相对论效应对大振幅电子等离子体振荡破裂影响的数值模拟

通过数值求解符拉索夫方程和泊松方程，研究了相对论效应和温度效应对等离子体振荡破裂的影响. 不考虑相对论效应情况下，初始扰动幅度较小时，不会发生等离子体振荡破裂，系统具有时间周期性. 此时电子温度的增加，会使得等离子体振荡最大幅度减小. 考虑相对论效应时，即使初始的等离子体电子密度扰动幅度不大，随着时间演化，相对论效应也能导致等离子体振荡破裂，而且初始电子密度扰动越小，产生等离子体振荡破裂所需时间越长. 在初始电子密度扰动较大时，无论考虑和不考虑相对论效应都会出现波破裂，但两者的结果有很大不同. 此外温度效应会降低能发生等离子体波破裂的阈值；等离子体波的相速度越大，能产生的波破裂现象也越明显. 关键词： 等离子体振荡 相对论效应 振荡破裂     

Estimates of the electron density in an ECR source of calcium plasma

A technique is proposed for estimating parameters of the plasma produced by a source based on the electron cyclotron resonance. The analysis is made for the ion cyclotron resonance (ICR) facility designed for separating calcium isotopes. It is assumed that the resonance condition for an extraordinary wave is fulfilled for electrons moving towards the wave. The plasma optical thickness, the transverse energy of resonance electrons, and its dependence on the longitudinal velocity are determined. The charged particle density in the plasma flow is estimated in terms of the balance of the electrons generated as a result of vapor ionization in the discharge zone and the electron losses due to longitudinal ambipolar diffusion.     

Controlling the generation of high frequency electromagnetic pulses with relativistic flying mirrors using an inhomogeneous plasma

A method for the controlled generation of intense high frequency electromagnetic fields by a breaking Langmuir wave (relativistic flying mirrors) in a gradually inhomogeneous plasma is proposed. The wave breaking threshold depends on the local plasma density gradient. Compression, chirping and frequency multiplication of an electromagnetic wave reflected from relativistic mirrors is demonstrated using Particle-In-Cell simulations. Adjusting the shape of the density profile enables control of the reflected light properties.     

Landau damping of electrons with bouncing motion in a radio-frequency plasma

One-dimensional particle simulations have been conducted to study the interaction between a radio-frequency electrostatic wave and electrons with bouncing motion. It is shown that bounce resonance heating can occur at the first few harmonics of the bounce frequency (nω_b,n=1,2,3,...). In the parameter regimes in which bounce resonance overlaps with Landau resonance, the higher harmonic bounce resonance may accelerate electrons at the velocity much lower than the wave phase velocity to Landau resonance region, enhancing Landau damping of the wave. Meanwhile, Landau resonance can increase the number of electrons in the lower harmonic bounce resonance region. Thus electrons can be efficiently heated. The result might be applicable for collisionless electron heating in low-temperature plasma discharges.     

Stochastic heating and acceleration of electrons in colliding laser fields in plasma

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.     

Electron energy spectrum in the field-ionized plasma

In this article, the effect of ionization on the energy spectrum of electrons within the interaction of a laser pulse with hydrogen atoms is investigated using particle-in-cell simulation codes. The results show that the behaviour of electrons' energy distribution function in the field-ionized plasma, which occurred due to the field ionization, compared with that in the pre-plasma strongly depends on the pulse shape. For short rise-time pulses (here 30 fs), due to the rapid enhancement of laser electric field, ionization occurs quickly, and as a result, there is not much difference in the electron energy in both the media. However, for pulses with rise time of 40 fs, in the pre-plasma state, the electron population reaches higher energies compared with the field-ionized plasma state. The main reason for this difference is the nonlinear wave breaking that happens earlier due to density inhomogeneity in the field-ionized plasma. On the other hand, at longer rise-time pulses (here 60 and 70 fs), electrons achieve higher energies in the field-ionized plasma than those in the case of pre-plasma. In this case, because of density fluctuations in the field-ionized plasma, the Raman backscattered radiations are seeded by a strong initial noise at the earlier times and the Mendonca condition for chaos threshold is met sooner. Therefore, the electrons gain more energy through the stochastic mechanism that is in agreement with chaotic nature of the motion.     

Nonlinear resonance of plasma waves in the thermal irregularities of ionospheric plasma

We study the effect of striction plasma density disturbances on the generation intensity of longitudional cold and plasma oscillations due to polarization of the magnetic field-aligned ionospheric plasma irregularities with δN_o<0 by a powerful radio wave. It is assumed that the plasma density level inside the irregularity intersects the upper-hybrid resonance level, in the vicinity of which the cold oscillations excited directly by a powerful radio wave are transformed to shorter-wave plasma oscillations. We consider the short plasma wave limit to reduce the problem to a system of two coupled equations for the cold wave induction and plasma wave electric field. The first equation is supplemented by a local source equal to the integral of the plasma wave electric field in the resonance region. The second equation involves the cold wave induction at the resonance point and describes the electric field of interacting waves in the resonance vicinity. We use simplifications connected with the small absorption of plasma waves propagating inside the irregularity and weak radiation of these waves outside the irregularity. These conditions correspond to the generation of eigenmodes of plasma oscillations trapped in the irregularity. We have obtained a resonance-type nonlinear equation for the electric field intensity (or energy flux) of eigenmode plasma waves with allowance for striction disturbances of the plasma density profile in the resonance region. It is shown that the striction expulsion of plasma is responsible for the occurrence of coefficients describing the change in the intensity of excitation and radiation of plasma waves at the irregularity boundary. Such an expulsion leads to variations of the efficient generation band of plasma eigenmodes with the total phase increment of the wave in the irregularity. It also leads to a change in the phase shift of the plasma wave reflected from the resonance. These coefficients and the nonlinear phase shift are expressed in terms of real wave functions of the nonlinear Airy equation which describes the electric field of the excited waves in the resonance vicinity when the dissipation is absent. Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, Troitsk, Moscow region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 3, pp. 270–297, March, 1998.     

Particle-in-cell investigation on the resonant absorption of a plasma surface wave

The resonant absorption of a plasma surface wave is supposed to be an important and efficient mechanism of power deposition for a surface wave plasma source. In this paper, by using the particle-in-cell method and Monte Carlo simulation, the resonance absorption mechanism is investigated. Simulation results demonstrate the existence of surface wave resonance and show the high efficiency of heating electrons. The positions of resonant points, the resonance width and the spatio-temporal evolution of the resonant electric field are presented, which accord well with the theoretical results. The paper also discusses the effect of pressure on the resonance electric field and the plasma density.     

飞秒激光-等离子体相互作用中快电子能量分布

 采用不同量程的电子谱仪与LiF热释光探测器相配合,测量了飞秒激光 等离子体相互作用中产生的快电子能量分布。结果显示快电子能量分布的一致性和多个重要特征与国外同类实验和计算机模拟结果相似。快电子能谱在低能处产生凹陷是由于冷电子的回流产生的;几种加速机制共同作用是能谱在100~350 keV范围内出现平台的原因;快电子的有效温度较好地满足共振吸收的温度定标律是由于反射激光加速与共振吸收机制均是通过朗道阻尼或波破对电子进行加速的。     

Emission of plasma escaping from a heterogeneous solid (granite) under an electric discharge near its surface

The electrical breakdown of air near a granite plate causes sequential release of several hundred plasma jets consisting of electrons, positively charged ions, and Si, O, and other atoms. The release duration of each jet does not exceed ～10 ns, and the interval between them varies from ～10 to ～300 μs. It is assumed that the shock wave increases the lattice strain in dislocation pile-ups to such values at which the ground and excited state levels begin to cross. This leads to transitions between electronic levels, breaking of interatomic bonds, and emission of positively charged ions and electrons.     

Resonant electromagnetic soliton in a dense plasma

A study is made of the propagation of an electromagnetic wave along a constant magnetic field B₀ in a high-density plasma under cyclotron resonance conditions when the interaction between the electrons and the wave is strongly nonlinear. It is found that the relativistic reduction in the electron gyrofrequency in the field of the electromagnetic pulse stabilizes the wave. A solution is obtained in the form of an envelope soliton and it is shown that its group velocity is considerably reduced.State University, Rostov-on-Don. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1994.     

The electric field of an electron in an electron–hole plasma with degenerate electrons: Possibility of formation of a superconductivity state

We consider the possibility of the formation of a superconductivity state either in a semiconductor or in an electron–hole plasma with degenerate electrons due to the attractive forces between the electrons as a result of the exchange effects through the electron–hole sound wave by an analogy to the phonon waves in a solid state. We have determined an interaction potential between two electrons in a degenerate electron–hole plasma. The potential appears to be attractive at distances much larger than the Debye radius and decreases as 1/r³. We discuss the conditions in which the bound electron state, the so‐called “Cooper Pair,” in a such field can be formed.     

On the possibility of natural formation of a transverse wave with a phase velocity lower than the velocity of light

The formation of a transverse wave with a phase velocity lower than the velocity of light, which can exist in an equilibrium plasma without a slow-wave structure in zero magnetic field, is described. It involves the transformation of a transverse wave with trapped electrons, traveling along the magnetic field, into a slow transverse wave after the removal of the magnetic field. During the evolution of the wave with trapped electrons, the magnetic induction decreases very slowly in the direction of the wave propagation. As a result, the velocity at which electrons are in resonant interaction with the wave increases; therefore, the electrons fall to the bottom of potential wells. Under the influence of the trapped electrons, the phase velocity of the wave decreases and becomes lower than the velocity of light. It becomes equal to the velocity at which the electrons are in resonance interaction with the wave at the instant when the magnetic field vanishes. It is demonstrated that a transverse wave with a velocity lower than the velocity of light can exist in an equilibrium plasma even after the magnetic field vanishes; in this case, the flow of trapped electrons serves as a slow-wave structure.     

The plasma oscillations spectrum of a periodically inhomogeneous 2D electron system near the perforation threshold

The transformation of the spectrum of plasma oscillations with the zero reduced wave vector in the spatially modulated two-dimensional electron system moving to the regime of isolated quasi-one-dimensional electron channels is theoretically investigated. The results provide an explanation of the well-known experimental observations of the plasma resonance transformation when a two-dimensional electron system crosses the continuity violation threshold.     

激光尾波场横向波破的粒子模拟

用粒子模拟方法，研究激光脉冲的横向宽度有限时对产生激光尾波场和电子加热的影响. 在 纵向和横向有质动力的作用下，电子密度的空间分布形成“马蹄型”的低密度区，这些低密 度区好像运动的透镜，使长脉冲激光自聚焦，而且随着激光的传播，“马蹄型”的曲率越来 越大，直到产生横向波破. 横向波破一方面使得波破时静电场极值远小于波破极限，另一方 面将更多的电子推入加速相位，静电场“俘获”的电子数目大大增加，但最大电子动能明显 减小. 关键词： 尾波场 有质动力 电子俘获 横向波破     

Resonant interaction of an electromagnetic wave with an inhomogeneous plasma slab

Resonant interaction at oblique incidence of an electromagnetic wave on an inhomogeneous plasma slab is studied. The time evolution of this interaction is solved numerically from two-fluid equations, adiabatic equation for electron pressure and from Maxwell equations. It is shown that the electromagnetic energy of an incident wave is transformed both into the heat energy and into the energy of plasma oscillations in the direction of density gradient. The distribution of the transformed energy between the heat energy and the energy of plasma oscillations is strongly dependent on the plasma temperature. The ratio of heat energy to the energy of plasma oscillations is growing with growing temperature. The plasma oscillations are generated by magnetic induction of the penetrating wave. In a cold plasma they are generated especially in the overdense region and their frequency is equal to local plasma frequency. The electric field in the direction of plasma gradient has a form of a wave packet whose envelope reaches a maximum at resonance. The characteristic wavelength in the wave packet decreases and the amplitude of the packet increases with the time.     

Cross-focusing of two laser beams in a plasma

Cross-focusing of two copropagating laser beams in a plasma is investigated using paraxial ray theory. If the lasers have a frequency difference equal to the electron plasma frequency, they can drive a large amplitude plasma wave. The ponderomotive force due to the plasma wave forces the plasma electrons outwards thereby generating a parabolic density profile giving rise to cross-focusing. The results show a decrease in threshold for focusing by two orders of magnitude as compared to focusing due to the ponderomotive force of the laser beams.     

Observation of pulsed fast electron precipitations and the cyclotron generation mechanism of burst activity in a decaying ECR discharge plasma

We have detected and investigated quasi-periodic series of pulsed energetic electron precipitations in the decaying plasma of a pulsed ECR discharge in a mirror axisymmetric magnetic trap. The observed particle ejections from the trap are interpreted as the result of resonant interaction between energetic electrons and a slow extraordinary wave propagating in the rarefied plasma across the external magnetic field. We have been able to explain the generation mechanism of the sequences of pulsed precipitations at the nonlinear instability growth phase in terms of a cyclotron maser model in which the instability threshold is exceeded through a reduction in electromagnetic energy losses characteristic of the plasma decay.     

Mode-coupling assisted electron accelerations by a plasma wave

The acceleration of electrons by a plasma wave in the presence of density ripple in plasma has been investigated. Plasma density ripple can excite higher harmonics of different phase velocities of the fundamental plasma wave. The combined role of the different harmonics of the plasma wave contributes significantly in electrons energy gain during acceleration by the fundamental plasma wave. Our calculation shows that the plasma electrons gain considerable energy during the acceleration by the plasma waves in the presence of a density ripple in plasma. The initial electron energy and the ripple density play an important role for the acceleration of an electron.     

Kinetic instability of twisted ion-acoustic mode with superthermal species of plasma

The electrostatic twisted ion-acoustic waves with finite orbital angular momentum states and associated kinetic instability are investigated in an electron-ion plasma. The plasma consisting of superthermal electrons and ions is modeled by using a non-gyrotropic Kappa distribution function in which the free energy source for wave excitation is provided by the relative directed motion of streaming electrons with respect to the ions. In the frame work of kinetic theory, the Vlasov-Poisson equations are employed to derive the expressions for dispersion relation and Landau damping rate under paraxial approximation. The results are analyzed for threshold condition of wave dispersion and instability growth rate in the presence of helical electric field structure. The relevance of study to the observed situations is also described.