Transport properties of heteroepitaxial films based on bismuth telluride in strong magnetic fields

The temperature and magnetic-field dependences of the galvanomagnetic properties of n-Bi₂Te₃ heteroepitaxial films in magnetic fields to 14 T at low temperatures are studied. It is shown that steps in the magnetic-field dependences of the quantum Hall effect and the plateau in the temperature dependences of the magnetoresistance of films are caused by topological surface states of Dirac fermions.     

Two-dimensional quantum-mechanical confinement of electrons in LED by strong magnetic fields

Two-dimensional quantum-mechanical confinement of electrons in a GaAlAs light emitting diode (LED) is realized by applying strong magnetic fields, in which electrons can move freely only in the direction of magnetic fields, and the dimensionality of the free carrier motion is reduced from three to one. With this confinement of electrons, considerable suppression of half-value width of the spectrum broadening from 80 to 60 Å at 80 K in the LED is observed. The systematic shift of the emission peak toward shorter wavelength is also observed in accordance with the theory, which takes into account the Landau level broadening due to finite electron mobility.     

Two-dimensional quantum-mechanical confinement of electrons in DH lasers by strong magnetic fields

Two-dimensional quantum-mechanical confinement of electrons in DH lasers is realized near room temperature by applying strong magnetic fields up to 32 T. In order to confirm the realization of two-dimensional quantum-mechanical confinement of electrons in such lasers, the spectral characteristics are measured. The observed shift of the lasing wavelength with magnetic fields is shown to correspond to the upward shift of the lowest Landau level with the increase of magnetic fields.     

Comparison of various safety guidelines for electronic article surveillance devices with pulsed magnetic fields

The paper uses the two methods suggested in both the ICNIRP and proposed IEEE safety guidelines for compliance testing of security systems based on time-varying magnetic fields being introduced for electronic article surveillance (EAS), radiofrequency identification, and other applications. For nonsinusoidal pulses that are often used, the two procedures are to treat the exposure as a multifrequency exposure with various frequency components or to calculate the peak induced current densities or electric fields treating the highest of the pulses of duration t(p) as a single frequency, half sinusoid of the same duration and frequency 1/(2t(p)). Using either of the procedures, the induced current densities (J) or electric fields (E) are compared to the basic restrictions on J or E for compliance testing. Using a heterogeneous, tissue-classified anatomic model of the human body, we calculate the induced J and E for the various tissues for a realistic, EAS system for two typical nonsinusoidal pulses to show that the two methods give substantially different results. While the approximate but simpler method of treating the pulse as a half sinusoid results in peak induced J or E that may be compliant with safety guidelines, the rigorous method of treating such exposures as multifrequency exposures gives induced current densities or electric fields that may be up to twice as large, thus making such systems potentially noncompliant with the safety guidelines. Since it is more accurate, it is suggested that safety assessment based on the Fourier analysis leading to multifrequency components be used for compliance testing of such devices.     

Effects of an axial magnetic field on the output power of a pulsed xenon laser

Data indicating the effects of an axial magnetic field on the output power of a pulsed xenon laser are presented. The output power increased significantly with increasing axial magnetic field only for low output power levels.     

低频脉冲磁场中磁屏蔽体磁饱和效应的试验研究

针对磁屏蔽体在低频脉冲磁场环境中可能存在的磁饱和问题,利用试验方法开展了磁饱和效应研究,证实了常规工程屏蔽体可在低频脉冲磁场环境中达到磁饱和状态,并通过观测屏蔽效能的变化获得了磁饱和规律,同时分析了磁饱和效应对屏蔽效能的影响及其与屏蔽体的材料磁导率、壳体厚度、外形尺寸等参数的关系.研究表明：磁屏蔽体屏蔽效能在磁饱和效应影响下,呈现出明显的动态变化特点,具有与屏蔽壳体磁导率类似的变化趋势;壳体厚度2 mm以内、长宽高为2 m×2 m×2 m左右的屏蔽体在上升时间为300μs、持续时间为1.2 ms的磁场环境中,达到磁饱和状态的磁化场强度约为10 mT,其磁饱和难易程度与磁导率及外形尺寸负相关,与壳体厚度正相关.试验研究结果与理论分析结论一致,可为磁屏蔽体的科学合理设计提供参考,具有较高的工程应用价值.     

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.     

A box coil for the stimulation of biological tissue and cells in vitro and in vivo by pulsed magnetic fields

An alternative coil system to the Helmoholtz coil-pair is described for the stimulation of biological tissue and cells: a relatively large box coil made of copper or aluminum sheet stock. The design is based on the principal determinant of the induced electric field, namely, the magnetic vector potential (A), in the equation, [formula: see text]. The second term in the equation is needed when boundaries of the conducting medium are in close proximity to the region of interest, such as in a culture dish. An electric surface charge builds up on the boundaries to generate an electric field which cancels [formula: see text] at the surface. The effectiveness of the new coil is demonstrated in a study of the outgrowth enhancement of axons from rat embryonic dorsal root ganglia.     

螺线管结构参数对脉冲磁场的影响

研究了线圈间距、匝数、个数以及不锈钢套筒对脉冲磁体产生磁场的影响规律。在储能电容和电压不变的前提下,研究结果表明：增加线圈间距会导致磁感应强度降低,磁力线包络增大,但总电流达到峰值时刻减小;增加线圈匝数,峰值电流明显减小,会降低磁感应强度,但有利于抑制磁力线包络;增加并联线圈个数,有利于产生较长的均匀区,但是在供能一定的条件下,磁场强度有所降低,同时总电流达到峰值时刻减小。总体来看,在一定均匀区长度的设计要求下,减少单个线圈匝数,增加并联线圈个数,能够得到磁感应强度更大、均匀性更好的磁场,但要考虑线圈承载电流的能力。另外,不对称的阴阳极金属结构会导致磁场不对称分布,且磁感应强度达到峰值时刻要晚于总电流达到峰值的时刻。     

Detrimental effects of electrical fields on cardiac muscle

The use of controlled electrical shock as a therapy to manage cardiac arrhythmia is a practice commonly used today. High intensity electrical fields are generated near the shock electrodes, and if the electrodes are placed directly on or inside the heart as is often the case, tissue injury and dysfunction may result if the shock intensity is too high. Many factors influence the degree of dysfunction, including the intensity of the shock poise, duration of the pulse, waveform shape, size and position of the electrodes, and physiological state of the heart. One of the most immediate indications of aberrant cardiac function is an abnormality in the electrocardiogram, which results from field-induced changes in cellular electrophysiology. This article reviews results obtained primarily from animal experiments which delineate the intensities of electrical field that produce electrical dysfunction at various structural levels of the heart. Possible mechanisms underlying the detrimental effects of electrical fields are presented with the main focus on electroporation of the cell membrane. Other mechanisms that are described include formation of oxygen-derived free radicals, conformational damage to ionic pumps/channels, barotrauma, and hyperthermia. Differences between cathodal and anodal shock effects, as well as factors which may ameliorate electrical field-induced cardiac dysfunction, are also discussed     

Influence of mode competition on the fast wavelength switching ofan SG-DBR laser

Because the sampled grating distributed Bragg reflector (SG-DBR) laser is among the most attractive sources for wavelength division multiplexing (WDM) systems, it is important and necessary to investigate its wavelength switching characteristics. This behavior will set the capability limits for reallocation in wavelength-routed optical networks. In this paper, that mode competition plays an important role in the wavelength switching dynamics of DBR-type tunable lasers is confirmed experimentally. By using a time-resolved spectrum technique, the loss-dependent mode competition behavior has been directly observed, for the first time, from measurements of wavelength switching on an SG-DBR laser     

Reconstruction of cardiac displacement patterns on the chest wall by laser speckle interferometry

Laser speckle interferometry has been used to determine the displacement pattern on the chest wall produced by the heart action during the systolic phase of the cardiac cycle. The time averaged specklegram, recorded in the image plane, is scanned by the pointwise method. From the values of the spacings of Young's fringes, measured at various points, the corresponding displacement pattern in the form of a 40 x 30 matrix, equal to that of the cardiac region, is reconstructed and color-coded by a PDP 11/23 image processor. This pattern shows significant variations in displacements at various locations. Depending on the clinical status of the subjects, further changes in the pattern are observed. The reconstructed cardiac displacement patterns, obtained by this new noncontact and noninvasive technique, provide information on the functioning of various parts of the heart in cardiac disorders which are in qualitative agreement with that of two-dimensional echocardiography.     

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     

Intersubband far-infrared emission and magnetotransport of 2D hole gas in a strong in-plane electric field and transverse magnetic fields

We observe FIR emission and magnetotransport phenomena of modulation doped p-GaAs/AlGaAs multi-quantum wells structures of different well width in a streaming regime. This FIR emission originates from radiative transitions between the upper quantum subbands, where hot holes are trapped by the emission of LO-phonons and the ground quantum state. The importance of the light-heavy states mixing with an increase of confinement effect is shown experimentally. The magnetoresistance in a strong in-plane electric field applied along the [110] crystallographic direction is found to be negative, which is explainedDby strong warping of the constant hole energy surfaces in two-dimensional quantum wells.     

Investigation of static and quasi-static fields inherent to the pulsed FDTD method

Demonstrates that trailing dc offsets, which can affect E- or H-fields in finite-difference time-domain simulations, are physically correct static solutions of Maxwell's equations instead of being numerically induced artifacts. It is shown that they are present on the grid when sources are used, which generates nondecaying charges. Static solutions are investigated by exciting electric and magnetic dipoles models with suitable waveforms.     

Observations of the effect of magnetic fields on the radiation from plasma-covered antennas

Measurements of the broadside radiation from an anisotropic plasma-covered slot are compared with theoretical predictions which neglect input impedance changes and coupling to electroacoustic or Tonks-Dattner resonances. For a magnetic field normal to the plasma layer, the results demonstrate transmission enhancement and underscore the necessity of a strong field (omega_{c} > omega). They also highlight some of the limitations of existing theoretical treatments. In particular, it is concluded that collisional damping, impedance variations, and electroacoustic resonances must be included for an accurate prediction of magnetic field effects. The experimental system consists of a narrow aperture in a large ground screen covered by a bank of long discharge tubes which are collectively equivalent to a plane layer. All measurements are performed at a fixed frequency of 565 Mc during the afterglow of the pulsed discharge.     

Production and investigation of TW proton beams from an annulardiode using strong radial magnetic insulation fields and a preformedanode plasma source

A magnetically insulated proton diode in extraction geometry which has achieved an ion beam power of 0.8 TW and an ion production efficiency of greater than 90% is described. Using a Pd-Ti metal hydride film on the anode surface and diverting part of the generator current through it with the help of plasma opening switches, a proton-rich plasma is preformed on the anode before any voltage occurs across the diode gap. Thus it is possible to apply very strong magnetic insulation, V_c/V_op>3. Ion current and diode voltage simultaneously reach a plateau for 40 ns, resulting in a nearly constant diode impedance for 60 ns. Thus, chromatic aberrations of the diode focusing system are minimized. The microscopic divergence of proton beamlets due to diode effects is determined to be 0.7°. Geometric aberrations of spherical and aspherical anode shape have been determined. The experimental results are compared to simulations with a 2.5 dimensional stationary PIC (particle-incell) code     

Effects of high space-charge fields on the response of microwavephotodetectors

We present simulation results, based on a one-dimensional p-i-n photodetector model, to explain the reductions in photodetector response observed at high optical powers. The nonlinear effects are attributed to the redistribution of the internal electric field due to space-charge effects created by the photocarriers. The corresponding modification of the position-dependent charge velocities results in high-frequency response reduction, roll-off reduction, and pulse narrowing. Electric field redistribution leads to two dimensional effects, which must be included for accurate modeling of milliampere photocurrents