The influence of reflections on the stability of gyrotron autooscillations

We study the influence of some types of reflections on the oscillatory processes in a gyrotron. The oscillation stability conditions in the presence of a reflected signal are estimated, the processes in a gyrotron with a fixed structure of the HF field are simulated numerically, and the enrichment of the signal spectrum in the presence of reflections is studied. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novogorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 10, pp. 1348–1357, october 1998.     

Nonstationary processes in a gyrotron with reflections from output-section inhomogeneities

We study the influence of some types of reflections on oscillatory processes in gyrotrons. Estimates of the conditions of oscillation stability in the presence of a reflected signal are given. The processes in a gyrotron with a fixed structure of the RF field are numerically simulated. The enhancement of the signal spectrum in the presence of reflections is studied. This work was presented at the Summer Workshop “Dynamic Days” (Nizhny Novgorod, June 30–July 2, 1998). Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 12, pp. 1616–1622, December, 1998.     

A numerical dispersion compensation technique for time recompression of Lamb wave signals

A Fourier domain numerical reconstruction technique has been created in order to eliminate the time spread of Lamb wave signals caused by their dispersive nature. This method allows a good time compaction of the echoes obtained from a Lamb wave inspection. In a pulse-echo setup, reflection peaks coming from targets located close one from each other that could not be separated or seen within raw signals are identified using this procedure. The utility of this new technique goes from simple signal analysis to imaging purposes such as the improvement of B-scan images or SAFT processing. It has been tested in three different situations with the S0 mode generated in a frequency bandwidth where it is highly dispersive. The reconstruction of a pure reflection coming from the edge of a plate, the separation of the echoes resulting from reflections on two targets near one each other and the effects of the presence of an obstacle between the emitter and the receiver are treated. Good results are obtained for every case studied.     

Charge stripes seen with X-rays in

Superstructure reflections due to the ordering of holes into stripes in have been studied with high energy X-ray diffraction. These reflections have been observed clearly for the first time in a sample which is superconducting at low temperatures ( K). The stripe peaks vanish above 62(5) K whereas the magnetic signal of the stripe ordering which has been seen with neutrons before is already suppressed at K. Our results confirm that the ordering of spins and holes is driven by the charges as it is found in the case of La_1.6-xNd_0.4Sr_xCuO₄ at the doping level of x =0.12. Received 27 April 1999     

Effect of photoconductivity and dielectric constant of the photorefractive materials on two-beam coupling gain and phase-shifts for a single unidirectional photorefractive ring resonator

Photoconductive dependence of two-beam coupling between the pump beam and the signal beam in photorefractive materials have been analyzed in case of non-degenerate wave mixing under the undepleted pump approximation method. During the two-wave mixing in photorefractive materials, steady state amplification of the signal beam and oscillation characteristics of a single unidirectional ring resonator has been studied. The domination of the two-beam coupling gain over the combined absorption and resonator losses such as Fresnel reflections from the crystal and imperfect mirrors builds up unidirectional oscillation. The buildup of such an oscillation leads to a saturation of the gain, which can be explained in terms of the photorefractive phase-shift. The existence of this phase-shift between the photorefractive index grating and the illumination intensity pattern, which is of characteristic of the photorefractive effect, leads to an energy transfer between the two beams. For a single unidirectional ring resonators, the effects of photoconductivity of the materials, two-beam energy coupling coefficient, dielectric constant, crystal thickness, and material's absorption coefficient on amplification of the two-beam coupling gain and photorefractive phase-shifts of the signal beam have also been studied in detail. It has been found that amplification of the signal beam and phase-shift can be enhanced by taking the photorefractive crystal having higher photoconductivity and lower dielectric constant, which improves performance of the resonators.     

基于波变量补偿的阻抗匹配时延双边遥操作

通信时延是遥操作系统中固有的问题,它会严重影响遥操作的性能,降低系统的稳定性和跟踪性。基于无源理论的波变量法可以保证遥操作系统在任意时延下稳定,是解决时延问题的一个重要方法。然而,波变量法带来的波反射会阻扰有用信号的传输,降低了主从端信号的跟踪性,严重时甚至会导致整个系统振荡。针对这一问题,提出了一种基于波变量补偿的阻抗匹配双边遥操作系统结构,旨在减少波反射,提高操作者的临场感和系统的跟踪性。通过仿真实验,结果表明所提方法能够保证固定时延条件下遥操作系统的稳定性,并具有较好的跟踪性。     

Compression and amplification of an ultrasonic pulse reflected from a one-dimensional layered structure

Compression of ultrasonic pulses reflected from layered structures is studied. A short pulse is emitted into water towards a structure consisting of solid plates backed with an air layer. Due to multiple reflections in the structure, the signal is elongated. The reflected signal is received by the same transducer and digitized. After that, the wave is reversed in time and emitted towards the layered structure for the second time; then, the reflected signal is received. Due to the invariance of the processes under the time reversal, the pulse is compressed by the structure: the reflected signal becomes shorter and acquires the waveform of the initial pulse. The possibility of an efficient compression of signals is demonstrated experimentally. Numerical simulations show that the use of more complex structures can considerably increase the compression ratio and produce short signals of a much higher amplitude than that emitted by the transducer. An efficient compression algorithm is proposed.     

二苯乙烯在CdS胶体界面和醇溶液中光氧化反应的ESR研究

采用ＥＳＲ方法研究了二苯乙烯在硫化镉超微纳米粒子界面光氧化过程产生的中间体及转化机理，并与均相直接光氧化和在甲基紫精存在下的光氧化进行了比较。结果表明，二苯乙烯的界面光氧化经历了超氧阴离子基和二苯乙烯正离子基的生成过程而且在ＣＤＳ胶粒表面有Ｈ原子的生成；羟基在Ｈ原子产生过程中起着重要的作用。     

Messung der Elektronenbeugungsintensitäten polykristalliner Aluminiumschichten bei tiefer Temperatur und Vergleich mit der dynamischen Theorie

By means of a retarding field apparatus the electron diffraction intensities of a polycrystalline Aluminium-foil have been measured for different electron energies between 17 and 50 kev. The specimen was kept at 154 °K and the thickness of the crystallites was about 280 Å. It has been shown that the high index reflections behave kinematically, while the first order reflections [e.g. (111), (200)] are much influenced by dynamical extinction effects which can be explained very well by the dynamical two beam approximation. Also the intensities of the higher orders [e.g. (222), (400)] of the strong reflections show marked deviations from the kinematical values. This intensity deficit can be interpreted by multiple scattering processes due to systematic interactions. The experimental results are compared with the second Bethe approximation (introduction of dynamic potentials) as well as with the second Born approximation where double scattering processes within the crystal are taken into account.     

Analysis of light out-coupling from microlens array

Microlens array has been known to be effective in enhancing the out coupling efficiency of LEDs, OLEDs and other thin film light emitting devices. However, mechanism of efficiency enhancement by use of microlens arrays is still ambiguous, apart from suggestions that it leads to randomization of photons. We have studied the photon dynamics in the presence of microlens arrays and the effect of various parameters e.g. microlens contact angle, absorption and substrate refractive index using ray tracing simulations. Microlens array leads to extraction of a portion of photons outside the escape cone at the cost of reduced extraction from within the escape cone. The reduced extraction from within the escape cone is compensated by multiple reflections from the back surface. Increase in microlens contact angle reduces the number of reflections required for out-coupling hence reducing the absorption in the device.     

Effect of fibre nonlinearities in a WDM transmission system

In this paper the combined effect of major fibre nonlinearities at low data rates like stimulated Raman scattering and four wave mixing has been studied in a WDM transmission system. Analysis has been carried out to evaluate the signal to noise ratio in the presence of amplifier spontaneous emission noise. The system parameters such as interchannel separation, interamplifier separation have been estimated so as to get maximum signal to noise ratio.     

The contribution of anion disorder to ionic conductivity on single crystals of β-PbF2

The effect of the different cooling processes on the disorder of flourine ions and ionic conductivity in β-PbF₂ has been studied by X-ray method and ionic conductivity measurements on single crystals below the transition temperature T_c. The spike-like diffuse scattering was observed along the <111>¹ directions around the Bragg reflections. The activation energies for the conduction process are 0.40 eV for the sample quenched from 970 K and 0.54 eV for the one from 720 K. The higher the quenching temperature is, the higher the conductivity and the lower the activation energy become. The dependence of conductivity on the different cooling processes is more evident in single crystals than in polycrystalline samples. The contribution of the different cooling processes to ionic conductivity can be quantitatively explained by the extent of ordering of mobile fluorine ions. Time dependence of ionic conductivity has not been observed.     

Degenerate two-wave mixing in photorefractive crystals with multiple internal reflections

The effect of multiple internal reflections on the stationary energy transfer in degenerate two-wave mixing, in a reflection geometry, with photorefractive crystals is studied. Numerical results obtained by using the shooting method are presented graphically. Generally it is found that the presence of multiple internal reflections reduces the amount of energy transferred between the two interacting beams. However, under some special conditions the energy transfer can be enhanced.     

Onset of special conditions of electron stream formation in a multifrequency magnetron

The initiation of an electron stream in a planar magnetron in the presence of a multifrequency signal is considered. Based on the trajectory analysis, it has been shown that only saddle equilibrium points can exist in the phase plane of this system. It is found that the saddle points can be of two types. The concept of a trajectory divergence domain has been introduced. The processes by which the form of the stream is changed radically as a function of signal parameters are described.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 8, pp. 1036–1043, August, 1996.     

Particle tunneling and scattering in a three-dimensional potential with a barrier

The particle tunneling through a 3-D rectangular potential barrier has been studied. The simplest model for multiple internal reflections has been assumed. The explicit expression for all the transmission and reflection probability amplitudes have been derived, as well as the tunneling and reflection phase times.      

Experimental study of a BaFBr:Eu image plate detector depending on dose,spectrum of pulse X-ray source,and scan number

The possibility of application of phosphor with memory effect for X-ray recording of fast (explosive) processes has been studied. Four different radiation sources were used in the series of experiments carried out. To obtain numerical X-ray images, a detector based on BaFBr:Eu Image Plate phosphor was used. It has been found that the summation of results of several scans after single exposition allows improving the signal-to-noise ratio and dynamic range of the readable signal. Spectra of pulse X-ray devices have been calculated on the base of calibration curves of radiation attenuation for different materials. Preliminary experiments on the registration of explosive processes have been conducted.     

Minimization of the fluctuation in the signal beam intensity of a nonlinear optical medium with a transmission grating

Fluctuation in the intensity of the output signal beam of a photorefractive optical resonator can be reduced significantly by employing pump beam positive feedback. The fractional intensity transfer which is independent of time in the absence of fluctuation becomes time dependent in the presence of fluctuation of the optical wave-mixing process. The influence of various controlling parameters on the relative fluctuation of the output signal intensity has also been studied.     

Second-harmonic generation at interfaces

Second-harmonic light can be generated at dye coated surfaces of fused silica. By changing either the angle of incidence or the frequency of the fundamental, phase matching of signals produced at the two surfaces of a plane-parallel plate has been achieved. The interaction of light from multiple internal reflections has also been studied.     

Observation of two-photon induced photoemission optogalvanic effect

Observation of laser induced two-photon photoemission optogalvanic (TPPOG) effect from tungsten electrode in a discharge cell using 564 nm radiation obtained from a pulsed dye laser is described. The magnitude of the POG signal is studied as a function of laser energy under various discharge parameters. Competition between one-photon and two-photon processes has been observed when nitrogen gas is used in the discharge cell.     

Chaotic Oscillations in a Nonlinear Lumped-Parameter Transmission Line

We present the results of numerical simulation of the complex dynamics of a nonlinear radio-technical line having reflections at the boundaries and excited by an external harmonic signal. It is shown that, with increase in the amplitude of the input signal, periodic oscillations at the external-forcing frequency become unstable and are changed to more complex regimes, either quasiperiodic or chaotic. The main scenarios of transition to chaos are studied. The influence of the modulation instability and soliton formation on the complex dynamics is discussed.