Theory of singular-phase reconstruction for an optical speckle field in the turbulent atmosphere

Analytical expressions are derived and computational algorithms are constructed for retrieving optical-field phase distribution under strong scintillation. The input data for the phase reconstruction are the wave-front slopes registered by a Hartmann sensor or shearing interferometer. The theory is based on representing the slope-vector field as the sum of its potential and solenoid components; it introduces the concept of phase-source and phase-vortex density and uses strict integral expressions relating these quantities to the wave-front slopes. To overcome the difficulties arising from the singular character of phase distribution, use is made of regularization of the wave-front slopes. The slopes can be measured with an ideal point wave-front sensor. It is shown that the slopes measured at the output of a nonideal sensor can be treated as regularized values of these slopes. Numerical simulation of phase unwrapping from the reference values of the wave-front slopes has shown that the algorithm designed for visualization of local phase singularities and those for phase reconstruction are very helpful in eliminating the measurement noise.     

Note: Ultra-high frequency ultra-low dc power consumption HEMT amplifier for quantum measurements in millikelvin temperature range

We have presented theory and experimentally demonstrated an efficient method for drastically reducing the power consumption of the rf/microwave amplifiers based on HEMT in unsaturated dc regime. Conceptual one-stage 10 dB-gain amplifier showed submicrowatt level of the power consumption (0.95 μW at frequency of 0.5 GHz) when cooled down to 300 mK. Proposed technique has a great potential to design the readout amplifiers for ultra-deep-cooled cryoelectronic quantum devices.     

From Dubins’ car to Reeds and Shepp’s mobile robot

In the paper, a control system with intermediate dynamics between Dubins’ car and Reeds and Shepp’s mobile robot is investigated. Time-limited reachable sets and reachable sets at given time are computed. Families of semipermeable curves that are useful for the detection of jumps of the value function of time-optimal control problem are constructed. Research is partly supported by the Russian Foundation for Fundamental Research under Grants 06-01-00414 and 07-01-96085.     

Effect of Molybdenum Additions on Compaction during Sintering of Fine-Grained Iron-Copper Pseudoalloys

The effect of added molybdenum powder on compaction and the properties of sintered fine-grained iron-copper pseudoalloys is studied. The original powder mixtures are prepared by mechanical alloying, and the original powder particle size in mixtures does not exceed 0.5 μm. Specimens are sintered in the range 600-1130°C. It is shown that addition of molybdenum powder to the original charge accelerates compaction of fine pseudoalloys in both the solid phase and liquid-phase sintering compared with compaction of the same pseudoalloys without adding molybdenum. After solid-phase sintering the maximum relative density of specimens is 98.8%, and after liquid-phase sintering it is 99.3%. The main reasons for acceleration of compaction are prolonged retention of a fine-grained structure of sintered specimens up to the melting temperature for the phase based on copper and mutual diffusion between iron and molybdenum; to a significant extent the latter occurs during specimen heating in the solid phase.     

Effect of machining conditions on specific tangential forces

This paper deals with different hypotheses regarding the resistance of deformable material that is subject to plastic strain. For analysing cutting processes, a new cutting layout with a single shear plane is proposed here to calculate the specific tangential forces. The specific tangential forces in the different shear zones were examined by experiment to verify one of the hypotheses in question. These analyses verified that the forces depend fundamentally on cutting parameters and cutting layouts. Hence, it is necessary to take account of a correlation between yield point and deformation temperature, when determining the mechanical properties of the cutting material during machining. It is suggested here to determine the mechanical properties from the conditions compensating the hardening and the softening of the cutting material.     

Sintering of Si3N4 Ceramics with Additives Containing Yttrium and Ytterbium Oxides with Microwave and Traditional Heating

Compaction processes, phase transformation, and porous structure evolution are compared for materials based on silicon nitride containing yttrium and ytterbium oxides under microwave and traditional heating conditions. Experiments by microwave heating are performed in a special gyrotron device operating at a frequency of 30 GHz in a nitrogen atmosphere at normal pressure with a constant heating rate of 30, 60, and 90 deg/min. Traditional sintering is carried out in a tubular continuous furnace with a tungsten muffle in a nitrogen atmosphere at normal pressure with a heating rate of 60 deg/min. It is established that under microwave heating conditions compaction of the test materials and the start of the phase transformation occur at temperatures 100-150 deg lower than those typical for traditional heating. In material with a reduced amount of oxide additives compaction is accompanied by an increase in the proportion of large pores which is the reason for a slow-down in compaction both with traditional heating and with microwave one. It is suggested that the reason for acceleration of mass transfer with microwave heating is the specific nature of the interaction of the microwave field with the liquid phase formed, promoting development of non-diffusion transfer of solid-phase elements dissolved within it and the mechanical effect on its particles.     

Effect of Refractory Boride Particles on Crystallization of the Amorphous Alloy Fe85B15

We have studied the characteristic features of the process of transition from the amorphous to the crystalline state in the presence of refractory boride particles of different crystallographic types (zirconium diboride ZrB₂ and lanthanum hexaboride LaB₆), introduced into ribbons of Fe₈₅B₁₅ amorphous alloy directly during ribbon formation. ZrB₂ particles in the base alloy have practically no effect on its thermal stability, defined by the temperature of onset of formation of primary α-Fe crystals, while in the presence of LaB₆ particles the thermal stability is considerably reduced. This qualitatively correlates with the Dankov principle of structural and dimensional correspondence considered in the theory of heterogeneous nucleation. We have shown that the major factor determining the high catalytic activity of LaB₆ particles with respect to nucleation of α-Fe crystallization centers is rapid chemical reaction within the zone where they come in contact with the Fe₈₅B₁₅ melt. The ZrB₂ particles which do not enter into such a reaction, do not affect the course of crystallization processes and the thermal stability of the base amorphous alloy.     

Interaction of electromagnetic waves in a waveguide with very thin wires

Absorption and scattering of electromagnetic radiation by thin metal wires in a waveguide are investigated experimentally and theoretically. It is shown that, when the wavelength many times exceeds the diameter of a cylindrical wire, this wire strongly absorbs the radiation and weakly scatters it, and the absorption efficiency factor depends on the wavelength only slightly. This phenomenon can be used for developing radiation absorbers in the meter-wavelength range.     

Surface segregation of Ti atoms during NiTi alloy sputtering

The paper addresses NiTi alloy sputtering by 9 keV He and Ar ions and discusses the experiment performed by V.S. Chernysh et al. about 10 years ago. The binary collision simulation has been applied to extract the concentrations of surface Ni and Ti atoms from the experimental data. The results of simulations favor segregation of Ti for both He and Ar ion bombardment. The effect of non-symmetric surface collisions (Ti on Ni and Ni on Ti) was found to be negligible. A pronounced effect of the interatomic (target–target) potential is noted.     

Investigation of Composition of Endometallofullerene Extracts

Gases evolved upon heating endometallofullerene (EMF) extracts with La or Y have been investigated. It has been shown that solvent molecules used to release EMF are observed in extracts up to high temperatures. It has been found that in the dimethylformamide (DMF) extract the EMF molecules are present mainly as the [EMF]^- anions, which were observed in electrospray mass spectra. The treatment of such extracts with iodine results in the anion reduction to neutral molecules.