The motion of negative ions in liquid helium in strong electric fields

The motion of negative ions in liquid helium at temperatures above 0.6 K is governed principally by the scattering of rotons. The mobility is defined as the ratio of the drift velocity,v to the electric fieldE for small electric fields. In stronger electric fields the ratioE/v usually increases as the velocity increases. However, it has been found recently that for negative ions under high pressures the ratioE/v decreases asv increases. This apparently anomalous behavior is explained by extending a theory of the low-field ionic mobility to higher drift velocities corresponding to stronger electric fields. Good quantitative agreement is then found with the recent experiments of McClintock and co-workers for drift velocities up to about3 m/sec.     

Thermal rectification in three-dimensional asymmetric nanostructure

Previously, thermal rectification has been reported in several low-dimensional shape-asymmetric nanomaterials. In this Letter, we demonstrate that a three-dimensional crystalline material with an asymmetric shape also displays as strong thermal rectification as low-dimensional materials do. The observed rectification is attributed to the stronger temperature dependence of vibration density of states in the narrower region of the asymmetric material, resulting from the small number of atomic degrees of freedom directly interacting with the thermostat. We also demonstrate that the often reported "device shape asymmetry" is not a sufficient condition for thermal rectification. Specifically, the size asymmetry in boundary thermal contacts is equally important toward determining the magnitude of thermal rectification. When the boundary thermal contacts retain the same size asymmetry as the nanomaterial, the overall system displays notable thermal rectification, in accordance with existing literature. However, when the wider region of the asymmetric nanomaterial is partially thermostatted by a smaller sized contact, thermal rectification decreases dramatically and even changes direction.     

Comparison of AC Losses of YBCO Circular Pancake Coils and Infinitely Long Stack Approximation

High-temperature superconducting (HTS) coils are key parts of many large-scale AC applications, such as transformers, superconducting magnetic energy storage, and motors. The estimation of AC losses of pancake coils is necessary for optimizing the design of HTS devices and cryogenic systems. To speed up the computation of AC losses, the numerical model of an infinitely long HTS stack is often utilized. An HTS stack is a good approximation of the circular coil only if the coil radius is sufficiently large, since AC losses will exhibit a stronger asymmetry along the radial direction for small values of coil radii. To assess the validation of an infinitely long stack approximation, the comparisons of AC losses between infinitely long stacks and circular coils with different radii are presented. The turn number varies from 10 to 80. We find that the AC losses of HTS circular coils will gradually increase to the same value as an infinitely long stack with increasing coil radii. A new parameter is proposed to quantitatively describe the correctness of infinitely long stack approximation. Finally, a method of AC loss estimation of HTS pancake coils is proposed.     

Drift velocity and deflection in non-perpendicular electric and magnetic fields

Accurate tracking in inhomogeneous and arbitrarily oriented magnetic fields requires the velocity and direction of the drifting ionization to be known to high precision. A semi-empirical method of calculating drift velocities and magnetic deflection angles is presented for non-perpendicular electric and magnetic fields. Data obtained using a drift chamber rotated in a B-field verify the calculation. The model is used to restore accuracy in the L3 forward/backward muon detector where non-orthogonal and inhomogeneous electric and magnetic fields are present.     

Solutions for a viscoelastic axisymmetric plane problem involving time-dependent boundary regions under mixed boundary condition

The stress and displacement fields for a viscoelastic axisymmetric plane problem involving time-dependent boundary regions under mixed boundary condition are presented in this paper. The viscoelastic fields are determined by solving two unknown functions; one is governed by a resulting second kind Volterra integral equation from the stress condition at the outer radius of an annular region, which is dependent on the other by the displacement condition at the inner radius. The integral equation has an analytical solution for the case of an infinite region with large enough outer radius while it can be solved using a numerical integral method for the case of a finite region. Numerical examples for the Boltzmann viscoelastic model are given, and the responses of the displacement and stresses are presented in detail to illustrate the effects of velocity of change in the inner radius and the magnitude of the outer radius. Meanwhile, the effect of the aspect ratio (void concentration parameter) on the viscoelastic fields is presented. When the ratio of the outer radius to the inner radius is large enough, the responses can be evaluated by directly adopting the resulting analytical solution to avoid a complex numerical procedure. The resulting solutions and computational results are helpful to a better understanding of mechanical behaviors for (large) excavation or finite void growth in viscoelastic media.     

Analysis of minor loop behavior during alternating flux reversal in rectangular-loop toroidal cores

The analysis of minor hysteresis loop behavior during partial alternating flux reversal in toroidal cores with a rectangular hysteresis loop is considered. A simple flux-reversal model is proposed on the basis of which, with the aid of dimensionless units, the asymmetry of minor hysteresis loops is derived. The drift of minor hysteresis loops during alternating flux reversal is investigated on the basis of loop asymmetry and by means of a proposed indirect method. The method involves compensation of the minor-loop drift by means of a dc component of the exciting field, the magnitude and sense of which are a measure of the drift. The experimental section deals partly with a qualitative comparison of the character of the actual asymmetry of a minor loop with that derived from the flux reversal model, and partly with comparing the actual minor-loop drift to the drift derived theoretically on the basis of the indirect method. Experimental results indicate that in the case of the 50 percent nickel-iron alloy, cold-rolled at a great reduction, there appear additional factors which affect the instability of the minor loop. This effect is further investigated, and a simplified mechanism is proposed for its stimulation. Finally, the proposed mechanism is verified experimentally.     

On certain regimes of inclusion drift in acoustic fields

The inclusion drift in acoustic fields is investigated by analytical methods. A formula for the total force acting on a spherical inclusion with allowance for the compressibility of the carrier phase and the inclusion is obtained. A formula for the frequency after which the total force changes its direction is derived. The total-force directional diagram proposed by the authors earlier is refined.     

气泡群振荡及噪声数值模拟研究

针对水中气泡群振荡及辐射噪声问题,基于均质混合流模型,考虑气泡动力学作用,建立数学模型;并采用拉格朗日有限体积法,对气泡群振荡过程及辐射噪声进行数值模拟。重点研究了初始气泡半径、含气率及气泡群尺度等对振荡辐射噪声的影响。结果表明,随着初始含气率、气泡群尺度增大,气泡辐射噪声频率逐渐减小、声压幅值逐渐增大即声强增大;初始气泡半径对辐射噪声频率影响较小,但声压幅值随其增大而减小。     

Asymmetry parameter and aggregate particles

We derive and examine the general expression for the scattering asymmetry parameter g. For aggregate particles, the asymmetry parameter is made up of two terms. One term accounts for interference effects of the electromagnetic fields radiating from the individual subsystems. The other term contains the effects of the interaction of the electromagnetic fields between these subsystems. Enhanced backscatter is one phenomenon resulting from these interactions. Numerical results demonstrate that interference effects play a dominant role when the separation distance between two-sphere aggregates is smaller than half the incident wavelength. As the separation distance becomes large, both interference and interaction effects drop off and the asymmetry parameter approaches that of the individual particle constituents.     

Measurements of the inhomogeneities of magnetic fields in a sector-type of slim electromagnet

A sector-type of slim electromagnet with iron pole pieces has been fabricated to obtain some precise information about the inhomogeneities of magnetic fields. Neither the eddy-current effect nor magnetic aftereffect, which are associated with the setting procedure and setting speed of the magnetic field, are observed in the present measurements. Sources producing field inhomogeneities along a central ray in the pole pieces with a sharp-cornered profile are the local saturation and hysteresis effect. The hysteresis effect is related to the magnetization-demagnetization process and is observed under a measuring precision better than 10^-4. The magnetic field distribution changes with the field strength independent of the magnetization-demagnetization process. Sources producing field inhomogeneities along a radial direction in the pole pieces with the sharp-cornered profile are the local saturation and the difference between leakage fluxes in pole-piece sides with the small and large curvature radii, which is due to the magnetic asymmetry of the pole-piece geometry along the radial direction. The B-constant design of pole pieces has excellent effects in avoiding field inhomogeneities due to the aforementioned sources, although the small cycling effect remains in field inhomogeneities along the central ray.     

Optimization of zero-reference grating considering tip distortions for scanning probe microscopy drift measurement

A nonperiodic grating composed of unevenly distributed step elements on a flat substrate can serve as the calibration grating for scanning probe microscopy (SPM) drift measurements. The grating named zero-reference grating (ZRG) is characterized by the number of step elements t, the number of total grating elements n and the maximum correlation peak contrast D. To optimize the ZRG structures considering tip distortions in SPM images, mathematical morphology and genetic algorithms are adopted. Results show that the optimal step elements tend to distribute orderly under the influence of large tip radius and the contrast D decreases with the increase of tip radius when t < or = n/2. On the contrary, the optimal ZRG structures and the contrast D are insensitive to tip distortions when t > n/2. These differences can be interpreted by the effective code mechanism due to tip dilation. For quantitative drift measurements, the parameter t is suggested to be larger than n/2 since the performances of such optimal ZRGs are more robust to tip distortions.     

Calibration of Coercive and Stray Fields of Commercial Magnetic Force Microscope Probes

Variable-field magnetic force microscope (MFM) is introduced to characterize the magnetic behavior of commercially available MFM probes that is relevant to interpret MFM imaging. A Nanotec Electronica S.L. microscope has been conveniently modified to apply magnetic fields in axial direction (up to 1.5 kOe) and in-plane direction (up to 2.0 kOe). Axial and transeverse hysteresis loops of the probes have been generated by measuring the changes in the MFM contrast observed when the magnetic field is applied. The variation of the MFM signal is ascribed to the modification of the magnetic state of the tips. This is enabled by the large coercitivity (~1.7 kOe) of the checked longitudinal recording media. The properties of the probes depend on the coating material, the macroscopic tip shape, and tip radius. In only a few cases, the magnetization of the probe can be oriented along in-plane orientation. In addition, the stray field of the tips has been deduced by measuring the influence of the probe in the magnetic state of the checked samples.     

Design and construction of the CDF central tracking chamber

We describe the design and construction of a large drift chamber of a novel design well adapted for operation in high magnetic fields and in the high track density environment of hadron colliders.     

Chiral pattern selection induced by a fokker-planck diffusion law

It is pointed out that in a reaction-diffusion system near a chiral symmetry-breaking instability, a weak drift term in the diffusion law can induce a systematic selection between macroscopic three-dimensional patterns of opposite chirality. When the cause of the drift term is an external field, the orientation of the field with respect to the system determines the final chiral choice. The possible appearance of a drift term in the diffusion law, arising from the intrinsic asymmetry of the underlying material is also considered. In a one-dimensional system with periodic boundary conditions it is shown that the drift term may account for systematic selection between waves rotating in opposite directions     

Quality judgments of technical fields: Bias,marginality, and the role of the elite

Most accounts of scientific and technological development stress the importance of quality judgments for particular technical fields. This study investigates social psychological and structural factors associated with such judgments for nineteen fields in nuclear waste and solar cell research. The results of the analysis indicate a tendency toward positive bias for fields in which researchers have been active, for this bias to be stronger in less innovative fields, and for elite membership to affect this bias in different ways depending on the nature of the system. In addition, there was no tendency for those with a high level of social contacts to others working in a field to display a positive bias, except in consensually innovative fields.     

Local optical parameters of spherical polydispersions: simple approximations

New analytical solutions for the local optical characteristics (extinction and absorption coefficients, asymmetry parameters of phase functions) of spherical polydispersions composed of comparatively large particles are derived. The geometric optics (GO) approximation is used to solve the problem. For the accuracy of the GO approximation to be improved, the edge effects were taken into account. A comparison with the data obtained by the use of the Mie theory shows a satisfactory accuracy of our analytical formulas. The simple formulas for the cloud local optical characteristics are derived.     

Tilted toroidal coils for superconducting magnetic energy storage systems

Superconducting magnetic energy storage (SMES) systems can be used to improve power supply quality and reliability. In addition, large amounts of power can be drawn from a small stored energy supply. Nevertheless, the strong electromagnetic force caused by high magnetic fields and coil current is a serious problem for SMES. To cope with this problem, we propose a new coil design, the tilted toroidal coil (TTC). The TTC, obtained from the toroidal field coil (TFC) system by varying two pitching angles, allows the balancing of the electromagnetic force in the major radius direction, maintaining the same manufacturing simplicity of the TFC system. After determining balanced configurations through computer simulation, we built an experimental device to confirm the feasibility of the balancing effect.     

Photonic-Band Structures in Macroporous Silicon

Macroporous Si was prepared by electrochemical etching at a linearly varying applied voltage. A relationship between the process parameters (illumination intensity, current density, and applied voltage) was derived in a diffusion–drift model. The experimental dependences of the ratio of the photohole concentration to the current density on the distance between the illuminated surface and pore bottom were shown to agree with the diffusion–drift model for nonequilibrium-hole transport provided that the anode thickness exceeds the diffusion length of holes and the pore radius is comparatively large. The photonic band gap of the two-dimensional macroporous Si structure was calculated by the plane-wave method. The transmittance of macroporous Si was measured. The effect of surface recombination on the lifetime of nonequilibrium charge carriers was assessed.     

Diffusion positive column of electric discharge in transverse magnetic field

The effect of a transverse magnetic field on the characteristics of planar diffusion positive column of electric discharge has been studied. It is shown that, as the magnetic induction increases, the distributions of plasma density and particle fluxes to walls become asymmetric; the density maximum shifts in the direction of Ampere’s force action, and the ion flux in this force direction can significantly exceed the reverse flux. It is established that there is a maximum value of magnetic induction, which bounds from above the region of magnetic fields in which a stationary state of the positive column is possible. In the region where a stationary state of the positive column is possible, each value of the magnetic induction corresponds to two positive-column regimes with different values of the electron energy, drift velocity, and electric field strength.