Electric convection in a nematic liquid crystal in a varying electric field

The emergence of electric convection in a nematic liquid crystal under the action of a varying electric field is analyzed in the 1D approximation. A new mode of external field variation with subharmonic oscillations is proposed. The behavior of several classes of synchronous perturbations and subharmonic oscillations is analyzed; the evolution of the fields of the director and of the volume charge is studied. Parametric instability regions are specified and the critical frequencies of transitions between response modes are determined. Stability maps for a nematic liquid crystal are plotted on the frequency-amplitude plane.     

Evaluation and properties of a model for the determination of internal electric fields in proteins from the Stark effect of spectral holes

In this paper, we further evaluate a model for the analysis of Stark-shifted spectral holes, which allows for the determination of electric fields generated at the site of probe molecules by their host environment. In contrast to the conventional treatment of the Stark effect, our model regards both the internal electric field and the externally applied electric field as perturbations to the Hamiltonian of the probe molecule. Diagonalization of the Hamiltonian and fitting to the Stark-shifted spectral holes allows for the extraction of the internal electric field. We apply this model to Stark-shifted spectral holes burnt in a biological system, namely, myoglobin. Determining internal electric fields at the active sites of biological systems is particularly interesting, as these fields may contribute substantially to the function of such systems. As input to the data analysis procedure, state energies, transition dipole moments, and the local field factor are needed. We demonstrate here how the number of electronic states, the choice of local field factor, and the energies of several excited states of the probe molecule affect the resulting internal electric fields.     

Imaging of conducting media by the electric field tomography method

The paper describes physical principles of electric field tomography that allows information on spatial distribution of conductivity and permittivity of an object to be obtained in a noncontact way using the quasi-static electric field. Specific features in implementation of the method are pointed out, and the results of imaging test objects are given.     

Equilibrium configurations of uncharged conducting liquid jets in a transverse electric field

Solutions for the problem on the equilibrium configurations of uncharged conducting liquid jets in a transverse electric field are obtained. These solutions correspond to finite-amplitude non-axisymmetric azimuthal deformations of the surface of a round jet: the jet is stretched along the field in its cross-section. The range of electric fields is determined for which solutions of the problem exist. If the electric field strength is over some critical value, the electrostatic equations have no solution, and the jet splits. The obtained solutions are qualitatively examined for stability under small azimuthal perturbations.     

Small-angle approximation for an ion beam in an external electromagnetic field

A small-angle approximation method is formulated for a curvilinear ion beam propagating in an electromagnetic field. The effects of particle velocity scatter and multiple elastic scattering on the beam path in a magnetic field, the electric field of a cylindrical capacitor, and mutually orthogonal electric and magnetic fields are considered. An analytical model for beam power compression is developed.     

Partitioned real-space density functional calculations of bielectrode systems under bias voltage and electric field

We have developed a new partitioned real-space density functional (PRDF) method and applied it to bielectrode systems under bias voltage and electric field. The electronic states, electrostatic potentials, local electric fields, and chemical potentials of biased jelliums and two Si5 clusters are fully self-consistently determined. We overcame an essential difficulty in obtaining the electronic states of biased systems from the conventional density functional method by dividing the entire system into subsystems of each electrode. The PRDF method can be applied to multielectrode systems and enables the investigation of ionic systems with computational efficiency.     

Hybrid magnetic field formulation based on the losses separation method for modified dynamic inverse Jiles-Atherton model

Dynamic formulation based on the losses separation method in conducting media for the inverse Jiles-Atherton model is proposed. This formulation is based on the concept of the Hybrid Magnetic Field model (HMF). The HMF consists of the modification of the effective field by introducing two counter-fields associated, respectively, with the eddy current and excess losses. Such a formulation is characterized by seven parameters with five parameters issued from the quasi-static Jiles-Atherton model. Thus, two new parameters related to these fields are added to that defined in the quasi-static model. The identification of these new parameters is based on the measurements of the volumetric energy density. To validate this formulation, measurements are carried out on grain non-oriented Fe-Si 3% electrical sheets.     

波导等离子体限幅器中气体的选择与触发条件计算

 为保护电子设备不受高功率微波损坏,在矩形波导中嵌入等离子体限幅器。计算了不同气体的微波击穿场强随气体压强以及微波频率的变化规律。在高气压条件下（1 333~133 320 Pa）,气体击穿场强随气压增大而增大,在计算的4种气体中Ne的击穿场强最小;低气压条件下（1.333 2~133.32 Pa）,气体击穿场强随气压增大而减小,且Xe具有最小击穿场强。高气压条件下气体的击穿场强明显高于低气压下的击穿场强。计算结果表明：当填充133.32 Pa的Xe时,限幅器能够在约30 km范围内,有效地防护10 GW级高功率微波对电子设备的损坏。     

Ab initio molecular dynamics in a finite homogeneous electric field

We treat homogeneous electric fields within density functional calculations with periodic boundary conditions. A nonlocal energy functional depending on the applied field is used within an ab initio molecular dynamics scheme. The reliability of the method is demonstrated in the case of bulk MgO for the Born effective charges, and the high- and low-frequency dielectric constants. We evaluate the static dielectric constant by performing a damped molecular dynamics in an electric field and avoiding the calculation of the dynamical matrix. Application of this method to vitreous silica shows good agreement with experiment and illustrates its potential for systems of large size.     

Long-term drifts of stray electric fields in a Paul trap

We investigate the evolution of quasi-static stray electric fields in a linear Paul trap over a period of several months. Depending on how these electric fields are initially induced, we observe very different timescales for the field drifts. Photo-induced electric fields decay on timescales of days. We interpret this as photo-electrically generated charges on insulating materials which decay via discharge currents. In contrast, stray fields due to the exposure of the ion trap to a beam of Ba atoms mainly exhibit slow dynamics on the order of months. We explain this observation as a consequence of a coating of the trap electrodes by the atomic beam. This may lead to contact potentials which can slowly drift over time due to atomic diffusion and chemical processes on the surface. In order not to perturb the field evolutions, we suppress the generation of additional charges and atomic coatings in the Paul trap during the measurements. For this, we shield the ion trap from ambient light and only allow the use of near-infrared lasers. Furthermore, we minimize the flux of atoms into the ion trap chamber. Long-term operation of our shielded trap led us to a regime of very low residual electric field drifts of less than 0.03 V/m per day.     

Attenuation of the Electric Potential and Field in Disordered Systems

We study the electric potential and field produced by disordered distributions of charge to see why clumps of charge do not produce large potentials or fields. The question is answered by evaluating the probability distribution of the electric potential and field in a totally disordered system that is overall electroneutral. An infinite system of point charges is called totally disordered if the locations of the points and the values of the charges are random. It is called electroneutral if the mean charge is zero. In one dimension, we show that the electric field is always small, of the order of the field of a single charge, and the spatial variations in potential are what can be produced by a single charge. In two and three dimensions, the electric field in similarly disordered electroneutral systems is usually small, with small variations. Interestingly, in two and three dimensional systems, the electric potential is usually very large, even though the electric field is not: large amounts of energy are needed to put together a typical disordered configuration of charges in two and three dimensions, but not in one dimension. If the system is locally electroneutral—as well as globally electroneutral—the potential is usually small in all dimensions. The properties considered here arise from the superposition of electric fields of quasi-static distributions of charge, as in non-metallic solids or ionic solutions. These properties are found in distributions of charge far from equilibrium.     

Perfect spin filtering controlled by an electric field in a bilayer graphene junction:Effect of layer-dependent exchange energy

Magneto transport of carriers with a spin-dependent gap in a ferromagnetic-gated bilayer of graphene is investigated.We focus on the effect of an energy gap induced by the mismatch of the exchange fields in the top and bottom layers of an AB-stacked graphene bilayer. The interplay of the electric and exchange fields causes the electron to acquire a spindependent energy gap. We find that, only in the case of the anti-parallel configuration, the effect of a magnetic-induced gap will give rise to perfect spin filtering controlled by the electric field. The resolution of the spin filter may be enhanced by varying the bias voltage. Perfect switching of the spin polarization from +100% to -100% by reversing the direction of electric field is predicted. Giant magnetoresistance is predicted to be easily realized when the applied electric field is smaller than the magnetic energy gap. It should be pointed out that the perfect spin filter is due to the layer-dependent exchange energy. This work points to the potential application of bilayer graphene in spintronics.     

RF and field measurements of the SSC-LINAC RFQ

The 53.667 MHz continuous-wave heavy ion RFQ has been designed and manufactured for the SSC-LINAC project.This four-rod RFQ accelerates ions with maximum mass to charge ratio of 7 from 3.728 keV/u to 143 keV/u.Measurements have been carried out to check the RF performance of the cavity and the quality of the electric field.The S11 of the power coupler is adjusted to better than-44 dB,and the Q0 of the cavity is 6440.The quality of the electric field is evaluated by the perturbation method.The measurement procedure and data analysis will be discussed in detail.The error due to gravity of the perturbation bead has been corrected by averaging the fields in different quadrants.As a result,the unflatness of the electric field is±2.5%,and the dipole field component distributes from 0%to 20%in different longitudinal positions,which indicates the asymmetry of the quadrupole field.The unflatness of the quadrupole field distribution represents a good agreement with the simulation results.High power RF test and beam commissioning of the RFQ are on schedule in early 2014.     

均匀外电场中无限长介质圆柱壳的场分布的计算和讨论

用圆柱坐标系中的分离变量法计算了位于均匀外电场中的无限长介质圆柱壳各区域的电势和电场,由计算结果分析了无限长介质圆柱壳对外电场的屏蔽效果,并指出均匀外电场中的无限长导体圆柱壳、无限长介质圆柱体、无限长导体圆柱及无限大均匀电介质中开有一无限长的圆柱形空腔的电势和电场都可以由均匀电场中的无限长介质圆柱壳电势及电场给出.     

PuH2分子激发态的外场效应

采用密度泛函(DFT)方法B3LYP/Gen,在Pu为SDD基组、H为6-311 G**基组水平上优化得到了分子轴方向不同电偶极场(-0.005~0.005 a.u.)作用下,二氢化钚的基态电子状态、几何结构和分子总能量.在优化构型下用同样的基组采用含时密度泛函(TDDFT)方法(TD-B3LYP)研究了同样外电场条件下对二氢化钚的激发能、振子强度、自旋污染和Pu原子正电荷的影响.计算结果表明,分子几何构型与电场大小和方向呈现较强的依赖,H-Pu-H的角度线性减少,分子总能量线性减少;自旋污染随电场增加线性增加;Pu原子正电荷随电场增加而线性减小;激发能随电场强度增加而减小,且对电场方向的依赖呈现近似对称性,满足Grozema关系.电场对振子强度的影响比较复杂,但仍满足跃迁选择定则.     

Electric field measurements and computational modeling at ultrahigh-field MRI

While magnetic resonance images essentially contain a map of the both circularly polarized components of the RF transverse magnetic fields (B(1) field), the thermal heat and electromagnetic power deposition is generated by the associated electric fields. Measurement of electric field distributions/intensities across a sample yields an indirect indication of possible cause of heating within the sample and potentially enables the detection of "hot spots," which can be present within inhomogeneous radiofrequency (RF) fields, such as the case with magnetic resonance imaging at high field strength. As a result, establishing a valid technique for direct measurements of the electric field and its correlation, obtained using computational electromagnetics, is essential in assessing (1) the safety of the RF coil designs and (2) the validity of the calculations. In this work, a probe was built and used to measure the transverse electric field (E(1) field) distributions within an empty 8 T (tuned to 340 MHz) RF head coil and within a saline water phantom loaded in the same coil. The measured E(1) field distributions were favorably compared to the distributions obtained utilizing a finite difference time domain in-house package.     

静电放电火花产生的电磁场树枝模型

 静电放电火花产生的电磁脉冲会对电子系统的正常工作造成严重的干扰，甚至造成系统的损伤。利用时域有限差分法建立了静电放电火花产生的电磁场的数值模型，模型中充分考虑了放电电极上的静电荷对电场的影响。把由此模型放电计算的电磁场值与由此解析方法得到的场值进行了比较，结果吻合良好。由此可以用此模型来研究静电放电火花产生的电磁场与电子系统的能量耦合问题。     

基于Rydberg原子的超宽频带射频传感器

Rydberg原子具有极大的极化率和微波跃迁偶极矩,对外界电磁场非常敏感,可实现基于Rydberg原子的超宽频带射频电场的高分辨高灵敏测量.通过Rydberg原子的全光学无损的电磁感应透明探测手段,可以实现基于原子的快速免校准宽频带(0.01-1000 GHz)外电场的精密测量.对于频率大于1 GHz的微波场,由微波场耦合相邻Rydberg能级形成的Autler-Townes分裂进行测量;而对于频率小于1 GHz的长波射频场,由Rydberg能级的射频边带能级进行测量.这种方法是基于原子能级参数,可溯源到基本物理常量,不依赖于外界参考;且对电场无干扰,易于实现微型化和集成化,具有广泛的应用前景.本文主要综述了基于Rydberg原子的外电场测量的最新研究进展,重点介绍长波长射频场的测量,包括电场强度、频率以及极化方向的测量,详细介绍了其测量原理和探测灵敏度,并讨论了其应用前景及未来发展方向.     

Electric field driven flow in natural porous media

Electric fields were applied to fluid-saturated packed sand beds (0.23+/-0.03 mm average pore diameter), and the effects on the mobility of the water molecules were monitored using stimulated echo (STE) and pulsed field gradient (PFG) experiments. The mean flow velocity, averaged over the entire sample, is expected to vanish in closed systems, but the PFG and time dependent signal decay was enhanced beyond the effects of thermal diffusion, due to velocity dispersion. The internal flow generated by the electric field was shown to be fully time-reversible upon inverting the electric field polarity (for total flow times of up to 0.4s), a strong indication that the NMR detected displacements were mainly due to electro-osmotic flow (EOF). However, a comparison of the velocity dispersion for different electrolyte concentrations showed that the measured effect scaled with the applied power VI (V = voltage, I = electric current), rather than with the voltage alone, contrary to the prediction of the basic model for EOF in a single capillary channel.     

Electrically stimulated motion of dislocations in a constant magnetic field

This paper reports on the results of investigations into the dislocation mobility in n-Si single crystals (N _d =5×10²⁴ m^?3) upon simultaneous exposure to electric (j=3×10⁵ A/m²) and magnetic (B≤1 T) fields. It is found that the introduction of dislocations (≈10⁹ m^?2) into dislocation-free silicon doped with phosphorus leads to the appearance of the paramagnetic component of the magnetic susceptibility. The paramagnetic component increases with an increase in the dopant concentration. Similar transformations in silicon account for the formation of magnetically sensitive impurity stoppers that respond to external magnetic perturbations. An analysis of the behavior of dislocations in electric and magnetic fields has revealed a parabolic dependence of the dislocation path length on the magnetic induction B. The effective charges and mobilities of dislocations are numerically calculated from the results obtained. A model is proposed according to which the observed increase in the dislocation mobility is associated with the decrease in the retarding power of magnetically sensitive stoppers due to a local change in the magnetic characteristics of the material and the spin-dependent reactions stimulated by a magnetic field.