海面大气边界层中聚焦光束漂移各向异性的实验研究

利用位置敏感型光电倍增管（PSPMT）,设计了测量聚焦光束漂移的实验装置。该装置在80 mm的有效探测面内实现了2维位置信号的直接探测,空间分辨率可达1 mm,明显优于四象限探测器和阵列探测器,最高采样频率可达80 kHz,且动态范围很大,优于一般的成像器件。在近海岸海面上5 m处的大气边界层中进行了距离为1 000 m的聚焦激光传输实验。测量结果表明：聚焦光斑的质心漂移具有各向异性,水平方向光斑漂移幅度一般介于5.61 mm和14.83 mm之间,垂直方向光斑漂移幅度介于3.54 mm和7.3 mm之间,两者之比的平均值为1.69;水平方向和垂直方向的光斑漂移功率谱密度（PSD）在低频段也存在差异,垂直方向光斑漂移的PSD比水平方向光斑漂移的PSD下降速率更快。     

海面大气边界层中聚焦光束漂移各向异性的实验研究

 利用位置敏感型光电倍增管（PSPMT）,设计了测量聚焦光束漂移的实验装置。该装置在80 mm的有效探测面内实现了2维位置信号的直接探测,空间分辨率可达1 mm,明显优于四象限探测器和阵列探测器,最高采样频率可达80 kHz,且动态范围很大,优于一般的成像器件。在近海岸海面上5 m处的大气边界层中进行了距离为1 000 m的聚焦激光传输实验。测量结果表明：聚焦光斑的质心漂移具有各向异性,水平方向光斑漂移幅度一般介于5.61 mm和14.83 mm之间,垂直方向光斑漂移幅度介于3.54 mm和7.3 mm之间,两者之比的平均值为1.69;水平方向和垂直方向的光斑漂移功率谱密度（PSD）在低频段也存在差异,垂直方向光斑漂移的PSD比水平方向光斑漂移的PSD下降速率更快。     

A new model for the cross spectrum and wavenumber-frequency spectrum of turbulent pressure fluctuations in a boundary layer

A model is developed for the statistical characteristics of the turbulent pressure field in a boundary layer. It is shown that the coherence scales are limited at low frequencies by the finiteness of the boundary layer thickness and at high frequencies by the effect of viscous forces acting in the flow. A relationship between the behavior of the coherence scales and that of the power spectra is demonstrated. The model is characterized by convertibility, which allows changes from cross spectra to wavenumber-frequency spectra and back via Fourier transformations. Calculations performed using the proposed model agree well with experimental results.     

Chiral fluctuations in MnSi above the Curie temperature

Polarized neutrons are used to determine the antisymmetric part of the magnetic susceptibility in noncentrosymmetric MnSi. The paramagnetic fluctuations are found to be incommensurate with the chemical lattice and to have a chiral character. We argue that antisymmetric interactions must be taken into account to properly describe the critical dynamics in MnSi above T(C). The possibility of directly measuring the polarization dependent part of the dynamical susceptibility in a large class of compounds by polarized inelastic neutron scattering is outlined as it can yield evidence for antisymmetric interactions such as spin-orbit coupling in metals as well as in insulators.     

Prediction of the acoustic pressure above periodically uneven facings in industrial workplaces

The aim of this work is to predict sound pressure in front of wall facings based on periodic sound scattering surface profiles. The method involves investigating plane wave reflections randomly incident upon an uneven surface. The waveguide approach is well suited to the geometries usually encountered in industrial workplaces. This method simplifies the profile geometry by using elementary rectangular volumes. The acoustic field in the profile interstices can then be expressed as the superposition of waveguide modes. In past work, walls considered are of infinite dimensions and are subjected to a periodic surface profile in only one direction. We therefore generalise this approach by extending its applicability to “double-periodic” wall facings. Free-field measurements have been taken and the observed agreement between numerical and experimental results supports the validity of the waveguide method.     

Saturation level of the intensity fluctuations of an optical beam in a turbulent atmosphere

It is shown that the saturation level of the variance of the intensity fluctuations of a coherent light beam propagating In the atmosphere depends to a considerable extent on the behavior of the high-frequency part of the fluctuation spectrum of the refractive index of the air.     

The energy spectrum of an electron beam after interaction with an RF field in a gyrotron

The electron distribution over residual energy after electron interaction with an rf field was determined using the method of decelerating electric field. It is demonstrated that, under the maximum efficiency conditions, electrons with an energy lower than 30% of their initial energy are absent in the spectrum. Hence, the efficiency of energy-recuperation gyrotrons can substantially be increased.     

Electronic absorption spectrum of CO+ in an ion beam

The He+He⁺¹ interactions have been studied, as a function of the internuclear separation R, in terms of the electronic forces acting on the nuclei and the change in the charge distribution. The analysis reveals that at large R the atomic densities are polarized inwards, causing an attractive force on each nucleus, while at small R the difference in the nature of the interactions in the ²Σ_u and ²Σ_g systems is noted. It is seen that the He+He⁺¹ (²Σ_u) interaction is less attractive than the He⁺¹+He⁺¹ interaction at lower values of R.     

Measurment of the energy spectrum of an electron beam extracted from an accelerator

The electron energy spectrum is one of the most important characteristics of an electron beam that is extracted from a linear accelerator. The most direct way to determine an electron spectrum would be to use a magnetic spectrometer and this method could also give results with high precision and effectiveness. In this article we describe our design of a new multilayer absorption method, which is based on the depth-dose curves method that can be used in most irradiation accelerators,and adds the Monte Carlo simulation and iterative algorithm in order to reconstruct the electron energy spectrum. In this article the energy spectrum was measured using these two methods, and good results were acquired. These results could be crosschecked, which made the results more reliable.     

Ultrashort broadband polarization beam splitter based on an asymmetrical directional coupler

An ultrashort polarization beam splitter (PBS) based on an asymmetrical directional coupler is proposed by utilizing the evanescent coupling between a strip-nanowire and a nanoslot waveguide. In order to be convenient for integration with other components, mode converters between the nanoslot waveguide and the strip-nanowire are introduced and merged into S-bends to achieve an ultracompact PBS. As an example a 6.9 μm long PBS based on a silicon-on-insulator platform is designed, and the length of the coupling region is as small as 1.3 μm. Numerical simulations show that the present PBS has a very broad band (>160 nm) for an extinction ratio of >10 dB.     

Energy spectrum of an electron beam multiply reflected from the target

The effect of inelastically scattered electrons on the pulsed-beam energy spectrum are studied for the case where they hit back the target upon reflection from the accelerating region. The beam is formed in a vacuum diode and transported to the target by the guiding magnetic field. The formulas for the total and differential fluxes of different-order absorbed and backscattered electrons are presented. It is shown that account of multiple electron backscattering gives rise to a considerable change in the behavior of the normalized heat-source function in the target: the energy loss near the surface of the irradiated target increases markedly due to its reduction in the deep layers of the material. High-Current Electronics Institute, the Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 2, pp. 49–53, February, 2000.     

Isotope separation by laser deflection of an atomic beam

Separation of isotopes of barium has been accomplished by laser deflection of a single isotopic component of an atomic beam. With a tunable narrow linewidth dye laser, small differences in absorption frequency of different barium isotopes on the 6s^{2 1}S₀₋ 6s6p¹P₁ 5536 Å resonance were exploited to deflect atoms of a single isotopic component of an atomic beam through an angle large enough to physically separate them from the atomic beam. It is shown that the principal limitation on separation efficiency, the fraction of the desired isotopic component which can be separated, is determined by the branching ratio from the excited state into metastable states. In barium, repeated absorptions and emissions on the 5536 Å transition eventually result in decay from the 6s6p¹P₁ state to the metastable 6s5d¹D₂ state. This was observed to occur for all but 3% of the¹³⁸Ba atoms. As a result, the efficiency of separation was about 0.7 for the 8 mrad atomic beam divergence employed. (Throughput was nearly 1 mg/day. No attempt was made to maximize this value.) The isotopic purity of the separated atoms was measured to be in excess of 0.9, limited only by instrumental uncertainty. The effects of near resonant atomic scattering and excitation exchange on isotopic purity are considered. Work performed under the auspices of the U.S. Energy Research & Development Administration.     

Interplay of localization and superconducting fluctuations above the critical point

A review of some developments in localization effects on conductivity and magnetoconductivity is given. The determination of inelastic scattering rates for electrons in thin disordered metallic films is emphasized. In two-dimensional disordered superconductors aboveT _c , the superconducting fluctuations play an essential role. Recent work on the interplay of localization and superconducting fluctuation effects in determining the magnetoconductivity and the inelastic rate is described.