐射产生的影响

通过对半导体太赫兹发射极在有和没有外加电场和磁场作用下发射光谱的测量。说明了外加电场和磁场对太赫兹电磁辐射的产生具有增强作用。采用反射式发射极在飞秒激光作用下辐射太赫兹脉冲的装置，同时利用电光取样方法探测太赫兹电场，得到了这些发射极的时域发射光谱，并通过快速傅里叶变换(FFT)得到了相应的频域光谱。实验表明，太赫兹时域发射光谱和频谱在外加电场、磁场作用下都有增强，但是所发射的频率成分和带宽都没有改变。借助于经典电磁理论的定性分析，认为太赫兹发射光谱在外加电场、磁场作用下的增强起源于半导体中载流子的加速运动受外加电场和磁场的影响。     

在环型自由电子激光器中电子运动的混沌特性

本文分析了相对论性电子在环型摆动器磁场,轴向磁场和由非中性电子束产生的平衡自电场和自磁场中的运动。通过数值计算画出Poincare截面映射图,表明当自场足够强时,这种运动变成混沌的。虽然现实的环型摆动器场和自场一样使电子运动方程成为不可积的,但自场使运动产生混沌的作用要比摆动器场强。轴向磁场有抑制混沌发生的作用。     

Multivalued hot electron distributions as spontaneous symmetry breaking

If hot electrons in many-valley semiconductors can spontaneously become redistributed between valleys, which are symmetrically oriented with respect to the applied electric field, the stable state is characterized by a multivalued electron distribution (MED) with anisotropic conductivities leading to strong transverse fields and negative differential conductivity (NDC) consequently. The condition for the existence of this effect, the stability of these nontrivial solutions of the transport equations, and the stratification of the sample into layers are considered. Experimental and theoretical results are reviewed; the influences of lattice temperature, magnetic fields, optical excitation, inhomogeneities, geometry, and crystallographic orientation of the samples are described.     

Reconfigurable Anisotropic Coatings via Magnetic Field‐Directed Assembly and Translocation of Locking Magnetic Chains

A method for the generation of remotely reconfigurable anisotropic coatings is developed. To form these coatings, locking magnetic nanoparticles (LMNPs) made of a superparamagnetic core and a two‐component polymer shell are employed. Two different polymers form phase‐separated coaxial shells. The outer shell provides repulsive interactions between the LMNPs while the inner shell exerts attractive forces between the particles. Applying a non‐uniform magnetic field, one gathers the particles together, pushing them to come in contact when the internal shells could effectively hold the particles together. When the magnetic field is turned off, the particles remain locked due to these strong interactions between internal shells. The shells are thus made stimuli‐responsive, so this locking can be made reversible and the chains can be disintegrated on demand. In a non‐uniform magnetic field, the assembled chains translocate, bind to the solid substrate and form anisotropic coatings with a “locked” anisotropic structure. The coatings can be constructed, aligned, realigned, degraded, and generated again on demand by changing the magnetic field and particle environment. The mechanism of the coating formation is explained using experimental observations and a theoretical model.     

Peculiarities of the electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration in the two-dimensional channel

The electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration are reported. We have observed anisotropy of the electron concentration and mobility measured at a low magnetic field in the [110] and $\left[ {1\bar 10} \right]$ crystallographic directions. It has been established by analysis of Shubnikov-de Haas oscillations that the conductivity through the two-dimensional electron channel InSb/AlInSb quantum well (QW) does not depend on the crystallographic direction. However, the magnetic-field dependences of the Hall coefficient and resistivity of the structures reveal a strong influence of the crystallographic directions. This has allowed one to conclude that the anisotropy of the electron transport parameters in the QW structures measured at low magnetic fields correspond to parasitic conductivity through the Al_0.09In_0.91Sb buffer layer with two pronounced anisotropic contributions: the influence of metallic In nanoclusters inhomogeneously distributed within the buffer layer and conductivity of the near-interface layer with a high anisotropic density of extended defects.     

Intense Sheet Electron Beam Transport in a Uniform Solenoidal Magnetic Field

In this paper, the transport of intense sheet electron beams in a uniform solenoidal magnetic field in high-power vacuum electronic devices is theoretically examined with the 3-D beam optics code MICHELLE. It is shown that a solenoidal magnetic field can be an effective transport mechanism for sheet electron beams, provided the beam tunnel is matched to the beam shape, and vice versa. The advantage of solenoidal magnetic field transport relative to periodic magnetic transport resides in the feasibility of transporting higher current density beams due to the higher average field strength achievable in practice and the lower susceptibility to field errors from mechanical misalignments. In addition, a solenoidally transported electron beam is not susceptible to voltage cutoff as in a periodic magnetic focusing system; hence, device efficiency is potentially higher.     

CHAOTIC ELECTRON TRAJECTORIES IN A CIRCULAR FREE ELECTRON LASER

The motion of a relativistic electron is analyzed in the field configuration consisting of a circular wiggler magnetic field, an axial magnetic field, and the equilibrium self-electric and self-magnetic fields produced by the non-neutral electron ring. By generating Poincare surface-of-section maps, it is shown that when the equilibrium self-fields is strong enough, the electron motions become chaotic. Although the realistic circular wiggler magnetic field destroys the inte-grability of the electron motion as the equilibrium self-fields do, the role the latter plays to make the motions become chaotic is stronger than the former does. In addition, the axial magnetic field can restrain the occurrence of the chaoticity.     

Applications of 1 MV field-emission transmission electron microscope

A newly developed 1 MV field-emission transmission electron microscope has recently been applied to the field of superconductivity by utilizing its bright and monochromatic field-emission electron beam. This microscope allows individual magnetic vortices inside high-Tc superconductors to be observed, thus, opening the way to investigate the unusual behaviour of vortices, which reflects the anisotropic layered structure of these superconducting materials. One example is the observation of the arrangements of chain vortex lines that are formed when a magnetic field is applied obliquely to the layer plane of the materials.