Dynamics and instability of electron phase-space tubes

Two-dimensional simulations of beam-driven turbulence in the auroral ionosphere have shown the formation and instability of phase-space tubes. These tubes are a generalization of electron phase-space holes in a one-dimensional plasma. In a strongly magnetized plasma, such tubes vibrate at frequencies below the bounce frequency of the trapping potential. A theory for these vibrations yields quantitative agreement with kinetic simulations. Furthermore, the theory predicts that the vibrations can become unstable when resonantly coupled to electrostatic whistlers-also in agreement with simulations.     

Gain recovery in semiconductor optical amplifiers

Pump-probe measurements are presented of gain compression and recovery in a series of four semiconductor optical amplifiers of different lengths but otherwise identical structures. A continuous wave probe beam from a tunable laser is used to measure the wavelength dependence of gain compression and recovery times. A good level of agreement is obtained when these results are compared with numerical simulations using a model that includes the material gain spectrum, saturation effects and the variation of optical intensities with longitudinal position in the device. In addition the experimental results are interpreted in terms of a recent theory that offers approximate analytic expressions connecting recovery time with transmission gain. The spectral dependence of the recovery time and the gain are used to verify the correlations between these quantities and their dependence on device length. The results for recovery time show a strong dependence on probe wavelength with a pronounced minimum which is coincident with the peak of the gain spectrum for each device. A rather weak correlation of speed with length is found.     

轴对称环状静电模的漂移波湍流参量激发理论研究

本文所论述的轴对称环状静电模是指环形磁约束等离子体(如托卡马克)中环向模数为零的近理想静电流体模,它包含有测地声模和基频率与之较低的声模;也含有所谓的‘近零频带状流’.本文根据冷离子流体模型在圆形磁面构成的准环坐标系中的表示,对涉及以上三种模式的漂移波湍流参量激发理论,在一级环形效应近似下,进行了系统讨论,并证明了带状流的四个新命题.利用对漂移波能谱的参数化描写,注意到由漂移波能谱径向有限宽度所引发的特性,如波能传播量的双Landau奇点,揭示了有限宽度对径向δ谱所得结果的重要修正:如,对近零频带状流和测地声模的参量激发条件带来的严格限制.此外,还讨论了密度带状流在高q条件下被激发的可能性.本文选用合理的物理参数.采用图示方法详细地讨论了有关的数值结果.分析表明,测地声模和近零频带状流的参量激发不可能发生在同一小半径处;如果测地声模被参量激发,也应能观察到密度带状流.     

Unstable mode of ion-acoustic waves with two temperature q-nonextensive distributed electrons

The linear characteristics of the unstable mode of ion-acoustic waves are examined in an electrostatic electron-ion plasma composed of streaming hot electrons, non-streaming cold electrons and dynamical positive ions. The plasma under consideration is modeled by using a non-gyrotropic nonextensive q-distribution function in which the free energy source for wave excitation is provided by the relative directed motion of streaming hot electrons with respect to the other plasma species. In the frame work of kinetic model, a linearized set of Vlasov–Poisson's equations are solved to obtain the analytical expressions for dispersion relation and Landau damping rate. The threshold condition for the unstable ionacoustic wave is derived to assess the stability of the wave in the presence of nonextensive effects. Growth in the wave spectrum and nontrivial effects of q-nonextensive parameter on the ion-acoustic waves can be of interest for the readers in the regions of Saturns' s magnetosphere.     

Ultrarelativistic excitation of beat waves in a hot magnetized plasma

Summary A theoretical investigation is made on the nonlinear relativistic excitation of an electrostatic electron plasma wave at the beat frequency of two high-power colinear laser beams in a hot, homogeneous and magnetized plasma. The relativistic Vlasov equation expressed in gyrokinetic variables has been employed to find the nonlinear response of the magnetized plasma electrons. It is noted that the power density associated with the excited beat wave is much higher for the relativistic consideration than that for the nonrelativistic consideration. The authors of this paper have agreed to not receive the proofs for correction.     

Observation of plasmon-polariton modes in two-dimensional electron systems

A manifestation of retardation effects, which were predicted theoretically more than 35 years ago, is revealed for the first time in the plasma excitation spectrum of a two-dimensional electron system with a high electron mobility. It is shown that a significant decrease in the resonant plasma frequency due to a hybridization of the plasma and light modes is observed in zero magnetic field. An unusual dependence of the frequency of the hybrid cyclotron-plasmon mode on the magnetic field has been observed in a perpendicular magnetic field. The experimental results are in good quantitative agreement with the theory.     

Parametric Mechanism of Anomalous Microwave Power Absorption in Experiments on Electron Cyclotron Heating in Toroidal Magnetic Traps

The nonlinear stage of the parametric decay instability of an extraordinary wave is analyzed in the presence of a nonmonotonic density profile. The decay excites an electron Bernstein wave, which is localized in the vicinity of a local density maximum, and an ion Bernstein wave, which leaves a nonlinear interaction region and is absorbed by ions in the vicinity of the harmonics of the ion cyclotron frequency. The main mechanism of instability saturation is considered to be a cascade of decays of a primary daughter electron Bernstein wave, which leads to the excitation of localized secondary electron Bernstein waves and ion cyclotron (Bernstein) waves. The localization of electron Bernstein waves causes a significant decrease in the secondary- decay excitation threshold, which is thought to provide saturation of the primary instability at the lowest level. The saturation of the primary parametric decay instability of a pump wave and the anomalous absorption of the pump power are analytically estimated. A numerical simulation is performed using the parameters that are typical of the experiments on the electron cyclotron resonance heating of plasma at the second resonance harmonic in TCV tokamak.     

Mode selective control of drift wave turbulence.

Experiments on spatiotemporal open-loop synchronization of drift wave turbulence in a magnetized cylindrical plasma are reported. The synchronization effect is modeled by a rotating current profile with prescribed mode structure. Numerical simulations of an extended Hasegawa-Wakatani model show good agreement with experimental results.     

On the reflection of coupled Rayleigh-like waves at surface defects in plates

The reflection of coupled Rayleigh-like waves from surface defects in elastic plates is investigated experimentally and analyzed on the basis of an analytical model and finite difference simulations. The propagation of Rayleigh-like waves in plates is characterized by an energy transfer to the opposite plate side and back over a distance called the beat length. Experimental results clearly show this beating effect and its dependency on the frequency-thickness product, and excellent agreement is obtained with existing analytical predictions. The propagation and scattering are modeled separately for the fundamental A(0) and S(0) Lamb modes that constitute the incident Rayleigh-like wave. The reflection coefficients from surface slots are investigated using finite difference simulations and the reflected Rayleigh-like wave is obtained by superposition. The theoretical model reveals strong dependencies of the reflected field on the ratio between excitation distance and beat length and on the cutoff frequencies of specific higher Lamb modes. Standard pulse-echo measurements allow for the detection of small defects from a remote transducer location. Good agreement is obtained between the predicted and measured amplitude spectra of the reflected Rayleigh-like wave. The developed model allows for the evaluation of defect location and damaged plate side using a combination of time-of-flight and frequency measurements.     

超声Lamb波二次谐波发生效率分析与模式选择

在导波模式展开分析方法的基础上,提出激发效率参量来定量表征超声Lamb波积累二次谐波的发生效率。以P92钢板为例,理论计算得到了与频散曲线对应的理论激发效率参量分布图谱,从图谱中选择理论激发效率参量大小不同的两种基频Lamb波模式:纵波型S1模式和交点型A2/S2模式,分别测量这两种基频Lamb波模式在钢板中传播时产生的二次谐波信号。理论计算和实验测量结果表明,这两种基频Lamb波模式的理论和实验激发效率参量的比值基本一致,且激发效率参量较大的纵波型S1模式能激发出效率更高的二次谐波信号。研究结果表明激发效率参量可以有效的用于Lamb波二次谐波发生效率的表征及模式选择。      

Flexural vibrations of a rectangular plate for the lower normal modes

Theoretical and experimental results for flexural waves of a rectangular plate with free ends are obtained. Both the natural frequencies and mode shapes are analyzed for the lower normal modes. To take into account the boundary conditions, a plane wave expansion method is used to solve the thin plate theory also known as the 2D Kirchhoff-Love equation. The excitation and detection of the normal modes of the out-of-plane waves are performed using non-contact electromagnetic-acoustic transducers. We conclude that this experimental technique is highly reliable due to the good agreement between theory and experiment.     

A model for steady-state large-volume plasma generation

A simple scheme for generating a uniform, steady-state, large-volume plasma is presented. The weakly magnetized plasma is created by direct ionization of the background gas by low-energy electrons generated from thermionic filaments. An annular arrangement of the filaments ensures a uniform plasma density in the radial direction as predicted by theory. Experiments have been performed to characterize the plasma generated in such a configuration. In order to explain the experimental observation, a bulk plasma theory based on plasma transport by means of cross-field diffusion is developed. As assumed in the theoretical model, the experimental measurements indicate a uniform plasma density along the axis. Both the theory and experiment indicate that the plasma density is a function of the square of the external magnetic field. The theory also predicts the plasma density to be proportional to the neutral density to the two-thirds power, in agreement with the experimental data. The experimental data agree well with theoretical prediction for a broad range of system parameters     

Magnetic reconnection in toroidal eta(i) mode turbulence

Based on three-dimensional simulations of the Braginskii equations we show that for typical plasma-edge parameters the saturation of electromagnetic toroidal eta(i) mode turbulence is controlled by the self-generation and subsequent annihilation of radial magnetic field perturbations. This should be contrasted with the electrostatic limit, where the growth of the linear eta(i) mode is terminated by the onset of sheared flow modes driven by the radial plasma streams. The impact of the saturation amplitude on the transport level is substantial and is not in accord with simple mixing length arguments, suggesting that electromagnetic effects should generally be included in simulations of eta(i) mode turbulence.     

Weak turbulence for a vibrating plate: can one hear a Kolmogorov spectrum?

We study the long-time evolution of waves of a thin elastic plate in the limit of small deformation so that modes of oscillations interact weakly. According to the theory of weak turbulence (successfully applied in the past to plasma, optics, and hydrodynamic waves), this nonlinear wave system evolves at long times with a slow transfer of energy from one mode to another. We derive a kinetic equation for the spectral transfer in terms of the second order moment. We show that such a theory describes the approach to an equilibrium wave spectrum and represents also an energy cascade, often called the Kolmogorov-Zakharov spectrum. We perform numerical simulations that confirm this scenario.     

Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven nanocontacts

Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.     

Double-line terawatt OPCPA laser system for exciting beat wave oscillations

A double-line terawatt beat laser (BEAT) is developed for exciting beat wave oscillations. BEAT consists of two oscillators and an amplification system including optical parametric chirped-pulse amplification (OPCPA) in which two individual pulses with wavelength separations of 10–35 nm are amplified, recompressed, and focused as a single beam. The recompressed pulse trace shows that a 150-fs pulse duration full width at half maximum was modulated at a beating period of 72 fs. This beating period matches a resonant excitation of plasma wave with an electron density of 2.5 × 10¹⁸ cm^?3, resulting in excitation of a beat wave in hydrogen plasma with wave amplitude of 15 GV/m. The multiple beating oscillations can amplify the plasma wave and improve its structure. This scheme would be ideal for stabilizing the plasma wave strength in the plasma cavity and for realizing a practical laser plasma accelerator.     

Acousto-optic Q-switched laser performances of Er^3＋：Yb^3＋：LuAl3（BO3）4 crystal at 1.5-1.6 μm

The cavity tuning characteristics of orthogonally polarized dual-frequency He-Ne laser at 1.15 μm are presented. Vectorial-extension model based on semi-classical laser theory reveals that cavity tuning characteristics are related to beat frequency, relative excitation, and type of Ne isotope. Distortions of cavity tuning curves become moderate with the increase of beat frequency because of the weakening of the crosssaturation effect. Distortions are enhanced with the increase of relative excitation because of the combined action of the self-saturation and cross-saturation effects. By adopting dual-isotope Ne instead of monoiso- toplic Ne, distortions are reduced because of the misalignment between peaks of the self-saturation and net gain coefficients. The theoretical calculations are in good agreement with the corresponding experimental results.     

微波与非均匀等离子体相互作用过程中坑子的形成与谐波发射

在稳态的等离子体装置中，利用微波和等离子体的相互作用，模拟了激光等离子体过程，研究了这个非线性相互作用过程中产生的坑子(caviton)，观察了坑子幅度增长，衰减及传播的演化过程。同时利用数值计算求解了在实验条件下的Zakharov方程，得到与实验较为符合的结果。实验测量了在相互作用过程中的二次谐波发射谱，并用参量衰变过程分析了二次谐波的频率红移。     

Oblique Electron Thermal Waves in a Magnetized Plasma

Propagations of an oblique electron thermal mode under the electron plasma frequency without boundary effects are investigated experimentally and theoretically in a magnetized plasma. The phase, ray, and group velocity surfaces of the electron thermal mode are obtained in a polar coordinate. The experimental observation of the electron mode radiated from a point source is found to be in fair accord with the theoretical wave fronts which are obtained from the ray velocity surface. Wave fronts and ray trajectories of an oblique electron thermal mode radiated from a point source are numerically obtained in an inhomogeneous magnetized plasma with a use of an electrostatic kinetic theory. The spatial numerical results are indicated mainly below the electron plasma and cyclotron frequencies. Reflections of the mode in the plasma density lower than the electron plasma frequency are made clear numerically.     

A new spiky soliton and an explosive mode of the nonlinear driftwave equation

A new type of nonlinear wave modes which occurs in the electrostatic drift waves in an inhomogeneous magnetized plasma is presented. The author predicts the existence of a new type of spiky solitary wave and an explosive mode with a negative potential as stationary solutions of this equation. These solutions are a consequence of a density gradient and not connected with a temperature gradient. These new nonlinear wave solutions appear to make a step forward in the general scheme of nonlinear normal modes for plasma waves. Using these nonlinear wave modes, the author can explain the solitary structure and the explosive event concerning nonlinear drift waves propagating in space