Amplification of picosecond pulses in a 140-GHz gyrotron-traveling wave tube

An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro-TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz in the HE?? mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz. Experimental results taken over a wide range of parameters show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.     

Higher-order wave interactions in a periodic medium

A singular perturbation analysis of the interaction of the fundamental plane wave progressing in the direction of the sinusoidally varying permittivity of an unbounded medium with the first four space harmonics progressing in the reverse direction is carried out. The periodic inhomogeneity introduces self and mutual interaction phenomena. The self interaction, in general, produces a phase shift and causes the mutual interaction to occur for frequencies higher than those predicted by the Bragg condition. The mutual interaction introduces a complex change in the wavenumber resulting in the interacting waves becoming evanescent. Analytical expressions for the frequency shift and the other wave characteristics in the interaction region are deduced. With the increase in the order of the interaction, the frequency shift becomes a more dominant effect than the wave evanescence.     

Study of modulation instability in a temporally incoherent photorefractive ring resonator below the threshold with incoherent light

Modulation instability (MI) and evolution of pattern formation inside a passive non-linear photorefractive ring resonator having cavity-length longer than the coherence length of the light circulating in the resonating cavity has been theoretically analyzed. Dynamics of the system below the threshold conditions have also been analyzed, numerically using linear perturbation theory for the Kerr-type non-linearity. The process of modulation instability in a temporally incoherent cavity below the cavity threshold and the influence of various parameters such as the cavity-length, intensity feedback of the perturbed wave field, amplitude of the incoming beam and spatial frequency on the gain factor and the spatial spectral density have also been studied. For the perturbed wave field, it has been found that the growth rate of MI increases with increase in the intensity feedback as well as the spatial frequency. It has also been found that increasing the intensity feedback of the perturbed wave field, for a given value of the spatial frequency, the spatial spectral density of the perturbation at the output of the non-linear medium increases with the shifting peak positions to the higher spatial frequency side.     

Plasma-maser instability of the ion acoustics wave in the presence of lower hybrid wave turbulence in inhomogeneous plasma

A theoretical study is made on the generation mechanism of ion acoustics wave in the presence of lower hybrid wave turbulence field in inhomogeneous plasma on the basis of plasma-maser interaction. The lower hybrid wave turbulence field is taken as the low-frequency turbulence field. The growth rate of test high frequency ion acoustics wave is obtained with the involvement of spatial density gradient parameter. A comparative study of the role of density gradient for the generation of ion acoustics wave on the basis of plasma-maser effect is presented. It is found that the density gradient influences the growth rate of ion acoustics wave.     

Stability of higher-order bragg interactions in active periodic media

The stability of waves in unbounded, longitudinally periodic media is studied for index and gain coupling. Time-independent periodic media are found to support both stable and absolutely unstable waves. The wave characteristics depend upon average gain or loss, coupling type, and Bragg order. The extended coupled waves (ECW) equations provide explicit values of threshold, frequency, and temporal growth rate for instabilities at all Bragg resonances through the dispersion relation. The first and second Bragg resonances are studied in detail since they are the archetypes for all odd and even resonances. Applications to multiharmonic periodicities and complex couplings are briefly discussed with particular not taken of possible reductions of the stability thresholds and removal of threshold degeneracies. Comparisons are made to the longitudinally bounded case of distributed feedback (DFB) lasers.     

Line narrowing and hole burning within the homogeneous linewidth: a new wave-mixing effect in two-level systems

The interaction of strong low-area pulses with two-level systems shows absorption line narrowing and hole burning within the homogeneous linewidth as a result of nonlinear wave mixing. The wave mixing results from the two-level electronic saturation nonlinearity and occurs, depending on the sign of the pulse area, as a strong absorption enhancement or gain at the transition frequency of the two-level system for resonant excitation.     

The fluorescence of a molecular gas excited a strong monochromatic standing wave radiation field

Among the different methods of nonlinear spectrosccpy the method of analysing the fluorescence light is useful in investigating weak absorbing molecular transitions. In this paper the fluorescence of a molecular gas in the resonator of a high-power single mode laser is investigated. By tuning the standing wave field frequency to centre of the absorption line one observes a narrow local minimum for the fluorescence. This minimum is determined by the homogeneously broadened line of the molecular transition and gives information of the different molecular relaxation processes. The influence of vibrational-rotational relaxations, velocity changing collisions and spatial inhomogeneous distribution of the population caused by the standing wave field on the width and contrast of the minimum is theoretically investigated. It will be shown, that vibrational-rotational and velocity changing collisions increase the intensity of the fluorescence. The spatial inhomogeneous distribution of population caused by the excitation decreases the contrast.Results have been partially presented in reference [12].     

Statistical characteristics of an intense wave behind a two-dimensional phase screen

An analysis of the parameters of nonlinear waves transmitted through a layer of a randomly inhomogeneous medium is carried out. The layer is modeled by a two-dimensional phase screen. Passing through the screen plane, the wave acquires a random phase shift. The wave front becomes distorted, and randomly located regions of ray convergence and divergence are formed, in which the nonlinear evolution of the wave alters profoundly. The problem is solved in the approximation of geometrical acoustics. The ray pattern of a plane wave transmitted through the regular screen is constructed. The solution that describes the spatial structure of the field and the evolution of an arbitrary temporal wave profile behind the screen is obtained. Statistical characteristics of the discontinuity amplitude are calculated for different distances from the screen. A random modulation is shown to result in a faster (in comparison with the case of a homogeneous medium) nonlinear attenuation of the wave and in the smoothing of the shock profile. The distribution function of the wave field parameters becomes broader because of random focusing effects.     

Formation of a detonation wave in the thermal decomposition of acetylene

The formation of a condensation detonation wave has been experimentally observed in the shock-induced thermal decomposition of acetylene. The stable detonation wave in the 20% C₂H₂ + 80% Ar mixture has been obtained at an initial pressure behind the shock wave of no less than 30 atm. The main kinetic characteristics of the pyrolysis of acetylene—the period of the induction of condensation and the growth rate constant of condensed particles—have been determined. The correlation of various stages of the process with the heat release in the condensation has been analyzed. It has been shown that the period of the particle growth induction is not accompanied by noticeable heat release. The subsequent condensation stages characterized by significant heat release occur very rapidly (faster than 10⁻⁵ s) in the so-called explosive condensation. The analysis of the results indicates that the reactions leading to the growth of large polyhydrocarbon molecules, which precede the formation of condensed carbon particles, constitute the limiting stage of the process, which determines the possibility of the formation of the condensation detonation wave in acetylene. An increase in the pressure is accompanied by the sharp narrowing of the induction region and the transition of the process to the condensation detonation wave.     

Wave growth rate in a cylindrical metal waveguide with ion-channel guiding of a relativistic electron beam

This paper addresses the formulae and numerical issues related to the possibility that fast wave may be grown when a relativistic electron beam through an ion channel in a cylindrical metal waveguide.To derive the dispersion equations of the beam-wave interaction,it solves relativistic Lorentz equation and Maxwell’s equations for appropriate boundary conditions.It has been found in this waveguide structure that the TM 0m modes are the rational operating modes of coupling between the electromagnetic modes and the betatron modes.The interaction of the dispersion curves of the electromagnetic TM 0m modes and the upper betatron modes is studied.The growth rates of the wave are obtained,and the effects of the beam radius,the beam energy,the plasma frequency,and the beam plasma frequency on the wave growth rate are numerically calculated and discussed.     

Traveling wave solutions of a reaction-diffusion model for bacterial growth

In this paper, we consider a reaction-diffusion model for the bacterial growth. Mathematical analysis on the traveling wave solutions of the model is performed. This includes traveling wave analysis and numerical simulations of wave front propagation for a special case. Specifically, we show that such solutions exist only for wave speeds greater than some minimum speed giving wave with a sharp front. The minimum speed is estimated and the wave profile is calculated and compared with different numerical methods.     

The interaction of high- and low-frequency waves in a free electronlaser

When a free electron laser (FEL) interaction occurs inside a waveguide, there are two possible operating frequencies at the two intersections of the beam line with the waveguide dispersion curve. Using a one-dimensional model valid for a low-gain oscillator, the mode competition between the high- and low-frequency waves is studied. It is found that the low-frequency wave can be nonlinearly suppressed, even when its linear growth rate is positive. When the frequency ratio is two, phase locking occurs and the nonlinear gain of the low-frequency wave can exceed its linear growth rate     

Single SH guided wave mode generation method for PPM EMATs

Using periodic permanent magnet(PPM) electromagnetic acoustic transducers(EMATs), different shear horizontal(SH) guided wave modes can form simultaneously in some situations, which can interfere with the inspection. The main cause of this phenomenon(typically named multiple modes) is related to the frequency bandwidth of excitation signals and the transducer spatial bandwidth. Simply narrowing the frequency bandwidth cannot effectively limit the number of different SH modes. Previous researches showed that unnecessary SH wave modes can be eliminated by using dual EMATs.However, in practical applications, it is more convenient to change the excitation frequency than to use dual EMATs. In this paper, the stress boundary conditions of the PPM-EMAT are analyzed, the analytical expression of SH guided wave is established, and the magnitude of SH guided wave mode under continuous tone and tone-burst input is obtained. A method to generate a single SH mode by re-selecting an operating point is proposed. Furthermore, the influence of the frequency bandwidth of the tone-burst signal is analyzed. Finally, a single SH mode excitation is achieved with tone-burst input.     

行波偏转器前置短磁聚焦条纹变像管理论设计与实验研究

条纹变像管因其超高时间分辨特性而成为实现皮秒至飞秒量级时间分辨的重要测量仪器.本文设计了一种同时兼顾高时空分辨的行波偏转器前置短磁聚焦条纹变像管.该管型通过减小电子渡越时间以抑制空间电荷效应、采用偏转器前置以及行波偏转方式提高偏转灵敏度,实现整管时空分辨率的大幅提升.利用CST微波工作室有限元法数值计算条纹变像管行波偏转器的通频带宽、偏转灵敏度,结果表明:本设计中的行波偏转器因其较高的通频带宽特性实现了偏转器上的电磁波相速度在很宽频率范围内与电子轴向群速度匹配,产生更有效偏转.利用CST粒子工作室模拟追踪光电子的运行轨迹,通过最佳像面上的时间调制传递函数和空间调制传递函数,计算得到其理论时间分辨率可达220 fs,空间分辨率高于100 lp/mm.同时根据像差定义给出追踪实际电子轨迹的像差计算方法,实现对变像管成像质量评价.最后利用紫外灯对其进行静态测试,获得静态空间分辨率优于35 lp/mm的结果.     

Stimulated Brillouin scattering in a plasma with ion-acoustic turbulence

A theory of stimulated Brillouin scattering (STBS) in a plasma with ion-acoustic turbulence is developed using concepts of parametric instability under conditions when equations of two-temperature hydrodynamics can be used to describe ion-acoustic perturbations of the electron density. The temporal growth rate of the absolute instability and the spatial gain of the scattered wave are determined. The dependence of the threshold density of the radiation flux on the angle between the scattering wave vector and the direction of anisotropy of the turbulent noise is described. A new effect of STBS forbiddenness caused by anomalous turbulent heating of the ions is predicted for a plasma with a high level of turbulent noise.     

Hydrodynamics and Platoon Formation for a Totally Asymmetric Exclusion Model with Particlewise Disorder

We consider a one-dimensional totally asymmetric exclusion model with quenched random jump rates associated with the particles, and an equivalent interface growth process on the square lattice. We obtain rigorous limit theorems for the shape of the interface, the motion of a tagged particle, and the macroscopic density profile on the hydrodynamic scale. The theorems are valid under almost every realization of the disordered rates. Under suitable conditions on the distribution of jump rates the model displays a disorder-dominated low-density phase where spatial inhomogeneities develop below the hydrodynamic resolution. The macroscopic signature of the phase transition is a density discontinuity at the front of the rarefaction wave moving out of an initial step-function profile. Numerical simulations of the density fluctuations ahead of the front suggest slow convergence to the predictions of a deterministic particle model on the real line, which contains only random velocities but no temporal noise.     

Dispersion relation and growth rate for a corrugated channel free-electron laser with a helical wiggler pump

The effects of corrugated ion channels on electron trajectories and spatial growth rate for a free-electron laser with a one-dimensional helical wiggler have been investigated. Analysis of the steady-state electron trajectories is performed by solving the equations of motion. Our results show that the presence of a corrugated channel shifts the resonance frequency to smaller values of ion channel frequency. The sixth-order dispersion equation describing the coupling between the electrostatic beam mode and the electromagnetic mode has also been derived. The dispersion relation characteristic is analyzed in detail by numerical solution. Results show that the growth rate of instability in the presence of corrugated ion channels can be greatly enhanced relative to the case of an uniform ion channel.     

Whistler mode based explanation for the fast reconnection rate measured in the mit versatile toroidal facility

Despite the widely discussed role of whistler waves in mediating magnetic reconnection (MR), the direct connection between such waves and the MR has not been demonstrated by comparing the characteristic temporal and spatial features of the waves and the MR process. Using the whistler wave dispersion relation, we theoretically predict the experimentally measured rise time (τ(rise)) of a few microseconds for the fast rising MR rate in the Versatile Toroidal Facility at MIT. The rise time is closely given by the inverse of the frequency bandwidth of the whistler waves generated in the evolving current sheet. The wave frequencies lie much above the ion cyclotron frequency, but they are limited to less than 0.1% of the electron cyclotron frequency in the argon plasma. The maximum normalized MR rate R=0.35 measured experimentally is precisely predicted by the angular dispersion of the whistler waves.     

Spatial growth rate and field profiles of symmetric mode in a rod dielectric Čerenkov maser with a magnetized plasma column

By numerically solving the exact dispersion equation, the dispersion relation of symmetrical TM waves propagating in a Čerenkov maser including a thin annular relativistic electron beam (TAREB), a strongly magnetized plasma column, and a dielectric rod is investigated. The effects of accelerating voltage, radii of TAREB and plasma column as well, on the frequency spectra and spatial growth-rate coefficient are presented. The axial electric field profiles during the wave amplification and the conditions under which the spatial growth rates are maximum are presented.     

布氏角放置的双折射滤光片在腔内倍频中的作用

详细研究了双折射滤光片在宽带增益激光器腔内倍频中的作用以及双折射滤光片在腔内位置对倍频效率的影响。以钛宝石激光器为例,实验分析了不同厚度的双折射滤光片对基频光线宽和倍频效率的影响;以及腔内倍频中,热效应的来源、双折射滤光片对热诱导相位延迟的补偿、双折射滤光片在腔内的最佳位置。实验发现,双折射滤光片不仅可以压缩基频光线宽,还可以有效补偿腔内元件的热致双折射,减小腔内损耗,大幅度提高腔内倍频效率和激光的稳定性。