Instability regions of an electron beam in superimposed periodic and uniform magnetic fields

The stability of an electron beam in a superimposed sinusoidal and uniform magnetic field is theoretically and experimentally investigated. The first instability region degenerates to a point within the first well-known passband for a zero uniform field component, whereas the second instability region coincides with the first stopband of the sinusoidal field. These considerations are of practical significance for periodic magnetically-focused electron beams.     

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.     

Hollow-beam focusing with electrostatic and periodic magnetic fields

A new method of focusing a hollow cylindrical electron beam is presented. The focusing system consists of a cylindrical center conductor inside the beam, a cylindrical outer conductor enclosing the beam, and a series of periodic magnets outside the tube. A radial electrostatic field between the conductors provides an outward force on the electrons. The periodic magnetic field produces an inward force on the electrons. The inward and outward forces can be adjusted to provide a balance of all the forces acting on the electrons at both boundaries of the beam by choosing the electric and magnetic fields properly. An approximate analysis has been made and is presented which gives necessary design information. A number of curves are presented which are useful in designing focusing systems of this type. Experimental results on a beam tester show that current transmission of over 90 per cent for perveance up to 11 micropervs can be obtained readily. The adjustments are not critical and the performance is very stable.     

Equivalence of periodic magnetic field to uniform magnetic field in electron beam focusing

The general solution of the electron trajectory equation in a periodic magnetic field is derived in the form of series expansion, assuming laminar electron flow and small perturbation. It is concluded that if the cathode is not very heavily immersed in a magnetic field, beam focusing by a periodic magnetic field would be almost equivalent to that by a uniform magnetic field except near the unstable region. Under these restrictions, each focusing system would give almost the same beam ripple (in magnitude and phase) under the same injection conditions into the focusing system. It is also found that the first stable region of beam perturbation in the periodic magnetic field becomes narrower as the flux threading the cathode increases or the period of the magnetic field decreases, as far as linearization of the path equation is possible.     

Electron beam characteristics in radially varying periodic magnetic fields

Periodic magnetic fields are being widely used for the light-weight focusing of beams in high-power traveling-wave tubes. In many tube designs, there exists a considerable amount of radial variation in the magnetic focusing field. The effect of this radial field variation is investigated analytically as an extension of the previous work in this field. The usual design curves of α vs β are plotted with three variable parameters: ripple, cathode shielding parameterK, and radial field variation parameter x0. It is noted that it is important to keep the magnetic-field strength constant at the beam edge over a considerable variation of the magnetic-field parameter x0.     

Electron-beam defocusing due to high-intensity RF fields

The electric fields associated with a "slow" electro-magnetic wave propagating along an electron beam will modulate this beam; the axial component of field will produce the usual "bunching," while the radial component will cause periodic perturbations in the radius of such an electron beam. This paper presents the results of analytical and experimental studies in an attempt to explain certain features of this defocusing effect of intense RF fields on the beam in a traveling-wave tube. In particular the effect of cathode flux on the RF defocusing of a beam in Brillouin flow is treated.     

Simulation and experimental investigation of a multi-pole multi-layer magnetorheological brake with superimposed magnetic fields

This paper presents comprehensive investigations on a new multi-pole multi-layer magnetorheological (MR) brake. This unique design features MR working gaps set between two-layer individual coils. Independent current supply was proposed to generate more flexible braking torque and lower power consumption. In this article, an exploded-view drawing of the proposed MR brake was presented, and theoretical analyses of the braking torque and temperature characteristic were conducted. Then, finite element analysis was performed to verify the effect of the magnetic field superposition. A prototype MR brake was fabricated and tested to evaluate the magnetic field superposition, preliminary dynamic behavior, temperature and the performance of individual input current. The results show that the magnetic field superposition has much influence on the braking torque, and individual current supply results in different power consumption and torque ranges. Moreover, the dynamic response performance of this brake is less affected by the slip speed. Furthermore, the maximum steady-state slip power of the proposed brake is about 160 W, and the greater the slip power is, the faster the temperature increases. The results also have verified the correctness of the structure and magnetic circuit design.     

Electric and magnetic fields of a wave in proximity of aprogressively phased periodic structure

Unexpected waves have been observed that were localized in the region of, and propagating along, a progressively phased periodic structure. These waves are related to a dominant TM₀ mode associated with and bound to a thin dielectric layer covering a conducting plane     

Near-field investigation of uniform periodic monopole arrays

Measurements of amplitude and phase of the fields close to a number of driven periodic arrays of monopoles have been made to determine frequency dependence of the propagation constant for such structures. The results have been analyzed, where possible, in terms of a single traveling wave along the structure, and dispersion curves are given which include attenuation as well as phase shift. These curves clearly illustrate the possibility of obtaining leading phase shift along the periodic structure that will produce backfire radiation. Experimental radiation patterns corroborate the near-field results. A simple theory is given for the backfire phasing in terms of the space-harmonic content of a Fourier series expansion for the fields of periodic structures. This theory predicts beam scanning for the periodic structures which is readily observed in the experimental models. The measured data are approximately applicable to tapered versions of the periodic structures which have log-periodic geometry. Examples of periodic structures that have frequency independent log-periodic counterparts as well as some that do not are discussed.     

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.     

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     

Imaging and detection of periodic, 5-100 Hz magnetic fields with epitaxial rare earth-iron garnet films

Sinusoidal magnetic fields with strengths as small as ~0.5 nT (RMS) and frequencies of 5 -100 Hz have been detected at room temperature by modulating the magneto-optical Kerr response of epitaxial rare earth-iron garnet films. Ferromagnetic nanoparticles with a mean size of 500 nm have also been detected in real time in a liquid flow stream at field strengths as low as 1 muT.     

Interband light absorption in size-confined systems in uniform electric fields

A simple approach to calculation of the interband absorption coefficient in a uniform electric field is developed. This approach provides a means for studying the special features of electroabsorption in a wide class of semiconductor systems on the basis of the most general relationships. The approach is used to study the electroabsorption in two-dimensional systems with different profiles of their one-dimensional potential, quantum wells, and superlattices in magnetic fields.     

Electron-beam metrology and inspection

Production of integrated circuits necessitates pattern inspection and measurement of critical dimensions to guarantee yield and quality. The shrinking of pattern size in microelectronics requires progressive employment of electron-beam techniques for this purpose. GaAs technology and integrated optics pose additional challenges.Critical dimensions, e.g. linewidth and pitch, as well as placement accuracy of patterns on wafers or masks are to be examined. The measurement accuracy of any e-beam technique is influenced by the quality of the instrument but also by the process of signal formation and by charging effects. High- and low-voltage operation are possible, both having a certain range of applications. Low-voltage operation requires the electron optics to be adapted, but minimizes charging and damage. The algorithms of peak detection, threshold and automatic threshold crossing as well as linear regression may be used for detection of the pattern edges.Inspection is necessary to monitor pattern fidelity, and in an automated form it is intended to be utilized for defect detection. Pattern-to-pattern comparison, pattern-to-data comparison, and feature extraction are the techniques which have been investigated so far.     

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.     

Sampling theorems for uniform and periodic nonuniform MIMO sampling of multiband signals

We examine a multiple-input multiple-output (MIMO) sampling scheme for a linear time-invariant continuous-time MIMO channel. The input signals are modeled as multiband signals with different spectral supports, and the channel outputs are sampled on either uniform or periodic nonuniform sampling sets, with possibly different but commensurate intervals on the different outputs. This scheme encompasses Papoulis' generalized sampling and several nonuniform sampling schemes as special cases. We derive necessary and sufficient conditions on the channel and the sampling rate that allow stable perfect reconstruction of the inputs or, equivalently, perfect inversion of the channel. From an implementation viewpoint, we note that it is desirable that the reconstruction filters have continuous frequency responses. We derive necessary and sufficient conditions that guarantee this continuity property. The frequency responses of the reconstruction filters are specified as solutions to a system of linear equations. Finally, we demonstrate that perfect reconstruction may be possible, even when the channel outputs are sampled at an average rate that does not allow the reconstruction of any output from its samples alone. In certain instances, this average rate can achieve the recently presented fundamental bounds on MIMO sampling density.     

Electron-beam microcolumn fabrication and testing

In this paper, we report the improvement of a process for full microfabrication of miniaturized electron lenses specially design for low-energy (100 eV) lithography tools. The main advantages of this technique are the following. It is batch processing oriented, meaning that lenses can be easily built in a full wafer fabrication. With this procedure it is possible to develop a completely integrated process for machining arrays of lenses. Lens bores are aligned using an electron lithography process, resulting in highly accurate positioning. Finally, the source lens chip has not only one but several sets of lenses with different aperture-sizes, each producing a different beam diameter. A scheme is proposed with which the appropriate lens can be selected by means of a deflection system.     

Finite element solution of time-harmonic modal fields in periodic structures

A weighted residual FEM formulation suitable for the electromagnetic model analysis of periodic structures to give numerical k- beta relationships and field patterns is introduced. A two-dimensional realisation of the formulation applying to open-sided ridged waveguides agrees well with analysis. The general method described could be applicable to practical travelling wave tube slow wave structures.<>     

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.”