Microwave emission from plasmas in InSb with and without magnetic fields

A theory of wave propagation through semiconductors, assuming general orientations of the carrier drift velocities, the propagation vector, and the RF electric field relative to an applied steady magnetic field, is derived. This is specialized to obtain a theory of pseudolongitudinal wave interactions in electron and hole streams in InSb in a transverse and in a parallel (or zero) magnetic field. The theory explains the various features of high electric-field microwave emission and RF oscillations observed by experimenters over several years in a wide variety of material conditions and field configurations. Explained in particular are the facts that: 1) starting n- or p-type materials with initial density ≲ 10¹⁴cm^-3have to be impact ionized to achieve electron or hole number densities adequate for microwave emission; 2) for impact-ionized plasmas, it takes a minimum transverse magnetic field (Bperp) on the order of 1.5-2.5 kG for the onset of microwave emission and the threshold current thereafter diminishes monotonically with increasingBperp; 3) p-type material with high enough initial density (nh≳ 1 - 3 × 10¹⁵cm^-3) is unstable with electron injecting contacts in the presence of 15-20 percent (or more) electron injection; and 4) in the absence ofBperp, a longitudinal magnetic field is necessary mainly to prevent pinching of the plasma at the high current densities needed for microwave emission, etc. RF oscillations observed in n- and p-type InSb with or withoutBperpare also explained. The dependence of the oscillation frequency on the magnitude ofBperpshows a remarkable correlation with experimental observations.     

Chaotic characteristics of electron motion in the static magnetic fields and self-fields of the electron beam

In this paper We study the properties of electron motion when a nonneutral electron beam goes through a static magnetic field. The self-electronic and magnetic fields also affects on the test electron. The Hamiltonian equations of electron motion are deduced and the Poincare surface of section and the Lyapunov exponent are used to prove that the electron motion becomes chaotic when the fields are strong enough.     

On VLF radiation fields along the static magnetic field from sources immersed in a magnetoplasma

The focusing of waves from radiating VLF sources immersed in a cold magnetoplasma is investigated for the case in which the focusing takes place along the static magnetic field direction. The explicit form of tensor Green's function valid along the field line is derived for the far field, and the frequency ranges where focusing may be expected are specified. The theory is applied to calculate the power flux along the field line for an electric dipole and a magnetic loop with two orientations with respect to the static magnetic field.     

30mT恒定磁场对神经细胞Na~+通道特性的影响

应用全细胞膜片钳技术,将急性分离的小鼠额叶皮层椎体神经元暴露于由磁铁产生的30mT恒定磁场中,研究磁场对神经元Na+通道特性的影响。实验发现,中等强度恒定磁场使神经细胞Na+通道激活电位向超极化方向移动,Na电流峰值增大。磁场作用可略微改变激活过程和显著改变失活过程,使激活和失活曲线发生移动。结果表明,30mT恒定磁场可改变神经元Na+通道特性,从而影响神经元的生理功能。     

Imaging magnetic fields

A scanning squid microscope (SSM) detects sub-nanotesla fields and nanoamperes of current with unsurpassed sensitivity. SSM can image magnetic fields at the surface of the sample under study. It is paving the way to an understanding of how materials that become superconducting at a rather high transition temperature do it. The author describes the SSM and how it is used to image the magnetic fields     

静态磁特性参数的测量研究

在原有冲击检流计测量静态磁参数的基础上进行了研究,设计了数字式静态磁性参数测量系统.该系统采用单片机和电子技术实现硬件测量电路和软件系统设计,由模拟信号转换为数字信号,并通过数码管显示.实验表明,该测量系统能够测量磁性材料的磁参数,与模拟式冲击检流计相比,精度和分辨率可达1%和0.000 1,不仅满足磁性材料质量检测,而还可应用于实验教学.     

Electric and magnetic fields

The content of the first of the two courses in electro-magnetic fields offered by the electrical engineering department at MIT to its juniors is summarized. The course deals with electroquasi-statics and magnetoquasistatics throughout, treating electrostatic and magnetostatic phenomena as special cases. This method of presentation allows the introduction of many more engineering examples into lecture demonstrations and homework than is possible in a treatment of statics followed by a treatment of the full dynamic equations, or by an overemphasis on static phenomena with only a belated discussion of quasistatic ideas. Details on several lecture demonstrations and examples are given, and it is shown how they are integrated into the "flow" of ideas. Further, our treatment of magnetization differs from the conventional one and the advantages of this treatment are discussed.     

Four-electron quantum dots in magnetic fields

Making use of hyperspherical coordinates, the energy spectrum of a four-electron configuration in a harmonic quantum dot (QD) as a function of the dot size and the strength of a magnetic field are investigated. Discontinuous ground-state transitions induced by an external magnetic field and the quantum dot size effect are shown. An important aspect of the size effect is the crossover of energy levels. The present results are useful to understand the optical and magnetic properties of QD materials.     

Superconducting cylinder in a static transverse magnetic field

A piecewise continuous distribution of the critical currents is proposed to make compatible the Maxwell equations and constitutive equations of superconducting media. A magnetic moment of the infinite superconducting cylinder is calculated on the base of the proposed shell model. Some generalizations of the calculation method are given for superconductors of more complicated shapes     

激光等离子体冕区自生磁场的实验研究

采用630.3nm光探针的Faraday施转法测量了激光等离子体冕区的自生磁场.实验结果证实了Colombant从理论上提出的“热力源”是产生自生磁场的重要源项之一.     

Neural stimulation with magnetic fields: analysis of inducedelectric fields

Spatial distributions of the derivative of the electric field induced in a planar semi-infinite tissue model by various current-carrying coils and their utility in neural stimulation are evaluated. Analytical expressions are obtained for the electric field and its spatial derivatives produced by an infinitely short current element. Fields and their derivatives for an arbitrarily shaped coil are then obtained by numerical summation of contributions from all the elements forming the coil. The simplicity of the solution and a very short computation time make this method particularly attractive for gaining a physical insight into the spatial behavior of the stimulating parameter and for the optimization of coils. Such analysis is useful as the first step before undertaking a more complex numerical analysis of a model more closely representing the tissue geometry and heterogeneity.     

可测量磁场的超导纳米导线

美国Illinois大学UrbanaChampaign校区的研究人员使用DNA分子支架研制出的超导纳米器件证实了一种新的量子干扰.这种器件可用来测量磁场和测定超导区域.一位研究人员说：“利用DNA的自我装配过程可为带有分子线度的电子器件制造出复杂的支架.”     

Steady magnetic fields in noninvasive electromagnetic flowmetry

Transcutaneous electromagnetic (EM) flowmetry measures, noninvasively, the induced voltage generated by the flow of blood through a region immersed in a magnetic field. Steady magnetic fields of less than 0.5 tesla (T) have been used to measure pulsatile popliteal, brachial, and bilateral common carotid blood flow in normal subjects, in patients with arteriovascular disease, and in subjects with arteriovenous fistula (AVF) surgically created for hemodialysis. In these studies, field magnitude and time of exposure were below the limits suggested by two exposure guidelines available. Flow rate was calculated from the measured voltages and geometrical and electrical parameters using equations developed for three-media (body segment-vessel-blood) cylindrical models, based on electromagnetic theory. The 68 measurements reported here are in the expected range.     

Conductivity of quantum wires in uniform magnetic fields

The features of the de conductivity of quantum wires in longitudinal and transverse magnetic fields are studied for degenerate and nondegenerate electron gas. The conductivity is calculated on the basis of the Kubo formalism with regard to the elastic scattering of charge carriers at long-wavelength lattice vibrations. The final theoretical results for the conductivity are compared to the experimental data. The suggested model of quantum wires allows, among other things, an interpretation of the nonmonotonic dependence of the transverse magnetoresistance on the magnetic field.     

De behavior of Si diodes in magnetic fields

The effect of applying a transverse magnetic field on forward biased Si p-n junctions is investigated, and the results obtained suggest the possibility of employing magnetic measurements to evaluate the physical properties of semiconductor devices.     

Representation of axisymmetric magnetic fields in computer programs

The standard expression for the off-axis fields in terms of the axial values and their differentials is examined. It is shown that if terms higher than the second are used, the accuracy becomes worse instead of better, unless the input data are mathematically perfect. It is not possible to remedy this by smoothing experimental data with a polynomial curve-fitting routine. Alternative expressions of the fields in terms of Legendre polynomials and of elliptic integrals are considered. It is shown that the elliptic integral method is the most convenient in practical cases, and a fast computer routine is given for the required K and E. In this method, the fields are defined by the parameters of a set of current loops, and the problem of finding these parameters is discussed. No general solution can be given, but a considerable class of cases is reduced to an explicit solution of a cubic equation by Cardan's method. It is shown that hybrid methods can also be used; the data is first represented by a set of current loops. A precise set of axial values is calculated from these loops, and the differential routine can then be used with acceptable accuracy, including terms up to the sixth order.     

Complete thermodynamic analysis of static electric fields and excess charge in metals

Eventually thermodynamics can predict the flow of electrons in electronic devices, but a prerequisite for such an analysis is information on the corresponding equilibrium condition. A suitable thermodynamic procedure for determining the equilibrium distribution of charge in a static electric field is developed from the electrochemical potential of the conduction electrons. The resulting equation is suitable for application to a metal or a semiconductor. The case of a metal is simpler and therefore has been chosen for this initial treatment. If the thickness of a sheet of a typical metal is greater than about 8 nm (about 20 atomic layers) the mathematical solution is relatively simple and leads to the well-known Debye layer of accumulated electrons. For thinner sheets, down to atomic dimensions, the solution is more difficult and leads to novel predictions, including a pronounced decrease in the maximum accumulation at the surface. These results are obtained on the assumption that the material is homogeneous. In particular, the surface layers are assumed to be basically the same material as the bulk of the specimen. The contrast with present mechanistic views is discussed.