Turbulent diffusion of magnetic fields in two-dimensional magnetohydrodynamic turbulence with stable stratification

We calculate the correction, due to nonlinear wave-wave interactions, to the Zel'dovich estimate for the turbulent diffusivity of magnetic fields in a model of two-dimensional magnetohydrodynamic turbulence in the presence of stable stratification. Such a model has some relevance to hydromagnetic turbulence in stellar interiors. The significance of this correction is that, unlike the lowest-order Zel'dovich balance, it is independent of the molecular resistivity eta and so will not vanish in the limit of a large magnetic Reynolds number, although the correction is O(sigma;{4}), where sigma is the wave slope, which necessarily is small. Thus, we are led to the counterintuitive result that the presence of stable stratification can actually increase the vertical flux of magnetic fields relative to that in 2D MHD turbulence without stratification.     

Flow, diffusion, and thermal convection in percolation clusters: NMR experiments and numerical FEM/FVM simulations.

Percolation objects were fabricated based on computer-generated, two- or three-dimensional templates. Random-site, semi-continuous swiss cheese, and semi-continuous inverse swiss-cheese percolation models above the percolation threshold were considered. The water-filled pore space was investigated by NMR imaging and, in the presence of a pressure gradient, NMR velocity mapping. The fractal dimension, the correlation length, and the percolation probability were evaluated both from the computer-generated templates and the corresponding NMR spin density maps. Based on velocity maps, the percolation backbones were determined. The fractal dimension of the backbones turned out to be smaller than that of the complete cluster. As a further relation of interest, the volume-averaged velocity was calculated as a function of the probe volume radius. In a certain scaling window, the resulting dependence can be represented by a power law the exponent of which was not yet considered in the theoretical literature. The experimental results favorably compare to computer simulations based on the finite-element method (FEM) or the finite-volume method (FVM). Percolation theory suggests a relationship between the anomalous diffusion exponent and the fractal dimension of the cluster, i.e., between a dynamic and a structural parameter. We examined interdiffusion between two compartments initially filled with H2O and D2O, respectively, by proton imaging. The results confirm the theoretical expectation. As a third transport mechanism, thermal convection in percolation clusters of different porosities was studied with the aid of NMR velocity mapping. The velocity distribution is related to the convection roll size distribution. Corresponding histograms consist of a power law part representing localized rolls, and a high-velocity cut-off for cluster-spanning rolls. The maximum velocity as a function of the porosity clearly visualizes the percolation transition.     

Simultaneous measurements of thermal diffusivity, thermal conductivity and thermopower with application to copper and ceramic superconductors

A home made experimental set-up allows us to measure the thermal conductivity, the thermopower and the thermal diffusivity simultaneously in the temperature range (20-300 K). Therefore the specific heat can be deduced. The role of a radiation shield is shown to be relevant. Tests of the system are made on a 99.9% pure Cu sample and two polycrystalline cuprate ceramics for illustration. Without any complicated optimisation, the technique indicates much promise already due to its efficiency and rapidity. Received 9 November 1998 and Received in final form 12 July 1999     

On the kinetic theory of diffusion,self-diffusion and thermal diffusion in adsorbed gases

Phosphorescence microwave double resonance experiments are reported on Zn porphin at 1·2 K. In glassy solution very broad resonance transitions are observed. However, for Zn porphin in a crystalline n-octane matrix—a system known for its sharp optical spectra (Shpolskii effect)—three pairs of microwave transitions with widths of a few MHz are found, all of them corresponding to a decrease in phosphorescence intensity. By studying the behaviour of the signals for various methods of preparation of the sample and as a function of the optical bandwidth of excitation and detection, one pair of transitions could be assigned to monomeric solute molecules. The corresponding zero-field splittings are |X - Z| = 1355, |Y - Z| = 806 MHz. It was further established that by ‘pumping’ either of these transitions a third one can be detected at the difference frequency, so that the order of the levels must be X> Y>Z (or reverse). The results indicate that the molecule no longer possesses a four-fold axis in the excited state. What one observes must be the lowest vibronic level of a Jahn-Teller unstable state, the degeneracy of which is removed by an anisotropic crystal field. From some preliminary E.S.R. experiments on zinc porphin in an EPA glass at 77K it is concluded that Z corresponds to that component where the angular momentum lies in the molecular plane; for a ππ* electronic state this is expected to be the lowest in energy of the three.     

BER analysis of BPSK-SIM-based SISO and MIMO FSO systems in strong turbulence with pointing errors

Free space optics (FSO) is one of the sprouting technologies in optical communication systems domain. It can be employed as an alternative for the conventional radio frequency (RF) links to work out the current limitations in communication systems. But, the major drawback in FSO communication is the effect of random environment conditions on its performance. In this paper, we analyze the bit error rate (BER) and outage performance of single-input single-output (SISO) and multiple-input multiple-output (MIMO) FSO systems in strong atmospheric turbulence using binary phase shift keying subcarrier intensity modulation (BPSK-SIM) signaling technique. The closed-form expressions are derived and the results are realized in terms of 2D and 3D plots.     

Rapid thermal diffusion of aluminum in silicon and its interaction with phosphorus

Aluminum diffusion profiles in silicon were produced under rapid thermal processing conditions using a Si-sandwich structure to avoid losses of aluminum by reaction with the quartz reactor. The impact of phosphorus on the diffusion behaviour of aluminum was investigated by predeposition of P into Al-diffused wafers and vice versa. Dopant profiles were determined by SIMS (Secondary-Ion-Mass Spectroscopy), electrochemical CV-and SRP (Spreading Resistance) analysis. The resulting profiles after Al predeposition at 1293 K exhibit high Al-surface concentrations up to the solid solubility limit of about 2×10²⁵ m^–3. It is shown that phosphorus has a great influence on the drivein behaviour of Al, leading to an accelerated Al diffusion in front of the P profile (enhanced Al diffusion caused by self-interstitial supersaturation) and an up-hill migration of Al in the high-concentration regime, which can be explained by field-assisted diffusion. A strong retardation of aluminum diffusion combined with concentrations well above the solidsolubility limit was observed during the Al predeposition into P-diffused wafers.     

Diffusion and thermal diffusion in binary mixtures of sulphur hexafluoride with noble gases

Diffusion coefficients and thermal diffusion factors are reported for binary mixtures of sulphur hexafluoride with noble gases. The results are compared with theoretical values calculated by means of the Chapman-Enskog theory, spherical potentials for the like interactions and multi-parameter anisotropic potentials for the unlike interactions.     

The formation and thermal stability of multilayer ohmic contacts to n-GaAs with TiBx and Mo diffusion barriers

Thermal degradation of Au/Mo/TiB_x/AuGe multilayer ohmic contacts with Mo and TiB _x diffusion barriers was studied. The contacts were employed in Gunn-effect diodes. Depth profiling of the components in the contacts was performed using Auger electron spectroscopy. The microrelief of the metal/semiconductor interface and contact surface morphology were examined with atomic force microscopy and scanning electron microscopy, respectively. The measurements were taken before and after argon annealing at T=400, 600, or 800°C for 60 s. The resistance of the Gunn diode mesa was also measured. Annealing at 400°C is shown not to affect the sandwich structure of the contacts. Annealing at 600°C causes structure rearrangement in the layers up to cracking. It is found that the thermal threshold of degradation of the Au/Mo/TiB_x /AuGe/GaAs structure depends on the resistance of the TiB_x layer to thermal effects. Reasons for the degradation of Mo and TiB_x antidiffusion properties are discussed.     

超强激光功率密度对等离子体中自生磁场和电子热传导的影响

应用多光子非线性康普顿散射模型、3维粒子模拟模型和数值计算方法,研究了超强激光与等离子体作用中自生磁场产生和电子热传导过程,提出了将非线性康普顿散射光作为改变等离子体自生磁场和电子热传导的新机制,给出了自生磁场最大饱和值和超热电子热传导的修正方程和数值计算结果。研究发现在时间为100~160范围内,自生磁场能量随入射激光功率密度增大而迅速增大,之后处于较高饱和阶段。增大的初始时刻较散射前提前了20,增大阶段的时间延长了30,饱和阶段增幅为40。入射激光功率密度为1019~1020W/cm2时,自生磁场强度最大模拟值为1.47104~3.75104T,单电子能谱峰值出现在3.3MeV和6.6MeV附近,能谱曲线在4~15 MeV和11~14.3MeV范围迅速衰减,在6.7MeV和13.2MeV以上时,超热电子有效温度为2.6MeV和4.5MeV,比无散射的理论值和拟合值均有一定增大。随入射激光强度增大,热流随激光脉冲一起向等离子体内流动的时间缩短,自生磁场限制热流的时间延长。并对所得结果给出了初步物理解释。     

Anisotropy of the heat conduction,thermal emf,and Lorentz number in degenerate semimetals at low temperatures

The anisotropy of the heat conduction, thermal emf, and Lorentz number of degenerate semimetals at low temperatures, whose carriers possess an anisotropic energy spectrum, is investigated. It is shown that the anisotropy of heat conduction decreases with the reduction in temperature. At ultralow temperatures the diffusion thermal emf decreases proportionately to the temperature. The insignificant anisotropy of the thermal emf is due to the inelasticity of the electron collision.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–53, July, 1979.     

The Broadening of He I lines including ion dynamic corrections,with application to λ4471Å

A theory is developed for the broadening of He I lines with forbidden components which is valid at low densities (where the forbidden line is reasonably well isolated). This theory takes into account the effects of dynamic ion broadening. Application to λ4471Å gives good agreement with experiment and with a recent calculation of Lee; reasons for agreement with Lee are discussed in detail. Comprehensive tables for 4471Å are presented. These have been extended further into the line wings than previous tabulations. Static ion theory is found to be adequate for densities greater than ～ 5 × 10¹⁵ cm^?3. A simple numerical profile for allowed lines, taking into account dynamic ion effects, is also presented.     

Two-dimensional field diffusion and magnetohydrodynamic flow in an electric explosion of a miniature single-turn solenoid in a megagauss magnetic field

Experimental and numerical studies are reported of high-temperature magnetohydrodynamic flows occurring in the explosion of miniature single-turn solenoids in an ultrastrong pulsed magnetic field with an amplitude of 300–400 T and a risetime of about 100 μs. Experiments with exploding single-turn solenoids (wall thickness of the order of 1 mm) using pulsed x-ray photography have revealed that the onset of deformation of the solenoid is delayed in fields up to 300–350 T and the initial geometric factor of the coil remains the same during the time taken by the current to rise to its maximum value. These phenomena are described for the first time in terms of the standard interpretation of the strength of a metal within the two-dimensional magnetohydrodynamic flow model with finite conductivity. Zh. Tekh. Fiz. 67, 32–47 (April 1997)     

Turbulent statistical characteristics associated to the north wind phenomenon in southern Brazil with application to turbulent diffusion

A parameterization for the transport processes in a shear driven planetary boundary layer (PBL) has been established employing turbulent statistical quantities measured during the north wind phenomenon in southern Brazil. Therefore, observed one-dimensional turbulent energy spectra are compared with a spectral model based on the Kolmogorov arguments. The good agreement obtained from this comparison leads to well defined formulations for the turbulent velocity variance, local decorrelation time scale and eddy diffusivity. Furthermore, for vertical regions in which the wind shear forcing is relevant, the eddy diffusivity derived from the north wind data presents a similar profile to those obtained from the non-extensive statistical mechanics theory. Finally, a validation for the present parameterization has been accomplished, using a Lagrangian stochastic dispersion model. The Prairie Grass data set, which presents high mean wind speed, is simulated. The analysis developed in this study shows that the turbulence parameterization constructed from wind data for north wind flow cases is able to describe the diffusion in a high wind speed, shear-dominated PBL.     

基于OpenFOAM的投影法磁流体求解器开发与验证

在开源计算流体力学C++工具包OpenFOAM环境下开发了低磁雷诺数条件下的磁流体求解器,并进行了验证。采用投影算法求解动量方程和压力泊松方程;采用非结构网格同位相容守恒算法求解电势泊松方程、感应电流和洛伦兹力;采用边界耦合方法求解流固耦合电势场。通过对均匀磁场下导电方管和导电圆管内的完全发展磁流体层流的数值模拟和解析解的对比,对求解器进行了验证。进一步对非均匀强磁场作用下导电方管和导电圆管内完全发展磁流体层流进行了数值模拟,并与ALEX实验结果进行了比较。数值解和实验结果吻合良好。所开发的求解器可用于复杂结构强磁场作用下磁流体的数值模拟研究。     

The Nernst-Ettingshausen coefficient in conductors with a narrow conduction band: Analysis and application of its results to HTSC materials

A theoretical analysis is made of the Nernst-Ettingshausen coefficient Q for the case of a narrow conduction band present in the band spectrum of a material. It is shown that the presence of such a band results in a qualitative change in the mechanism responsible for the Nernst-Ettingshausen effect as compared to the classical case of a broad conduction band and that the behavior of this coefficient reveals a number of specific features that are different from the case of the classical theory of transport coefficients in semiconductors and metals. It is demonstrated that the pattern of the Q(T) relation in the case of a narrow band is drastically affected by the asymmetry of the dispersion curve, whereas the other features of the band spectrum and of the properties of the carrier system, including the character of the energy dependence of the relaxation time, are less significant and, in a first approximation, can be disregarded. The calculated Q(T) curves are in qualitative agreement with the experimental relationships obtained for doped HTSCs of the YBa₂Cu₃O_y system. The possibility of using this approach for a complex analysis of the experimental temperature dependences of the four transport coefficients in the normal phase of HTSC materials is demonstrated.     

Preparation of cold ions in strong magnetic field and its application to gas-phase NMR spectroscopy

Nuclear Magnetic Resonance (NMR) technique is widely used as a powerful tool to study the physical and chemical properties of materials. However, this technique is limited to the materials in condensed phases. To extend this technique to the gas-phase molecular ions, we are developing a gas-phase NMR apparatus. In this note, we describe the basic principle of the NMR detection for molecular ions in the gas phase based on a Stern-Gerlach type experiment in a Penning trap and outline the apparatus under development. We also present the experimental procedures and the results on the formation and the manipulation of cold ions under a strong magnetic field, which are the key techniques to detect the NMR by the present method.     

High precision thermal modeling of complex systems with application to the flyby and Pioneer anomaly

Thermal modeling of complex systems faces the problems of an effective digitalization of the detailed geometry and properties of the system, calculation of the thermal flows and temperature maps, treatment of the thermal radiation including possible multiple reflections, inclusion of additional external influences, extraction of the radiation pressure from calculated surface data as well as computational effectiveness. In previous publications [1, 2] the solution to these problems have been outlined and a first application to the Pioneer spacecraft have been shown. Here we like to present the application of our thermal modeling to the Rosetta flyby anomaly as well as to the Pioneer anomaly. The analysis outlines that thermal recoil pressure is not the cause of the Rosetta flyby anomaly but likely resolves the anomalous acceleration observed for Pioneer 10.     

Interaction of a laser with a qubit in thermal motion and its application to robust and efficient readout

We present a detailed theoretical and experimental study on the optical control of a trapped-ion qubit subject to thermally induced fluctuations of the Rabi frequency. The coupling fluctuations are caused by thermal excitation on three harmonic oscillator modes. We develop an effective Maxwell–Boltzmann theory which leads to a replacement of several quantized oscillator modes by an effective continuous probability distribution function for the Rabi frequency. The model is experimentally verified for driving the quadrupole transition with resonant square pulses. This allows for the determination of the ion temperature with an accuracy of better than 2% of the temperature pertaining to the Doppler cooling limit T _D over a range from 0.5T _D to 5T _D . The theory is then applied successfully to model experimental data for rapid adiabatic passage (RAP) pulses. We apply the model and the obtained experimental parameters to elucidate the robustness and efficiency of the RAP process by means of numerical simulations.     

Renormalization of phonon frequencies by conduction electrons in layer systems with application to YBa2Cu3O7

It is generally assumed that there are free electronic carriers associated with the CuO₂ planes in high-temperature superconductors. Regarding these carriers as confined to these planes their influence on phonon frequencies is studied within the framework of a shell model of lattice dynamics taking into account the long-range part of the electron-phonon coupling. We find a complete suppression of the LO-TO splitting for modes polarized along the planes by metallic screening and a strong coupling of electronic plasmon and particle-hole excitations especially to some To-phonon branches polarized perpendicular to the conducting planes, particularly pronounced in systems with at least two CuO₂ planes per unit cell. Numerical results are presented for YBa₂Cu₃O₇.