Influence of a magnetic field on the light-induced ion drift

It is shown that the velocity component transverse to the radiation propagation direction can arise in the light-induced ion drift (LIID) after applying an external magnetic field to a weakly ionized gas. It is predicted that the projection of the ion drift velocity onto the radiation direction changes its sign with an increase in the magnetic field, resulting in the anomalous LIID.     

Impressed by light: Mechanical action of radiation on atomic motion

Intense light can exert a mechanical action on the motion of atoms in two quite distinct ways. The resonance radiation force is based upon the exchange of photon momentum between an atom and the radiation field. The phenomenon of light-induced drift results from the difference in mobility of excited and ground-state atoms in a buffer gas. We derive the relevant equations of motion for the atomic distribution function, by applying a systematic formulation of time-dependent degenerate perturbation theory. For the resonance radiation force, we obtain a Fokker-Planck equation for the velocity distribution of atoms in one or several light beams, while including the degeneracy of the atomic levels. The derivation generalizes previous results, and it clarifies the physical origin of the various terms. Likewise we obtain a diffusion-type equation for the density of atoms which contains light-induced drift, and we obtain explicit solutions in several model cases of practical interest. These solutions demonstrate the front formation that produces the effect of the optical piston in an optically thick system. Finally we discuss the differences and the analogies between the two effects.     

Noninversive partial velocity amplification of radiation by ions due to their rotation in a magnetic field

It is shown that upon the application of an external magnetic field, a gas of ionized particles may experience noninversive partial velocity amplification of radiation by ions due to their Larmor rotation. In this case, virtually all ions may be in the ground state. It may happen that approximately half the number of ions in the medium amplify the incident radiation. The integrated absorption coefficient remains positive due to the enhancement of absorption of radiation by the other half of ions. Noninversive amplification of radiation takes place when the condition ω_c?Γ²/kv _T is satisfied ωw_c is the cyclotron frequency of ions in the magnetic field; Γ is the homogeneous half-width of the absorption line for ions, and kv _T is the Doppler width). In the case of interaction of atomic ions with radiation in the optical range, this corresponds to magnetic fields B?600 G (for the ion mass M～10 amu). Noninversive partial velocity amplification of radiation is a “latent” effect in the sense that it disappears upon averaging over all velocity directions of ions. This effect is associated with the emergence of phase incursion of the induced dipole moment oscillations for ions moving in circular cyclotron orbits, which depends on the ion velocity.     

On the collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field

The collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field is investigated theoretically. It is shown numerically that the transfer effect is weak. This makes it possible to use simpler approximate one-dimensional quantum kinetic equations instead of three-dimensional equations to solve spectroscopy and light-induced gas kinetics problems, where it is important to take account of the velocity dependence of the collision frequency. It is shown for anomalous light-induced drift, calculations of which are most sensitive to neglecting the transfer effect, that in a wide range of spectroscopy and light-induced gas kinetics problems the transfer of nonequilibrium can be neglected without risking the loss of important fine details of the phenomena being described.     

Nonlocal ion transport in a weakly ionized nonequilibrium plasma

Nonlocal ion transport in a weakly ionized plasma with a strong electric field is analyzed. It is assumed that charge-exchange interactions are the main mechanism of ion scattering. Ion density and drift velocity are determined for nonuniform time varying electric field by using both the direct solution of the kinetic equation and the Chapman-Enskog-type approach. The ion mean velocity is given by an integro-differential operator applied to the electric field. Ion density and drift velocity exhibit resonant behaviour when ω≃kW0, which corresponds to the resonance between ions moving with average velocity W₀ and wave traveling with the phase velocity ω/k     

Anfachung und Dämpfung magneto-akustischer Wellen in MHD-Kanälen und ihr Einfluß auf die mittlere Stromdichte und die mittlere Hall-Feldstärke

Magneto-acoustic waves generated by fluctuations in the Hall parameter, the electric conductivity and the stream velocity are theoretically investigated in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both magnetic field and stream velocity. The investigations hold for seeded rare gas plasmas at any degree of seed ionization but are resticted to waves propagating in parallel or antiparallel direction to the current density vector and in parallel or antiparallel direction to the stream velocity vector and to wave lengths which are small in comparsion to the interaction length which occurs as a characteristic wave length. The influence of these waves on the mean current density and the mean Hall field intensity is calculated in case of small amplitudes and low degree of seed ionization up to second order terms. Omitting Ohmic heating the dispersion equation can be solved exactly. A phase shift exists between the fluctuations in gas density and gas velocity. The phase velocity and the amplification rate depend on the wave length. Typical results are represented in a diagram. For both types of waves the phase velocity slightly rises with increasing wave length, while the amplification rate decreases. Waves propagating in opposite direction to the current density vector are amplified, if the electron velocity exceeds a critical value. They reduce the mean current density and the mean Hall field intensity. Waves propagating in opposite direction to the stream velocity vector are also amplified except for very high degrees of seed ionization. The threshold current density is greater than that for the waves of the first type approximately by the Hall parameter as factor. At extremely high degree of seed ionization the phase velocity is directed opposite to the direction occuring at weakly ionized seed. Waves of the second type decrease the mean current density, but increase the mean Hall field intensity.     

Drift of gases induced by nonmonochromatic light

The phenomenon of light-induced drift of gases influenced by a nonmonochromatic light with different spectral properties is under consideration. The dependence of the drift velocity on the spectrum shape, bandwidth, detunings and light intensity is studied. The largest drift velocity is found to be obtained at the bandwidth of the spectrum being of the order of the Doppler atomic transition width, and when the wings of the spectrum drop rapidly. It is shown that nonmonochromatic light of the same intensity as a monochromatic one allows to reach a much larger drift velocity. Principal possibilities to obtain supersonic light-induced drift velocities are shown.     

Dissipative instability in an aerosol plasma

Dissipative instability in a weakly ionized aerosol plasma has been studied with allowance for a finite electric conductivity of the medium, electron and ion diffusion, and friction of the aerosol component against a neutral gas. Instability is caused by the relative drift of the aerosol and ion components. Estimates of the basic parameters of instability (threshold, characteristic wavelengths, and increments) in experiments with dust crystals indicate that this instability can be an important additional factor upon the formation of regular structures in an aerosol plasma.     

On the ion drift in gas mixtures

The features of the velocity distribution function of ions during their drift in a mixture of different gases are analyzed. Examples of drift of heavy ions in light gas, a mixture of two gases with equal concentrations, and drift of light ions in heavy gas are considered. It was shown that the transition to discharge in mixtures of different gases allows the formation of an ion flux with characteristics unattainable for discharge in single-component gas under typical conditions under which experiments with dust structures in plasma are performed.     

Orientation of stereoisomers by electromagnetic field

A magnetic orientation is shown to appear during light-induced drift of stereoisomers relative to a buffer gas. The magnetic orientation can be initiated even by linearly polarized or nonpolarized radiation.     

Charging of a conducting sphere moving in a weakly ionized gas under an arbitrarily oriented external uniform electric field

A charging conducting sphere moving in a weakly ionized gas is investigated. An external uniform electric field is applied with arbitrary orientation relative to the gas flow. The ion current is obtained analytically and investigated numerically in ballistic assumption. It is shown that charging regimes depend not only on the net charge of the sphere but also on the gas flow type, and the parameter ξ_± – the ratio of ion drift velocity far from the sphere to the gas velocity. The cases |ξ_±|<1 and |ξ_±|>1 yield two different charging regimes for Stokes and potential flows. For the potential flow, the ion current has been found analytically in continuous ξ_±-parameter space. The stationary charge of an isolated sphere is also calculated numerically as a function of α. It achieves maximum magnitudes in direct (α=0) and back (α=π) flows respectively.     

Drift dissipative instability in a two temperature plasma

The presence of a small amount of relatively cold electrons in an otherwise hot plasma reduces the ion sound speed in the medium and hence reduces the growth rate of the drift dissipative ion acoustic mode in an inhomogeneous weakly ionized plasma. This is expected to improve the confinement time in certain magnetic confinement schemes. The propagation of a small but finite amplitude mode in the presence of ion viscosity is also investigated by using reductive perturbation method. It is shown that, when the damping due to ion viscosity is stronger than the growth due to collisions, there exists a stationary shock solution.     

激发光线宽对原子光致漂移速率的影响

在光致漂移效应的研究中,激发光线宽会改变原子激发的速度选择性,进而影响漂移速率的大小.本文以原子光致漂移速率方程理论为基础,利用强碰撞模型描述原子与缓冲气体的碰撞作用,运用数值方法对速率方程进行求解计算,研究了激发光线宽对原子漂移速率的影响.研究结果表明,其他条件相同时,随着线宽的增大,漂移速率的值呈现先增大后减小的趋势.存在一个最佳的激发光线宽,使得原子的漂移速率达到最大值.最佳线宽与激发光功率密度、温度和缓冲气体压强有关.为了获得最佳的光致漂移效果,激发光应工作在最佳线宽条件下.当激发光线宽在最佳线宽附近波动时,设置激发光线宽略大于最佳线宽可减少线宽波动对漂移速率的影响,对获得较大漂移速率更为有利.     

Nonlinear deformations of a radiating shell moving with acceleration

The two-dimensional nonstationary motion of a cold dense shell accelerated under the action of rarefied hot gas pressure is modeled. The influence of radiative processes on manifestations of cumulative effects predicted by the inertia model is analyzed. It is assumed that the major medium component is hydrogen with small admixtures of oxygen, nitrogen, carbon, silicon, and iron. Radiative energy loss caused by photorecombinations and excitation followed by impurity ion metastable level deexcitation is taken into account. An approximation to the cooling function is suggested and implemented. This approximation can be used to perform calculations over wide ionization degree and gas temperature ranges. The formation of a shell during gas expansion ionized and heated by a source of ultraviolet radiation is studied. The characteristic time of shell appearance and its gas dynamic parameters are determined. The distribution of plasma temperature is shown to be nonmonotonic and have a maximum close to the ionization front. An increase in small perturbations of the velocity of a shell is shown to cause the formation of radial fibers and the concentration of gas mass and momentum in them. The structure of condensates formed is, however, much more complex than that predicted by the model of a thin layer of incompressible matter. In particular, condensation includes “fingerlike” thickening and a more extended region with a lower density. It also follows from the calculations that radiative cooling contributes to shell expansion in the radial direction but does not change the integral characteristics of condensations substantially.     

Oscillations of the free electron concentration in a semiconductor under the action of a femtosecond pulse

The interaction of a femtosecond light pulse with a semiconductor is studied in the approximation of an optically thin layer taking into account the dependence of the absorption coefficient on the light-induced electric field. A possibility of the development of spatial oscillations of semiconductor characteristics (concentrations of free electrons and ionized donors and the strength of the light-induced electric field) under the action of a Gaussian beam is demonstrated on the basis of computer simulation and linear analysis.     

A remark on the drift dissipative instability of weakly ionized plasma

The author takes into account ion motion along the magnetic lines in the dispersion equation. It is shown that the drift branch of waves joins the ion acoustic branch in the weakly ionized plasma, too, and that the term describing this motion has a stabilizing effect. These results are independent on the relation of the wave length to the mean free path of the electrons.The author would like to express his sincere thanks to Dr. J. Václavík for his valuable comments.     

Computer simulation of the drift of solid particles caused by resonance gas oscillations in the open channel

Computer simulation methods are used to study the drift of solid spherical particles suspended in the open channel due to longitudinal oscillations of the gas column. The gas velocity field consists of periodic nonlinear waves and an acoustic flow. The particles move under the action of the Stokes force. Particle distributions in the wave field of the open channel are obtained in the vicinity of the first, second and third eigen-frequencies of the gas column. The particle’s spatial distribution features attributed to the particle’s drift under the action of acoustic radiation are revealed.     

Anfachung und Dämpfung von Ionisationswellen in MHD-Kanälen und ihr Einfluß auf die effektive elektrische Leitfähigkeit,den effektiven Hall-Parameter und die mittlere Elektronentemperatur

The phase velocity, the amplification rate and the critical Hall parameter are theoretically determined for ionization waves in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both the magnetic field and the stream velocity. The investigations hold for seeded rare gases at any degree of seed ionization. The critical Hall parameter β_c depends on the degree of ionization, the ionization energy and the temperatures of electron gas T₀ and neutral gas T_g · β_c is always greater than one, if 0 < T₀ — T_g ? T₀ holds. The three-dimensional treatment indicates the existence of waves with a nonvanishing wave vector component in the direction of the magnetic field. The influence of ionization waves on mean current density, mean Hall field intensity and mean electron temperature is determined up to second order terms in the relative fluctuations of the electron temperature. The amplification of ionization waves reduces the effective electric conductivity, the effective Hall parameter and the mean electron temperature compared to the undisturbed state. Similar results are also obtained for steady state homogeneous isotropic turbulence and a special case of axially symmetric turbulence. Furthermore, a component of the electric field in direction or in opposite direction to the magnetic field vector may be generated by non isotropic and non homogeneous turbulence.     

Theoretical investigation of tunable polarized broadband terahertz radiation from magnetized gas plasma

The mechanism of terahertz(THz) pulse generation with a static magnetic field imposed on a gas plasma is theoretically investigated. The investigation demonstrates that the static magnetic field alters the electron motion during the optical field ionization of gas, leading to a two-dimensional asymmetric acceleration process of the ionized electrons. Simulation results reveal that elliptically or circularly polarized broadband THz radiation can be generated with an external static magnetic field imposed along the propagation direction of the two-color laser. The polarization of the THz radiation can be tuned by the strength of the external static magnetic field.