Static characteristics of a conduction and radiation dominated electric arc in nitrogen

The time-dependent energy and circuit equations are solved numerically to obtain temperature profiles, current-voltage characteristics and electric field strength vs axis temperature diagrams in the asymptotic region of a wall-stabilized electric arc operated on nitrogen. The numerical calculations demonstrate the splitting of the rising branch of the stationary solutions in the current-voltage characteristic. This phenomenon was earlier found experimentally for air. The computations were made for various channel radii and for various series resistances. We explain the steady-state characteristics and convergence to the single branch of solutions on the basis of the linearized Lyapunov stability theory (LLST). Comparison with available results of other authors shows that the analytical solution based on simple polynomial approximations to thermodynamic and transport functions differs significantly from numerical computations.The author is grateful to Dr. Z. Sedláek and Dr. M. Hrabovský for their stimulating advices, reminders and helpful comments.     

Absolute negative conductivity and spontaneous current generation in semiconductor superlattices with hot electrons

We study transport through a semiconductor superlattice with an electric field parallel to and a magnetic field perpendicular to the growth axis. Using a semiclassical balance equation model with elastic and inelastic scattering, we find that (1) the current-voltage characteristic becomes multistable in a large magnetic field and (2) "hot" electrons display novel features in their current-voltage characteristics, including absolute negative conductivity and a spontaneous dc current at zero bias. We discuss experimental situations providing hot electrons to observe these effects.     

石墨烯量子磁输运性质的电场效应

最近研究发现石墨烯在一维周期性电学或磁学调制势下, 其扩散电导率会出现Weiss振荡. 本文进一步探索了面外加垂直磁场和面内加横向电场以及一维周期性弱调制电学势的多场耦合作用下, 石墨烯的量子磁输运性质, 结果表明: Weiss振荡振幅和电导率数值都随着静电场的增加而增加. 有趣的是, 当电场与磁场的比值达到某一临界值, 即时, 输运电导率的Weiss振荡突然消失. 这一奇特现象在传统的二维电子气体中是不存在的, 因此可以归因于石墨烯载流子外加电磁场的反常相对论性能谱. 关键词：石墨烯; 静电场; Weiss振荡; 磁输运性质     

石墨烯量子磁输运性质的电场效应

最近研究发现石墨烯在一维周期性电学或磁学调制势下, 其扩散电导率会出现Weiss振荡. 本文进一步探索了面外加垂直磁场和面内加横向电场以及一维周期性弱调制电学势的多场耦合作用下, 石墨烯的量子磁输运性质, 结果表明: Weiss振荡振幅和电导率数值都随着静电场的增加而增加. 有趣的是, 当电场与磁场的比值达到某一临界值, 即时, 输运电导率的Weiss振荡突然消失. 这一奇特现象在传统的二维电子气体中是不存在的, 因此可以归因于石墨烯载流子外加电磁场的反常相对论性能谱. 关键词：石墨烯; 静电场; Weiss振荡; 磁输运性质     

Current-voltage characteristic of a narrow-band semiconductor taking into account ionization of impurities

The current-voltage characteristic of a material with a narrow conduction band in a strong electric field is studied taking into account ionization of deeply lying impurities in the model, in which collisions of charge carriers are described in the Fokker-Planck approximation. The results are compared with analogous results obtained in the v approximation. It is shown that the results obtained earlier in the v approximation coincide with our results to within ∼ 5%. It is noted that the Fokker-Planck model additionally provides the temperature dependence of the current-voltage characteristic.     

MHD mixed convection stagnation point flow of a hybrid nanofluid past a vertical flat plate with convective boundary condition

The steady magnetohydrodynamics (MHD) mixed convection stagnation point flow of a hybrid nanofluid past a vertical flat plate with convective boundary condition is investigated in this study. By choosing appropriate similarity variables, the partial differential equations are transformed into a system of linear equations which are solved afterwards by using the boundary value problem solver, namely bvp4c in Matlab software. The solutions of the ordinary differential equations are proven to have two branches which are upper and lower solutions in a certain range of the mixed convection parameter. This leads to the execution of a stability analysis that verifies the achievable solution of the upper branch which is considered stable while the lower branch is unstable. The findings exposed that the fluid velocity increases with the increasing value of the magnetic parameter while the fluid temperature is decreased contradictory.     

Gas dynamics and thermal-ionization instability of the cathode region of a glow discharge. Part II

The influence of the gas flow structure in the cathode sheath of a glow discharge on the discharge stability is studied numerically. The electric parameters are calculated in a diffusion-drift model that consistently takes into account associative dissociation as an additional electron source. The model also includes equations describing both the thermal mode of the cathode and the nonequilibrium physicochemical gas dynamics of a moderately rarefied gas. It is shown that, in a pulsed discharge, the increasing branch of the current-voltage characteristic, which is associated with the gas rarefaction behind the cathode shock wave, can change to a descending branch associated with the intensification of associative ionization. This gives rise to cathode sheath instability. The results of calculations agree well with experiments.     

分离比对混合流体Rayleigh-Bénard对流解的影响

混合流体Rayleigh-Bénard对流是研究非平衡对流的非线性动力学特性的典型模型之一.基于流体力学方程组的数值模拟,首先探讨了矩形腔体中具有强Soret效应(分离比Ψ=-0.60)的混合流体行波对流的分叉特性及斑图演化,沿着分叉曲线的上部分支,随着相对瑞利数的增加,此系统依次出现了局部行波对流、具有缺陷的行波对流、行波对流、摆动行波对流及定常对流5种行波对流解.然后,研究了分离比Ψ对对流解的影响,与弱Soret效应(Ψ=-0.11)时的对流解相比较,强Soret效应(Ψ=-0.60)时出现的对流解更丰富.由于有强Soret效应的对流的复杂性,Ψ=-0.60时的对流解与Ψ=-0.20,-0.4时的对流解不同.     

On resistive nonlocal Josephson electrodynamics

New solutions of one-dimensional nonlocal Josephson electrodynamics are proposed that describe the steady and nonsteady Abrikosov-Josephson vortex states of the resistive model; these solutions are based on the superposition principle of the vortex structures whose properties are determined by the nonlinear interaction of the vortices. The stability of the current-voltage characteristic (1.13) is shown and the relaxation-oscillation mode of establishing the corresponding state is investigated. The laws governing the annihilation and dispersal of the interacting Abrikosov-Josephson vortices are examined. Zh. éksp. Teor. Fiz. 112, 1396–1408 (October 1997)     

Numerical model of the anode region of high-current electric arcs

A two-dimensional, two-temperature axisymmetric numerical model has been formulated for the flow-affected region and the boundary layer in front of high-intensity electric arc anodes. The plasma flow is laminar, steady, incompressible, and the plasma composition is found from the diffusion equation because chemical nonequilibrium is expected. Computational results are obtained for an atmospheric pressure argon arc considering two different situations: a free-burning electric arc and an arc with a constrictor tube. The solutions indicate two different anode attachments modes-a constricted and a diffuse attachment. It is found that under the conditions considered in the calculations, the gradient-induced current densities become significant at distances in the order of 1 mm from the anode surface. The thermal anode boundary layer is compressed with increasing current. The thickness of the thermal boundary layer for the constricted mode is approximately three times smaller than for the diffuse mode. A reversal of the electric field strength occurs over the entire thickness of the boundary layer in all calculated cases. A satisfactory agreement is reached between the calculated heat flux values and experimental results obtained for a 200-A free-burning electric arc     

Stable spatially localized solutions and holes in optical bistability

We investigate the spatial behavior of the electric field for optical bistability in the good cavity limit. We show that stable spatially localized solutions exist in one and two dimensions. In addition we find that stable one-dimensional hole solutions also arise in this driven non-equilibrium system with dissipation and dispersion thus demonstrating that stable holes are not restricted to dispersive systems, but can emerge in strongly dissipative systems as well. A critical comparison with the properties of stable localized solutions in other driven dissipative systems is also included.     

Negative differential resistance in a one-dimensional molecular wire with odd number of atoms

We have investigated the effects of electron-phonon coupling on the current-voltage characteristics of a one-dimensional molecular wire with odd number of atoms. The wire has been modelled using the Su-Schreiffer-Heeger (SSH) Hamiltonian and the current-voltage characteristics have been obtained using the Landauer’s formalism. In the presence of strong electron-lattice coupling, we find that there are regions of negative differential resistance (NDR) at some critical bias, due to the degeneracy in the energies of the frontier molecular orbitals. The presence of the applied bias and the electron-lattice coupling results in the delocalization of these low-lying molecular states leading to the NDR behaviour.     

Dynamic behaviour of electric arc gas discharge

The time-dependent energy and circuit equations are solved numerically to obtain temperature profiles, current-voltage characteristics and electric field strength vs axial temperature diagrams in the asymptotic region of a wall-stabilized electric arc operated in nitrogen. Dynamic current-voltage characteristics and transient phenomena of the formation of the stationary state during the spark ignition phase are studied. The arc timeconstant for free decay is computed and compared with approximate analytical results. It is found that for fixed initial conditions the arc response to the variable applied voltage is several times longer than the arc time-constant for free decay.The author is grateful to Dr. Z. Sedláek, Dr. M. Hrabovský and Dr. R. Klíma for their stimulating advices and helpful comments.     

Relativistic effects in the time evolution of an one-dimensional model atom in a laser pulse

We define 1D Volkov states as solutions of the one-dimensional Dirac equation in a time dependent electric field, similar to the Volkov solutions in the three dimensional case. They are eigenspinors of the momentum operator and reduce in the absence of the field to free solutions of positive or negative energy. Then we add a time independent attractive Gausssian potential and, by integrating the Dirac equation for a laser pulse of Gaussian shape, we determine the state which coincides initially with the ground state of the system in the absence of the electric field. Our main objective is the study of the population dynamics on the Volkov states during the pulse action. For different values of the laser pulse intensity and two values of the potential depth, we find that the Volkov states which evolve from free solutions of negative energy are practically not populated, in contrast to the population on free negative energy states.     

Autowave Regimes of Thundercloud Electrification

We consider the evolution of the electric field and charge in a one-dimensional electrohydrodynamic (EHD) distributed-parameter system which serves as a simple model of a thundercloud. A diffusion equation for the electric field is proposed which, under reasonable assumptions on the nonlinear dependence of the dissipation current on the electric field (caused by the corona effect around aerosol particles in strong fields), has autowave solutions describing the dynamics of spatially separated electric-charge regions in the thundercloud. The nonlinear set of equations describing the electric-field dynamics in an ensemble of colliding aerosol particles and light ions is reduced to a rather simple basic model which admits solutions in the form of traveling fronts and traveling pulses. We find the asymptotic values of the velocity and thickness of the space-charge front, which are determined by the diffusion and the separation and conductivity currents. Using these values, we estimate the electrostatic-energy growth rate in a thunderstorm cell at the stage of its intensive electrification. A possibility is shown of the formation of a traveling pulse of field and conductivity in such an EHD system with allowance for inductive and noninductive mechanisms of aerosol-particle charging.     

石墨烯量子磁输运性质的电场效应

最近研究发现石墨烯在一维周期性电学或磁学调制势下,其扩散电导率会出现Weiss振荡.本文进一步探索了面外加垂直磁场和面内加横向电场以及一维周期性弱调制电学势的多场耦合作用下,石墨烯的量子磁输运性质,结果表明:Weiss振荡振幅和电导率数值都随着静电场的增加而增加.有趣的是,当电场与磁场的比值达到某一临界值,即β_1=E/(ν_F·B)=1时,输运电导率的Weiss振荡突然消失.这一奇特现象在传统的二维电子气体中是不存在的,因此可以归因于石墨烯载流子外加电磁场的反常相对论性能谱.     

The influence of local phonon modes in a wide-band matrix on the tunnel current-voltage characteristics of quasi-zero-dimensional structures

Experimental results on the visualization of the density of states in InAs/GaSa(001) quantum dots that were obtained by tunnel atomic-force microscopy in an ultrahigh vacuum are presented. A one-dimensional (1D) model of dissipative quantum tunneling is proposed for describing experimental current-voltage characteristics of a tunnel contact between an atomic force microscope probe and the surface of InAs/GaAs (001) quantum dots. It was found that the influence of two local modes of the wide-band matrix on the probability of 1D dissipative tunneling leads to the appearance of several randomly spaced peaks in the field dependence. It was shown that the theoretical dependence agrees qualitatively with experimental the current-voltage characteristic of the atomic force microscope tip and the surface of InAs/GaAs(001) quantum dots.     

Soliton solutions of the non linear Schrödinger equation with an external time-dependent field

A functional transformation between solutions of the one-dimensional nonlinear Schrödinger equation with time- and coordinate-dependent coefficients and solutions of the conventional nonlinear Schrödinger equation (NSE) is constructed. Exact solutions of the NSE with a homogeneous time-dependent external electric field and the NSE with oscillator potential are obtained.     

Analysis of droplet transfer mode and forming process of weld bead in CO2 laser–MAG hybrid welding process

In this paper, CO₂ laser–metal active gas (MAG) hybrid welding technique is used to weld high strength steel and the optimized process parameters are obtained. Using LD Pumped laser with an emission wavelength of 532 nm to overcome the strong interference from the welding arc, a computer-based system is developed to collect and visualize the waveforms of the electrical welding parameters and metal transfer processes in laser–MAG. The welding electric signals of hybrid welding processes are quantitatively described and analyzed using the ANALYSATOR HANNOVER. The effect of distance between laser and arc (D_LA) on weld bead geometry, forming process of weld shape, electric signals, arc characteristic and droplet transfer behavior is investigated. It is found that arc characteristic, droplet transfer mode and final weld bead geometry are strongly affected by the distance between laser and arc. The weld bead geometry is changed from “cocktail cup” to “cone-shaped” with the increasing D_LA. The droplet transfer mode is changed from globular transfer to projected transfer with the increasing D_LA. Projected transfer mode is an advantage for the stability of hybrid welding processes.