Absolute parametric instability of low-frequency waves in a 2D nonuniform anisotropic warm plasma

Using the separation method, absolute parametric instability (API) of electrostatic waves in a magnetized pumped warm plasma is investigated. In this case the effect of static strong magnetic field is considered. The problem of strong magnetic field is solved in two-dimensional (2D) nonuniform plane plasma. Equations which describe the spatial part of the electric potential are obtained. Also, the growth rates and conditions of the parametric instability for periodic and aperiodic cases are obtained. It is found that the spatial nonuniformity of the plasma exerts a stabilizing effect on the API. It is shown that the growth rates of periodic and aperiodic API in warm plasma are less when compared to that in cold plasma.     

Self-Focusing and Channeling of Wave Beams in a Nonuniform Magnetic Field under Conditions of Ionization Nonlinearity

We study experimentally the effect of ionization self-channeling of waves at the whistler frequencies in a nonuniform magnetic field. It is shown that the formed plasma nonuniformity localizes the radiation from a short high-frequency source inside a discharge channel stretched along an external magnetic field. We found a possibility to control the parameters of the formed plasma-wave channel as well as the dispersion characteristics and structure of wave fields in wide limits by varying the magnetic field in a specified spatial region. We propose a method for the formation of a plasma resonator and test this method in the laboratory experiment. The spatial plasma and field distributions in this resonator are similar to those along a geomagnetic field tube of the magnetospheric resonator. We reveal the plasma instability in such a resonator in the vicinity of the frequency of electron bounce oscillations between magnetic mirrors.     

Effect of drifting carriers on longitudinal electro-kinetic waves in ion-implanted semiconductor plasmas

We present an analytical study of wave spectra of electro-kinetic waves propagating through semiconductor plasma, whose main constituents are drifting electrons, holes and non-drifting negatively charged colloids. By employing the hydrodynamical model of multi-component plasma, a compact dispersion relation for the same is derived. This dispersion relation is used to study slow electro-kinetic wave phenomena and resultant instability numerically. We find some important modifications in the wave spectra of the slow electro-kinetic branch. It is found that the drift velocities of electrons and holes are responsible for converting two aperiodic modes into periodic ones. The applied dc electric field increases the phase velocities of contra-propagating modes. The amplification coefficients of propagating modes can be optimized by tuning the amplitude of applied electric field and wave number. It is hoped that the results of this investigation should be useful in understanding the wave spectra of slow electro-kinetic waves in ion-implanted semiconductor plasma subjected to a dc electric field along the direction of wave propagation.     

Thermal instability of a reconnecting current layer as a trigger for solar flares

The stability of small perturbations of a reconnecting current layer (CL) in a plasma with a strong magnetic field has been investigated in the approximation of dissipative magnetohydrodynamics. The case where the wavevector of the perturbations is parallel to the electric current in the CL has been considered. The suppression of plasma heat conduction by a magnetic field perturbation inside the CL is shown to be responsible for the instability. At the linear stage of instability development, the perturbations grow with the characteristic radiative plasma cooling time calculated in the approximation of an optically thin plasma with cosmic abundances of elements. The formation of a periodic structure of cold and hot magnetic flux tubes, viz., filaments, located across the direction of the electric current, should be expected at the nonlinear stage of the instability in the CL. The proposed mechanism of the thermal CL instability can explain the sequential brightening (ignition) in the arcades of magnetic loops in solar flares.     

Effect of a periodic magnetic field on waves in a cold plasma

The effect of a spatially-uniform periodic magnetic field on waves propagating perpendicular to it in a cold plasma is considered. Assuming that the pump field is weak, the method of multiple time scales is used to treat the problem. The results show that, unlike the pump electric field, the pump magnetic field does not produce any parametric excitation of the transverse waves, no matter what the value of the frequency mismatch is.     

Relativistic warm plasma waveguide under the effects of plasma electrons and HF electrical field on Buneman instability

The stabilization effect of a strong HF (pump) electrical field and plasma electrons on a two-stream (Buneman) instability in a plane relativistic warm plasma waveguide is investigated; using the separation method to solve the two-fluid plasma model we separate the problem into two parts. The “temporal” (dynamical) part enables us to determine the frequencies and growth rates of unstable waves; this part within the redefinition of natural frequencies coincides with the system describing HF suppression of Buneman instability in uniform unbounded plasma. Natural frequencies of oscillations and spatial distribution of the amplitude of the self-consistent electrical field are determined from the solution of a boundary-value problem (“space part”) taking into account specific spatial distribution of plasma density. Plasma electrons are considered to have a relativistic thermal velocity. It is shown that the growth rate of instability in relativistic warm plasma is reduced compared to non-relativistic (cold or warm) plasma and relativistic cold plasma. In addition, it is found that the plasma electrons have no effect on the solution of the space part of the problem.     

Parametric channeling and collapse of beams of charged particles with internal structure in crystals: 1. Features of parametric channeling of molecular ions in crystal channels

A new type of self-organized orientational motion of charged particles with internal energy structure (molecules, atoms, and in some cases nuclei and electrons) in crystals, i.e., parametric channeling, is proposed and studied in detail. A feature of this mode of motion is associated with the strong parametric coupling of orientational oscillations controlled by the interaction of the moving particle charge with averaged fields of crystal plane axes and vibrations caused by intraparticle processes. The features of parametric processes during channeling of molecular diatomic ions are studied. It is shown that this effect can cause “parametric collapse,” i.e., beam self-cooling, compression, and a decrease in its phase-space volume. For molecules whose axis is perpendicular to the channel axes, periodic parametric collapse occurs. For molecules oriented along the channel, beam self-cooling is an asymptotic, aperiodic, and irreversible process.     

Dynamics and instability of current-carrying microbeams in a longitudinal magnetic field

The dynamics and instability of current-carrying slender microbeams immersed in a longitudinal magnetic field is investigated by considering the material length scale effect of the microbeam. On the basis of modified couple stress theory, a theoretical model considering the effect of Lorentz forces is developed to analyze the free vibration and possible instability of the microbeam. Using the differential quadrature method, the governing equations of motion are solved and the lowest three natural frequencies are determined. The obtained results reveal that the electric current and the longitudinal magnetic field tend to reduce the microbeam's flexural stiffness. It is therefore shown that the lowest natural frequencies would decrease with increasing magnetic field parameter. The mode shapes of the microbeam are found to be generally three-dimensional spatial in the presence of the longitudinal magnetic field. It is interesting that buckling instability would concurrently occur in the first mode or in the higher-order modes when the magnetic field parameter becomes sufficiently large.     

Magnetoelectric effect in garnet films with the induced magnetic anisotropy in a nonuniform electric field

The mechanism underlying the strong magnetoelectric effect in epitaxial films of magnetic garnet is proposed. This mechanism is not related to the existence of the electrically charged domain walls. It results from the effect of a nonuniform electric field on the induced magnetic anisotropy and depends on the crystallographic orientation of the films.     

Parametric excitation of upper hybrid and diffusion modes in a magnetized plasma

This paper presents the criteria for the parametric instability in which a pump wave with electric field normal to the external magnetic field excites an upper hybrid mode and a diffusion mode in a magnetized plasma.     

飞秒激光等离子体中电子热传导现象的数值模拟

采用相对论电磁粒子模拟程序研究了飞秒激光等离子体相互作用中产生的电流密度、电场和自生磁场的发展演化过程。介绍了电子的非局域热输运的基本特性以及激光加热过程中温度烧蚀前沿稠密等离子体子区的预热效应、临界面附近的限流效应,以及冕区的反扩散与限流效应,得到了经典Spitzer-Harm理论描述的电子热传导随自生磁场的演化情形。数值模拟表明:在线性强激光作用下,由于电子初始时刻的无规则热运动,在等离子体上激发电磁不稳定性,而不稳定性激发的强电磁场使电子束在非常短的距离内沉积能量,同时对在激光有质动力推开电子时形成的超热电子能量输运产生抑制作用。     

Frequency shift and anomalous resistivity of turbulent plasmas in external electric and magnetic fields

Accounting for the modified orbits of plasma particles due to constant external electric and magnetic fields, a general expression for the velocity space diffusion tensor of a turbulent plasma is derived. The nonlinear frequency shift and the anomalous resistivity in the presence of external fields are calculated. It is shown that the effect of a strong external electric field on the frequency shift is to reduce its magnitude. Furthermore, the dependence of the anomalous resistivity on the external magnetic field is obtained.     

Instability of a spherical drop in the field of an electrical dipole

Analytical calculations show that, as a field in which an initially spherical charged conducting incompressible drop is placed becomes more and more nonuniform, coupling between the drop’s oscillation modes grows and the threshold of stability against the electrical field pressure declines. When an electrostatic parameter characterizing the electric field pressure exceeds a value that is critical for a certain mode to be unstable, the amplitude of this mode exponentially grows in an aperiodic manner and the amplitudes of modes coupled with this mode build up in an oscillatory manner, each mode having its own instability growth rate. In all cases, there exists a threshold value of the dimensionless electric parameter above which all oscillation modes are unstable.     

Formation of the microscopic structure of high-voltage nanosecond diffuse discharges in sharply nonuniform geometry

The possibility of the development of ionization instability in the avalanche and plasma phases (of the long-wave and short-wave type, respectively) is demonstrated for high-voltage nanosecond discharges in sharply nonuniform geometry. Specific implementation is determined by the electric field distribution in the cathode region and by the emissivity of the cathode. The spatial structure of the discharge formed in both cases is self-similar.     

电子束在周期场中的稳定性

本文讨论了强流电子束通过周期性静电场和静磁场的稳定性。说明了圆柱形电子束通过简谐电磁场和周期性非简谐场例如锯齿形,矩形,等腰梯形等静电场的稳定和不稳定区域。将各种场的主要不稳定区进行比较,表明锯齿形场的主要不稳定区宽度最小。指出当空间电荷效应不可略去时,电子束在稳定区内并不是无条件稳定而是有条件稳定的。这与空间电荷电流大小以及电子束的振幅有关。     

Parametric excitation of hybrid mode in magnetised piezoelectric semiconductors

Using the hydrodynamic model of homogeneous plasma, the parametric decay of a laser beam into an acoustic wave and another electromagnetic wave has been studied in heavily dopedn-type piezoelectric semiconductors in the presence of a transverse magnetostatic field. This decay process results in the parametric excitation of the hybrid mode. The threshold electric field necessary for the onset of instability equals to zero. The magnetostatic field couples the acoustic and the electromagnetic waves and in its absence the instability disappears. The growth rate increases with the square of the magnetic field.     

Collective processes in a microsecond relativistic electron beam: Basic rules and mechanisms

Collective processes taking place in the space charge of a microsecond relativistic electron beam with magnetic insulation are considered. The space-time characteristics of the low-frequency and high-frequency oscillations are found, and the effect of the magnetic compression of the beam near the cathode on these oscillations is studied. It is shown that the basic source of the low-frequency oscillations is the collective motion of the space charge, which takes place in crossed electric and magnetic fields near the cathode, while the primary reason for the high-frequency oscillations is two-stream instability in the beam. The possibility of suppressing both types of oscillations by compressing relativistic electron beams near the cathode is demonstrated. The effect of nonuniform magnetic fields, including their effect on the cathode plasma motion, is elucidated.     

Localized superconductivity in superconductor-ferromagnet hybrid structures

The results of theoretical and experimental investigations of the peculiarities of the nucleation of superconductivity in the presence of a nonuniform magnetic field induced by ferromagnetic films or dots with out-of-plane magnetic anisotropy are reviewed. It is shown that the phase transition line of superconductor-ferromagnet hybrids in the H-T plane (H is an external magnetic field and T is temperature) is determined significantly by the spatial distribution of nonuniform magnetic field. It allows to control the thermodynamic and transport properties of superconductor-ferromagnet hybrids by tuning the magnetic state of the ferromagnet.     

Three-dimensional structures of MHD flow in a disk generator

Three-dimensional nonuniform plasmas and boundary layers have been studied numerically under an MHD interaction. The nonuniform plasma of “streamer” owing to weak ionization of seed material has a spiral structure in the r-&thetas; plane, and the plasma becomes almost uniform between the walls in the r-z plane. This structure is almost the same as that in our previous paper (1997), where the gas (heavy particle) properties are assumed to be invariant and steady. In addition to the streamer, the nonuniform plasma of “domain” owing to weak ionization of noble gas is revealed. The domain has the structure perpendicular to the streamer. In a strong MHD interaction case, the static pressure considerably increases in the upstream region of a generation channel, and the pseudo-shock waves appear in the generator, but the plasma is almost uniform along the &thetas; direction. The boundary layer in the strong MHD interaction is considerably thick, and in the broad region near the wall the Hall current flows reversely. In the weak MHD interaction case, the plasma forms a nonuniform structure along the &thetas; direction, and the Hall current does not always flow in the opposite direction even on the insulator wall since the azimuthal electric field is not zero