对称强磁场产生装置设计及磁场仿真

在对称线圈上施加强脉冲电流,在脉冲电流的激励下,线圈中产生感应脉冲磁场,从而实现同极性磁场撞击.在撞击过程中,磁能会以一种波的形式激发出去,从而产生激磁波.激磁波是一种磁性波,其能量主要通过磁场的变化进行传递.利用有限元的方法对对称线圈的内部外部分别进行仿真,讨论并分析了磁场在空间和时间上的变化.     

The influence of magnetic fields on dielectric surface flashover

The influence of low-amplitude magnetic fields, in a variety of configurations, on pulsed dielectric surface flashover has been investigated. These variations include DC magnetic fields; pulsed magnetic fields simulating conditions for magnetic self-insulation; and different environments (vacuum, ambient, gas, plasma), geometries, dielectric materials, and orientations of the magnetic field. For field amplitudes of 0.3 T, typically a doubling of the flashover voltage is observed, if the E×B drift is away from the surface. For flashover in vacuum, it is sufficient to place permanent magnets in the cathode vicinity to increase the flashover voltage. The observations are consistent with the saturated surface secondary avalanche model and electron-induced gas desorption. The pulse shape of light emission during the prebreakdown phase depends on the orientation and amplitude of the magnetic field and shows that the electron trajectories above the surface are altered by magnetic fields     

Maximum pulsed electromagnetic field limits based on peripheralnerve stimulation: application to IEEE/ANSI C95.1 electromagnetic fieldstandards

This communication proposes a rationale for maximum pulsed magnetic field limits in the electromagnetic-field standards of IEEE/ANSI C95.1. The peak limits, intended to protect against peripheral nerve excitation by pulsed fields, are adapted from existing standards for patient exposure in magnetic resonance imaging (MRI) examinations     

Analysis and measurements of magnetic field exposures for healthcare workers in selected MR environments.

There are concerns about workers repeatedly exposed to magnetic fields exceeding regulatory limits with respect to modern magnetic resonance imaging (MRI). As a result, there is need for an ambulatory magnetic field dosimeter capable of measuring these fields in and around an MRI scanner in order to evaluate the regulatory guidelines and determine any underlying exposure risks. This study presents results of tri-axial measurements using an ambulatory magnetic field dosimeter worn by workers during normal working shifts. We recorded and analyzed magnetic field exposures in and around 1.5 T, 2 T, and 4 T magnets during routine patient procedures. The data was integrated and averaged over time and evaluated against the latest exposure standards. Time-varying magnetic fields occur when individuals move through spatially non-uniform static magnetic fields or during gradient-pulsed magnetic fields or a combination of both. Our previous numerical analysis shows that at certain positions surrounding the MRI scanner ends, such fields may induce current densities and electric fields that may exceed the relevant EU, ICNIRP, and IEEE standards. A high-speed acquisition version of the dosimeter measured gradient- pulsed fields at positions accessible by MRI workers near the scanner ends, and the results were evaluated and compared against the numerical simulations and the standards. Our measurements confirm that workers can be exposed to magnetic fields exceeding the guidelines at positions near the gradient coil ends during clinical imaging and a high degree of correlation exists with the numerical results. While the time-weighted average magnetic field exposures in 1.5 T, 2 T, and 4 T were all within the regulatory limits during static magnetic field measurements, the peak limits for the head can be exceeded in some circumstances. This study presents a small number of routine shifts of data that provide indicative results of magnetic field exposure in real situations.     

Galvanomagnetic phenomena in p-Hg1−x MnxTe solid solutions

Galvanomagnetic effects in homogeneous samples of p-Hg_1−x Mn_xTe solid solutions were investigated in stationary and pulsed magnetic fields up to 200 kOe in the temperature range 1.8–300 K. The magnetopolar origin of the anomalous dependence of the Hall coefficient on the magnetic field at liquid-helium temperatures is confirmed. The shift in the inversion point of the Hall coefficient agrees with the model of the bound magnetic polaron. Freezing of holes to acceptor levels in high magnetic fields was observed for all samples. Fiz. Tekh. Poluprovodn. 31, 326–328 (March 1997)     

Application of a nonharmonic magnetic-field distribution for focusing of intense electron flows in magnetic periodic focusing systems

The results of theoretical and experimental investigation of focusing of intense extended electron flows in periodic magnetic fields with a nonsinusoidal distribution are presented. An easily realizable nonharmonic distribution that has three local maxima on the magnetic-field half-period and ensures small fluctuations of the boundary of an intense electron flow at high values of the field parameter is found. The results of application of magnetic periodic focusing systems (MPFSs) with the obtained distributions in pulsed traveling-wave tubes with a slow-wave structure integrated with the MPFS pole pieces are presented.     

Anisotropic magnetoresistance of the epitaxial single-crystal Fe(001) film of nanoscale thickness

The dependence of the anisotropic magnetoresistance (AMS) of the single-crystal Fe(001) film grown with the aid of the pulsed laser evaporation on the direction and magnitude of the external magnetic field is analyzed. Three ranges of magnetic fields with different types of variation in AMS with the direction of magnetic field are revealed. The discontinuities of resistance and the hysteresis are demonstrated in a certain range of magnetic fields. The anisotropy field is estimated using the experimental results.     

Faraday rotation effects of Mn2+-modifiedTb2O3-B2O3 glass in pulsedmagnetic field

We present the dynamical Faraday rotation of a terbium metaborate glass (25Tb₂O₃-75B₂O₃) modified with Mn²⁺ ions using pulsed magnetic fields, which reached up to ~16 T. The superexchange couplings of Tb³⁺-O ^2--Tb³⁺ or Tb³⁺-O^2--Mn²⁺ were magnetooptically observed at lower temperature. In particular, the enhancement of the Faraday rotation effect was obtained at 15 K with repetitional operation of the 16-T pulsed magnetic fields, resulting in the destruction of the dimer couplings. When the operated glasses were heated back to 300 K, only the Tb₂O₃-B₂O₃:Mn²⁺ exhibited a significant change in the Faraday rotation     

A test for teratological effects of power frequency magnetic fieldson chick embryos

An analysis of thirteen studies of the teratological effects of pulsed magnetic fields on chick embryos from ten independent laboratories permits no clear conclusions. It is noted that comparatively little has been done to follow up on the reports by J. Juutilainen et al. (Rad. Environ. Biophys., vol.25, p.65-7, 1986) on the effects of extremely-low-frequency, sinusoidal magnetic fields on the malformation rate in chick embryos. An attempt to follow up on their results using similar but not identical exposures of 10-μT, 50-Hz magnetic fields, which produced negative results, is reported     

Ionization-induced nonequivalent absorption in a birefringentsilica fiber

The response of a single-mode highly birefringent fiber, operating at a wavelength of 1.3 μm and irradiated by a source of 200 MeV pulsed protons, is presented. The fiber fast and slow axes of birefringence were optically excited and observed to exhibit transient absorption at different rates which scaled linearly over the pulsed exposure range of 2.6-17.4 krad (Si). Mechanisms possibly affecting the fiber birefringence are discussed including the introduction of transient electric and magnetic fields arising from the ionizing radiation     

Immunity tests of implantable cardiac pacemaker against CW and pulsed ELF fields: experimental and numerical results

The aim of the present work is to investigate pacemaker (PMK) immunity against high-level extremely low frequency (typically 50 Hz) magnetic fields that can be found in industrial environment where high power machines operate. To that purpose, a test bench has been set up using a Helmholtz coil for producing extremely low frequency magnetic fields and a trunk simulator rightly fed by cardiac signals. A widely used PMK was tested, and results have been reported, under different operating conditions, for both continuous waves and variously pulsed excitations. A numerical tool has also been developed to analyze the realized test bench, based on a proper discretization of a human trunk simulation and on the construction of an equivalent three-dimensional (3-D) network. Numerical results have showed a good agreement with the experimental results.     

Demagnetization of Trapped Field in High TC Superconductors Using Pulsed Field Methods

This paper examines the demagnetization of a trapped magnetic field in a high T_C superconducting (HTS) monolith using a single, short, high-amplitude field pulse. With a single optimized pulse, the peak trapped field can be reduced to one-third of the maximum saturated value. Alternatively, the spatial average of the trapped field could be fixed very close to zero. After demagnetization, the trapped field has both positive and negative values at various locations in the HTS monolith. Transient responses of the HTS to pulsed fields with different amplitudes are also reported and discussed. It does not appear that the residual trapped field can be reduced to zero throughout the monolith using pulsed field demagnetization methods in a fixed sample/coil geometry. Bean's critical state model is used to explain the measured equilibrium field distributions. Transient response of the HTS to an externally applied field is qualitatively explained by magnetic diffusion. Drive circuit characteristics and design feasibility issues are also addressed     

Submillimeter EPR of Co:Rb2MgF4 and anomalous g-values

We found new effect which changes g-values of pair and trimer Co²⁺ spins by EPR of Rb₂Co_0.3Mg_0.7F₄ at 220, 370, 762.2 and 693.6 GHz in pulsed high magnetic fields. The anomalous g-values come from mixing of 1LS and strong exchange interaction.     

Resonant-phonon terahertz quantum-cascade laser operating at 2.1 THz (/spl lambda//spl sime/141 /spl mu/m)

The development of quantum-cascade lasers (QCLs) at 2.1 THz (/spl lambda//spl sime/141 /spl mu/m), which is the longest wavelength QCL to date without the assistance of magnetic fields, is reported. This laser uses a structure based on resonant-phonon depopulation, and a metal-metal waveguide to obtain high modal confinement with low waveguide losses. Lasing was observed up to a heatsink temperature of 72 K in pulsed mode and 40 K in continuous-wave (CW) mode, and 1.2 mW of power was obtained in CW mode at 17 K.     

自动检测系统中的电流测量技术

根据自动检测系统中电流测量技术极少被采用的现状，提出利用电流产生的磁场进行电流测量的概念，详细阐述了一种基于电子束在被测电流生成的磁场中发生偏转的原理进行测量的非侵入式电流传感器，并讨论了这种电流测量技术所面临的问题，提出了相应的解决方案。最后提出有关电流数据和其他相关信号的分析方法以及电流测量的意义。     

一种高空核电磁脉冲磁场测量技术

针对高空核电磁脉冲（high altitude electromagnetic pulse，HEMP）试验研究，分析了电小环天线测量原理和频率特性，建立了传感器数值模型，研究了其脉冲接收特性，设计了一种B-dot传感器，研制了一套脉冲磁场测量系统.结果表明:当环面半径小于20 mm，传感器的上限截止频率达到720 MHz以上，标定结果显示测量系统整机的上升时间小于1 ns.应用该系统测试HEMP模拟器的电磁脉冲环境，并与电场测量结果进行了比对和验证，结果与理论相符，该系统具备HEMP模拟器产生的快前沿脉冲磁场的测量能力，同时可推广应用于其他强脉冲磁场测量.     

High‐Density Periodically Ordered Magnetic Cobalt Ferrite Nanodot Arrays by Template‐Assisted Pulsed Laser Deposition

A novel nanopatterning method using pulsed laser deposition through an ultrathin anodic aluminium oxide (AAO) membrane mask is proposed to synthesize well‐ordered nanodot arrays of magnetic CoFe₂O₄ that feature a wide range of applications like sensors, drug delivery, and data storage. This technique allows the adjustment of the array dimension from ～35 to ～300 nm in diameter and ～65 to ～500 nm in inter‐dot distance. The dot density can be as high as 0.21 Terabit in.^?2. The microstructure of the nanodots is characterized by SEM, TEM, and XRD and their magnetic properties are confirmed by well‐defined magnetic force microscopy contrasts and by hysteresis loops recorded by a superconducting quantum interference device. Moreover, the high stability of the AAO mask enables the epitaxial growth of nanodots at a temperature as high as 550 °C. The epitaxial dots demonstrate unique complex magnetic domains such as bubble and stripe domains, which are switchable by external magnetic fields. This patterning method creates opportunities for studying novel physics in oxide nanomagnets and may find applications in spintronic devices.     

Improved field localization in transcranial magnetic stimulation of the brain with the utilization of a conductive shield plate in the stimulator

In this paper, a carefully designed conductive shield plate is presented, which helps to improve localization of the electric field distribution induced by transcranial magnetic stimulation for neuron stimulation. The shield plate is introduced between a figure-of-eight coil and the head. In order to accurately predict the field distribution inside the brain and to examine the effects of the shield plate, a realistic head model is constructed from magnetic resonance image data with the help of image processing tools and the finite-element method in three dimensions is employed. Finally, to show the improvements obtained, the results are compared with two conventional coil designs. It is found that an incorporation of the shield plate into the coil, effectively improves the induced field localization by more than 50%, and prevents other parts of the brain from exposure to high pulsed magnetic fields.     

New design concept of the magnet system generating the high pulsed field in combination with the bias field of the superconducting magnet

A new design concept of the axisymmetric magnet system generating the very high pulsed magnetic field which is superimposed on the bias magnetic field of the superconducting magnet is presented. The pulsed magnet consists of two coaxial coils which are wound in opposite directions. The geometry of both pulsed coils, i.e. the working (inner) one and the compensating (outer) one is designed in such a way that the mutual coupling between the small pulsed magnet and the outer superconducting magnet is practically zero. This configuration prevents the rise of the high induced voltage on the current leads of the superconducting magnet when the pulsed magnet is being energised, hence resulting naturally in protection of the system (superconducting magnet and the current source) against possible damage. Further, it is predicted that the stray field of the pulsed magnet, which gives rise e.g. to the eddy currents in the winding of the superconducting magnet, is considerably decreased. The simple theory enabling the design of the geometry of the compensating pulsed coil is derived. The advantages of this new concept are demonstrated on the results of the theoretical analysis using, as an example, one of the pulsed coils that were designed and fabricated in the Clarendon Laboratory, in connection with the Oxford Instrument superconducting magnet (Clarendon hybrid outer) which can generate a steady magnetic field up to 10 T in a room temperature working space with a diameter of 240 mm.     

The cyclotron resonance of holes in InGaAs/GaAs heterostructures with quantum wells in quantizing magnetic fields

The spectra of the cyclotron resonance of holes in the InGaAs/GaAs selectively doped heterostructures with quantum wells are studied in pulsed magnetic fields as high as 50 T at 4.2 K. The previously observed effect of the inverted (compared with the results of the single-particle calculation of the Landau levels) ratio of the spectral weight of two split components of the line of the cyclotron resonance, which is attributed to the effects of the exchange interaction of holes, is confirmed. It is found that the ratios of intensities of the components of the line of the cyclotron resonance profoundly differ on the ascending and descending branches of the magnetic field pulse, which may be associated with a long time of the spin relaxation of holes between the two lowest Landau levels, which constitute tens of milliseconds.