Observations of ion-beam formation in a current-free double layer

With nonperturbative laser-induced fluorescence measurements of ion flow, we confirm numerical simulations of spontaneous electric double-layer (DL) formation in a current-free expanding plasma. Measurements in two different experiments confirm that the DL is localized to the region of rapidly diverging magnetic field. The measurements indicate that the trapped ion population is a single Maxwellian, that the spatial gradient of the energy of ions accelerated through the DL matches the magnetic field gradient, and that DL formation is triggered when the ion-neutral collisional mean-free path exceeds the magnetic field gradient scale length.     

Theory for formation of a low-pressure, current-free double layer

A diffusion-controlled theory is developed for the formation of a low-pressure, current-free double layer just inside an upstream insulating source chamber connected to a larger diameter, downstream chamber. The double layer is described using four groups of charged particles: thermal ions, monoenergetic accelerated ions flowing downstream, accelerated electrons flowing upstream, and thermal electrons. The condition of particle balance upstream is found to determine the double layer potential. The double layer disappears at very low pressures due to loss of ionization balance upstream and due to energy relaxation processes for ionizing electrons at higher pressures, in good agreement with experiments.     

Electron energy distribution of a current-free double layer: Druyvesteyn theory and experiments

Electron energy distributions upstream of a current-free double layer (CFDL) contained in a low-collisional plasma are modeled and compared with experimental results. The experimentally observed electron energy probability functions (EEPFs) with a depleted tail above the energy corresponding to the potential drop of the CFDL can be well approximated by a Druyvesteyn distribution function. Theoretical effective electron temperatures for the Druyvesteyn distribution are in good agreement with the values obtained from the experimental EEPFs over the range of pressure where the CFDL is observed.     

Attenuation of Richtmyer-Meshkov instability growth of fluid layer via double shock

Suppression of the hydrodynamic instabilities involved in the inertial confinement fusion has attracted much attention but remains a challenge. In this work, we report the first theoretical analysis and experimental validation on attenuating the instability growth of a shock-accelerated fluid layer through a second shock impact. An analytical model is established to predict linear growth rates of the perturbations at two interfaces of the layer by considering both the effects of interface coupling and reverberating waves. Theoretically, there are nine possibilities for simultaneously attenuating the instability growths of perturbations at the two interfaces. Accordingly, shock-tube experiments are specially designed and conducted, and nine possibilities are all realized by experiments, which verifies the reliability of the analytical model and also demonstrates the feasibility of attenuating the instability growth of a fluid layer via double shock.     

双层结构液冷机箱设计

本文设计了一种可安装32～36个标准模块(ASAAC)模块的液冷机箱,该液冷机箱采用双层式结构。液冷机箱主体结构由上、中、下冷板及左侧冷板组成,各冷板内均设置有冷却液流道,用于机箱的冷却。上、中、下三块冷板之间的流量分配和连接由左侧冷板完成,左侧冷板上装有一进一出快速自密封接头,用于和液冷源的连接。通过实验测试了液冷机箱冷板均匀性、模块最高温度及不同类型模块负载对散热性能的影响。实验表明,该液冷机箱具有换热性能好、可靠性高等优点,可满足实际应用需要。     

Time-resolved double ionization with few cycle laser pulses

Ionization of D2 launches a vibrational wave packet on the ground state of D+2. Removal of the second electron places a pair of D+ ions onto a Coulombic potential. Measuring the D+ kinetic energy determines the time delay between the first and the second ionization. Caught between a falling ionization and a rapidly rising intensity, the typical lifetime of the D+2 intermediate is less than 5 fs when an intense 8.6 fs laser pulse is used. We simulate Coulomb explosion imaging of the ground state wave function of D2 by a 4 fs optical pulse and compare with our experimental observations.     

On the double ionization mechanism

A resonant mechanism for double ionization processes is proposed.     

Multiphoton double ionization of barium

The multiphoton double ionization of Ba from ~280 to 700 nm was investigated using laser pulses 5 ns long of peak intensity ~10¹⁰ W/cm². The spectrum consists of a number of strong resonances, which can be assigned to Ba⁺ transitions. Most of the assignments have been verified by pump-probe techniques. Thus, the Ba⁺⁺ observed is due to sequential ionization. The multiphoton ionization probability is highest for λ~500 nm, which matches a series of strong Ba and Ba⁺ transitions leading to double ionization     

Acoustic reflection of rib-stiffened double plates coated with a viscoelastic layer

To predict sound-absorbing performance of anechoic materials,the acoustic reflection problem of a viscoelastic layer backed with periodically rib-stiffened infinite double plates is studied in this paper.The reason why structural theories of plates are not applicable to viscoelastic plates is explained through comparing dispersion and attenuation curves of flexural waves with those of Lamb waves.As a result,（visco-）elastic theory is adopted to deal with（visco-）elastic plates,and ribs are treated by structural theories of plates.The coupling between ribs and plates are solved by Hull＇s method,and solution of the reflected field is obtained.The accuracy of present method is validated by comparing with the results by the structural theories of plates.The influence of a backing on the acoustic reflection of the viscoelatic layer is analyzed by computing reflection coefficients.Performances of different viscoelastic materials are evaluated by the average reflection coefficients.The computational results show that,influence of a backing on the acoustic reflection cannot be suppressed by the viscoelastic materials in low frequencies.The resonance is determined by the coupling of the fluid layer and the double plates.And ribs,which are coupled with the double plates,mainly reduce the acoustic reflection.     

Nonsequential double ionization as a completely classical photoelectric effect

We introduce a unified and simplified theory of atomic double ionization. Our results show that at high laser intensities (I>/=10(14) W/cm(2)) purely classical correlation is strong enough to account for all of the main features observed in experiments to date.     

Rosenzweig instability in a thin layer of a magnetic fluid

A simple mathematical model of the initial stage of nonlinear evolution of the Rosenzweig instability in a thin layer of a nonlinearly magnetized viscous ferrofluid coating a horizontal nonmagnetizable plate is constructed on the basis of the system of equations and boundary conditions of ferrofluid dynamics. A dispersion relation is derived and analyzed using the linearized equations of this model. The critical magnetization of the initial layer with a flat free surface, the threshold wavenumber, and the characteristic time of evolution of the most rapidly growing mode are determined. The equation for the neutral stability curve, which is applicable for any physically admissible law of magnetization of a ferrofluid, is derived analytically.     

Current layers and filaments in a semiconductor model with an impact ionization induced instability

Dissipative structures associated with an instability in a semiconductor far from equilibrium are studied. A generation-recombination mechanism, which effects anS-shaped current-voltage characteristics, is coupled to diffusion and drift of the electrons. The spectrum of linear recombination-diffusion modes is computed for the homogeneous steady state with negative differential conductivity. The obtained soft mode instability gives rise to the bifurcation of a family of transversally modulated inhomogeneous steady states and longitudinal travelling waves. The inhomogeneous steady states are calculated from the full nonlinear transport equations for plane and cylindrical geometries. They correspond to oscillatory and solitary concentration profiles, including depletion and accumulation layers and cylindrical filaments. Conditions for the formation of kink-shaped coexistence profiles are established in terms of equal area rules. The current-voltage characteristics are extended to include inhomogeneous current states. Nonequilibrium phase transitions between various branches of these characteristics are associated with switching through filamentation.     

Softening induced instability of a stretched cohesive granular layer

We report on a cellular pattern which spontaneously forms at the surface of a thin layer of a cohesive granular material submitted to in-plane stretching. We present a simple model in which the mechanism responsible of the instability is the "strain softening" exhibited by humid granular materials above a typical strain. Our analysis indicates that such a type of instability should be observed in any system presenting a negative stress sensitivity to strain perturbations.     

Non-sequential double ionization of helium-like atom

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Marangoni instability of a liquid layer with insoluble surfactant under heat flux modulation

We investigate the parametric excitation of Marangoni convection by a periodic flux modulation in a liquid layer with insoluble surfactant absorbed on the nondeformable free surface. The stability analysis of the convective system is performed for arbitrary wave numbers of the disturbances. An interesting feature of the onset of convection is the existence of bifurcating neutral curves with double minima, one of which corresponds to a quasi-periodic solution, and the other one corresponds to a subharmonic solution. The evolution of the subharmonic instability region depending on the amplitude of the external heat flux modulation and the frequency of the modulation is studied. The quasi-periodic neutral curve is close to the oscillatory neutral curve of the nonmodulated problem.     

Transverse instability of a plane front of fast impact ionization waves

The transverse instability of a plane front of fast impact ionization waves in p ⁺-n-n ⁺ semiconductor structures with a finite concentration of donors N in the n layer has been theoretically analyzed. It is assumed that the high velocity u of impact ionization waves is ensured owing to the avalanche multiplication of the uniform background of electrons and holes whose concentration ??_b ahead of the front is high enough for the continuum approximation to be applicable. The problem of the calculation of the growth rate s of a small harmonic perturbation with wavenumber k is reduced to the eigenvalue problem for a specific homogeneous Volterra equation of the second kind containing the sum of double and triple integrals of an unknown eigenfunction. This problem has been solved by the method of successive approximations. It has been shown that the function s(k) for small k values increases monotonically in agreement with the analytical theory reported in Thermal Engineering 58 (13), 1119 (2011), reaches a maximum s _M at k = k _M, then decreases, and becomes negative at k > k ₀₁. This behavior of the function s(k) for short-wavelength perturbations is due to a decrease in the distortion of the field owing to a finite thickness of the space charge region of the front and ??smearing?? of perturbation of concentrations owing to the transverse transport of charge carriers. The similarity laws for perturbations with k ? k _M have been established: at fixed ??_b values and the maximum field strength on the front E _0M, the growth rate s depends only on the ratio k/N and the boundary wavenumber k ₀₁ ?? N. The parameters s _M, k _M, and k ₀₁, which determine the perturbation growth dynamics and the upper boundary of the instability region for impact ionization waves, have been presented as functions of E _0M. These dependences indicate that the model of a plane impact ionization wave is insufficient for describing the operation of avalanche voltage sharpers and that fronts of fast streamers in the continuum approximation should be stable with respect to transverse perturbations in agreement with the previously reported numerical simulation results. The results have been confirmed by the numerical simulation of the evolution of small harmonic perturbations of the steady-state plane impact ionization wave.