Radiation Emission of Fast Electrons in Collisions with “Ion‐Sphere” in Dense Plasmas

Radiative emission of fast electrons in collision with an “ion‐sphere” electron distribution in dense plasmas is under consideration. The electron structure of the ion sphere is calculated ab initio using self‐consistent solution of both bound and free electron distribution inside the sphere. Two radiation channels are included: emission of the colliding electron itself in static potential (conventional or static Bremsstrahlung) and the emission of “ion sphere” medium due to its polarization by the colliding electron (polarization Bremsstrahlung). The last one is calculated in the frame of local plasma density approximation. Interference between conventional and polarization Bremsstrahlung is taken into account. It is shown that spectral cross section of the process has characteristic features depending on plasma density and ionization stage of plasma ions. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anode plasma structure in a gas discharge with neutral density depleted by ionization

We consider the anode plasma structure in a gas discharge with density of neutral atoms (neutrals) depleted by strong ionization. We obtain analytical solutions of the quasi-neutrality equation for the potential distribution and a condition for the existence of anode plasma in the one-dimensional case for arbitrary potential dependences of the neutral depletion frequency and the electron density. We consider the special cases of a constant neutral depletion frequency, ionization by Maxwellian electrons, and ionization by an intense electron beam under the conditions of collisionless ion motion and Boltzmann thermal electron distribution. The solutions for the first two cases at zero depletion parameter, i.e., at constant gas density, match those obtained in [1] by a power series expansion. In the case of ionization by Maxwellian electrons, the formation of anode plasma at reasonable working-gas flow rates is shown to be possible only at a fairly high electron temperature (if, e.g., xenon is used as the working gas, then T _e ≥ 5 eV). Steady-state solutions of the quasi-neutrality equation under ionization by an intense electron beam exist only if the ratio of the electron beam density to the maximum thermal electron density does not exceed a certain limiting value.     

影响单电子非线性汤姆孙散射因素的研究

应用电子汤姆孙散射的经典理论，通过理论分析和计算机模拟，研究了超短超强激光脉冲作用下电子产生的辐射脉冲的性质.计算表明，在这种情况下，电子的辐射通常以阿秒脉冲列的形式出现.讨论了不同激光场参数（包括激光强度、脉宽、初相位和偏振态）、不同电子初始状态（初始速度和位置）对辐射脉冲的时间和空间特性的影响.通常在相对论光强条件下，激光强度越大，电子辐射越强，脉宽越窄，中心频率越大，并且方向性越好；电子在线偏振激光中产生的辐射效率，比在同样强度下圆偏振激光中产生的效率更高；无论入射光是线偏振光，还是圆偏振光,辐射场呈现较复杂的偏振态, 并且它与辐射方向有关.当电子具有一定的初始能量时，通常辐射场的振幅随电子初始能量的增大而增大.不管电子的初始能量以及运动方向如何，做相对论运动的电子产生的辐射趋向于出现在靠近电子运动方向的角度区域.     

Determination of Plasma Frequency and the Pulse Relaxation Time in Doped Semiconductors by FIR Surface Waves

The Surface Electromagnetic waves method was applied for determination of plasma frequency and pulse relaxation time in doped A³B⁵ semiconductors. The influence of plasmon phonon interaction, nonparabolity of the conduction band, electron scattering peculiarities, presence of disturbed layers are under consideration.     

Scattering of relativistic electrons by a focused laser pulse

The problem of the motion of a classical relativistic electron in a focused high-intensity laser pulse is solved. A new three-dimensional model of the electromagnetic field, which is an exact solution of Maxwell’s equations, is proposed to describe a stationary laser beam. An extension of the model is proposed. This extension describes a laser pulse of finite duration and is an approximate solution of Maxwell’s equations. The equations for the average motion of an electron in the field of a laser pulse, described by our model, are derived assuming weak spatial and temporal nonuniformities of the field. It is shown that, to a first approximation in the parameters of the nonuniformities, the average (ponderomotive) force acting on a particle is described by the gradient of the ponderomotive potential, but it loses its potential character even in second order. It is found that the three-dimensional ponderomotive potential is asymmetric. The trajectories of relativistic electrons moving in a laser field are obtained and the cross sections for scattering of electrons by a stationary laser beam are calculated. It is shown that reflection of electrons from the laser pulse and the surfing effect are present in the model studied. It is found that for certain impact parameters of the incident electrons the asymmetic ponderomotive potential can manifest itself effectively as an attractive potential. It is also shown that even in the case of a symmetric potential the scattering cross section contains singularities, known as rainbow scattering. The results are applicable for fields characterized by large (compared to 1) values of the dimensionless parameter η² = e ²〈E ²〉/m ²ω² and arbitrary electron energies.     

三角激光脉冲尾波加速粒子模拟

电子俘获是激光尾波场加速电子的主要机理，增大电子的初速度可以使更多的电子被尾波场俘获.提出三角脉冲激发尾波加速电子的方案，三角脉冲平缓上升沿激发受激Raman散射，用以初步加速电子，三角脉冲陡峭下降沿激发尾波场，将更多的电子加速到接近光速.2D3V粒子模拟结果证实了这一点.同时表明：脉冲长度为几个等离子体波长的超强激光在稀薄等离子体中传播时，还激发侧向Raman散射.在侧向受激Raman散射中，静电波增长最快的波矢模式为k_p=(2ω_p/ω₀ 关键词： 有质动力 电子俘获 前向受激Raman散射 侧向受激Raman散射     

Calculation of hot electron distribution functions and of the threshold field in the gunn effect

The Boltzmann equation is formulated for the (000) electrons in GaAs in the highenergy approximation taking into consideration acoustic, polar and nonequivalent intervalley scattering to the 〈100〉 satellite valleys and the nonparabolicity of the central valley. The simplified form of the equation permits a qualitative discussion of the shape of the electron density distribution without explicitly computing the distribution function and illustrates the effect of the various scattering mechanisms as a function of the electric field. Examples of electron density distribution curves are calculated for a few typical field values, and the result is used to derive the fielddependent population of the satellite valleys and the average drift-velocity in the vicinity of the Gunn threshold. For an intervalley deformation potential constant of 5×10⁸ eV/cm the threshold is found to be 2.7 kV/cm in the parabolic case increasing to 3.5 kV/cm when nonparabolicity is included. Comparison with experimental data is made.     

Optical characteristics and parameters of a gas discharge plasma based on a mixture of mercury dibromide vapor and helium

We present the results of studies of the optical characteristics and parameters of a gas discharge plasma produced by a barrier discharge at atmospheric pressure, based on a mixture of mercury dibromide vapor and helium (the working medium of a small (emitting area 8 cm²) exciplex gas discharge emitter). An average emission power of 70 mW (λ_max = 502 nm) was achieved for pump pulse repetition frequency 6 kHz, voltage pulse amplitude 2.3 kV, and current pulse amplitude 8 A of duration ~100 nsec. We established the plasma parameters: the electron energy distribution functions, the transport characteristics, the discharge power losses per unit pressure going toward electron processes, the electron concentration and temperature, and also the rate constants for processes of elastic and inelastic scattering of electrons by the components of the working mixture as a function of the parameter E/N, where E is the electric field strength and N is the total concentration of mercury dibromide molecules and helium atoms.     

Experiment on the electromagnetic radiation excited in an electron beam‐ion channel system

An experimental study on electromagnetic (EM) radiation in an electron beam‐ion channel system is reported, which indicates the same result predicted by our previous theory. The system is formed in an arc plasma jet with a plasma density of 10¹⁷m^?3 , and the electron beam is driven by a voltage pulse of 20 kV. The result shows that the system can excite EM radiation in the range of the plasma frequency. The scheme is also hopeful to be used for generating high‐frequency and wide‐band EM radiation up to terahertz by enhancing the plasma density.     

Electron temperature (T e) measurements by Thomson scattering system

Thomson scattering technique based on high power laser has already proved its superoirity in measuring the electron temperature (T _e and density (n _e) in fusion plasma devices like tokamaks. The method is a direct and unambiguous one, widely used for the localised and simultaneous measurements of the above parameters. In Thomson scattering experiment, the light scattered by the plasma electrons is used for the measurements. The plasma electron temperature is measured from the Doppler shifted scattered spectrum and density from the total scattered intensity. A single point Thomson scattering system involving a Q-switched ruby laser and PMTs as the detector is deployed in ADITYA tokamak to give the plasma electron parameters. The system is capable of providing the parameters T _e from 30 eV to 1 keV and n _e from 5 × 10¹²cm⁻³−5 × 10¹³cm⁻³. The system is also able to give the parameter profile from the plasma center (Z=0 cm) to a vertical position of Z=+22 cm to Z=−14 cm, with a spatial resolution of 1 cm on shot to shot basis. This paper discusses the initial measurements of the plasma temperature from ADITYA.     

稠密热等离子体的电子传导不透明度研究

 采用相对论自洽场平均离子势模型，电子简并效应采用Fermi–Dirac分布描述，参数结构因子用来考虑离子结构的影响，电子对离子的散射截面采用分波分析，利用推广的Ziman公式对稠密热等离子体的电子传导不透明度作了大量计算，并将结果与实验值和其它理论值作了比较。     

Level shifts and inelastic electron scattering in dense plasmas

A completely quantum mechanical formalism has been developed to describe the high density plasma effects on fundamental atomic parameters. Both the bound and free electrons are treated by a method which in principle is similar to Hartree's self-consistent field method. The free plasma electrons' wavefunction is obtained from the Schrödinger equation with the effective pottential representing the spherically averaged Coulomb interaction with bound and free electrons. Results are given for level shifts, coefficients of transition probabilities, and electron collision cross sections of Ne⁺⁹ for temperatures of 200 and 500 eV for an electron density range of 1–6 × 10²⁴ cm^?3.     

一种考虑几何屏蔽效应的计算“电子-分子”散射总截面的可加性规则修正方法

在考虑分子内原子间的几何屏蔽效应随电子入射能量变化的基础上, 提出了一种能够在中、高能区准确计算“电子-分子”散射总截面的可加性规则修正方法. 利用这一修正后的可加性规则并使用“电子-C, H, O, N原子”散射总截面的实验数据, 在50—5000 eV内计算了电子被NO, N₂O, NO₂和C₂H₆分子散射的总截面, 且将计算结果与实验结果及其他理论结果进行了比较. 结果表明, 利用这一方法修正过的可加性规则进行计 关键词： 电子散射 可加性规则 总截面 几何屏蔽效应     

Generation of a dense electron beam in a thin film using an ultraintense femtosecond laser pulse

Electron dynamics in a thin target irradiated with femtosecond laser pulses at an intensity of 10²⁰ W/cm² is studied in the framework of the kinetic theory of laser plasma based on the construction of propagators (in classical limit) for electron and ion distribution functions in plasma. The calculations are performed for real densities and charges of plasma ions. Electrons are partly ejected from the target. The laser pulse energy is predominantly absorbed by electrons, and the electrons are accelerated to relatively high energies.     

Nonlinear Thomson scattering of an ultrashort laser pulse

The nonlinear scattering of an ultrashort laser pulse by free electrons is considered. The pulse is described in the “Mexican hat” wavelet basis. The equation of motion for a charged particle in the field of a plane electromagnetic wave has an exact solution allowing, together with the instant spectrum approximation, the calculation of the intensity of nonlinear Thomson scattering for a high-intensity laser pulse. The spectral distribution of scattered radiation for the entire pulse duration is found by integrating with respect to time. The maximum of the emission spectrum of a free electron calculated in 10¹⁹–10²¹ W/cm² fields lies in the UV spectral region between 3 and 12 eV. A part of the continuous spectrum achieves high photon energies. One percent of the scattered energy for the field intensity 10²⁰ W/cm² is concentrated in the range ħω > 2.7 × 10² eV, for a field intensity of 10²¹ W/cm² in the range ħΩ > 7.9 × 10² eV, and for an intensity of 10²² W/cm² in the range ħΩ > 2.45 × 10⁵ eV. These results allow us to estimate nonlinear scattering as a source of hard X-rays.     

高能电子与超强激光束作用产生的阿秒脉冲列

利用非线性汤姆孙散射的理论，从理论和数值模拟上研究了单电子在横向穿越高斯激光束束 腰时所辐射的x射线阿秒脉冲列的性质. 主要分析了电子以初始能量γ₀=1M eV—100M eV横向穿越激光振幅参数为a₀=1—10的高斯光束束腰获得的阿秒辐射脉冲的 时间 和空间性质. 计算表明，辐射呈现脉冲列的形式. 脉冲列的包络宽度取决于激光强度、束腰 的宽度以及入射电子能量. 电子的初始能量比激光强度对电子辐射脉冲的影响更大. 辐射脉 宽、脉冲间隔和脉冲包络宽度都正比于1/γ²₀，辐射功率正比于 γ⁶₀，辐射能 量正比于γ⁴₀. 当改变激光振幅a₀时，辐射功率正比 于a²₀、辐射包络中单 个脉冲脉宽正比于1/a₀、脉冲之间的间隔正比于a₀. 当保持激光强 度不变，而改变光束 束腰半径w₀时，辐射的脉冲数量、包络和辐射能量正比于w₀. 当 激光功率保 持不变时而改变激光强度和束腰半径时，脉冲包络宽度和最大辐射能量都基本不变. 当激光 振幅参数a₀=1，电子初始能量为10MeV时，激光束腰为两个激光波长时，电子 辐 射脉冲包络宽度只有14×10^-3τ₀（τ₀为入 射激光周期），达到几个阿秒的量级. 关键词： 阿秒脉冲 非线性汤姆孙散射 高斯激光光束     

氢负离子在变形球面附近的光剥离

利用理论模型成像方法, 对氢负离子在变形球面附近的光剥离进行了研究. 首先, 推导出了光剥离电子通量的计算公式, 然后对电子通量分布和光剥离截面进行了计算.结果表明: 平面效应只在一定范围内对氢负离子的光剥离过程产生影响. 在距离z轴比较近的区域, 球面效应起主要作用, 电子通量分布和光剥离截面与只有球面存在的情况一致, 此时平面效应可以忽略; 距离z轴较远的区域, 平面效应和球面效应共同起作用, 此时变形球面对光剥离过程会产生比较大的影响. 当球面半径和氢负离子到球面之间的距离给定, 随着入射光子的能量增大, 光剥离电子通量的振幅先增大后减小, 然后又慢慢增大, 振荡频率增大.当固定球面与氢负离子之间的距离, 随着球面半径的增大, 光剥离电子的通量趋向于只存在球面的情况.因此, 可以通过改变入射光子能量和球面的半径对氢负离子的光剥离进行调控. 本文的结果对于研究负离子体系在曲面附近的光剥离及光剥离 显微问题的实验研究具有一定的参考价值. 关键词： 理论模型成像方法 光剥离电子通量 光剥离截面 变形球面     

Die Störung der Resonanzstreuung von γ-Strahlung einer Kr79-Quelle an Br79 durch gaskinetische Stöße in der Quelle

γ-quanta emitted by gaseous Kr⁷⁹-sources were scattered resonantly from Br⁷⁹-nuclei. By increasing the density in the source the effective cross section for resonance scattering was influenced due to gaskinetic collisions of the recoiling Br⁷⁹-nuclei with atoms before emission of theγ-quantum. The cross section for gaskinetic collisions was determined to beσ _st=(12.0±1.8) 10^?16 cm² in agreement with the theoretical value obtained from a hard sphere model. This technique, similar to the usual “Doppler shift technique”, yields, if combined with the measurement of the absolute cross section for resonance scattering, spin and lifetime of the levels at 398 and 606 keV. The resonance scattering from the 834 keV level shows an unexpected behaviour in complete contradiction to the hard sphere model, indicating that the velocity of the recoiling nuclei is increased in a dense source atmosphere.     

Neutron yield upon the irradiation of thin TiD<Subscript>2</Subscript> foils with a superintense laser pulse

The yield of neutrons from the thermonuclear-fusion reaction D(d, n)³He induced in a thin skin layer by the interaction of a high-intensity laser pulse of picosecond duration with thin TiD₂ foils is calculated. A multiple ionization of titanium atoms at the leading edge of the laser pulse is considered. The heating of free electrons proceeds via induced inverse bremsstrahlung in elastic electron scattering on multiply charged titanium ions. The electron temperature is calculated. It proves to be about 10 keV at the laser-pulse intensity of 5×10¹⁸ W/cm² at the peak. The neutron yield is estimated at 10⁴ per laser pulse. These results are in qualitative agreement with experimental data.     

Absorption of electromagnetic energy by slow electrons under scattering from Coulomb centers

Simple analytical expressions are obtained for the rate of the inverse stimulated bremsstrahlung absorption under electron scattering from a Coulomb center with charge Z in the presence of the electromagnetic field. The initial and final values of electron energy are assumed to be small compared to the Rydberg energy Z ² (atomic units are used throughout). Single-photon processes of absorption and induced radiation of photon by electron are treated. It is assumed that the electromagnetic field frequency ω is rather low, so that the condition Zω/p ³ ? 1, where p is the electron momentum, and the condition ?ω ? p ² are valid. However, this frequency is assumed to be fairly high compared to the electron-Coulomb center collision frequency: ω ? v _nei. The dependences of the rates of photon absorption and induced radiation on the angle θ between the direction of incident electron and the electromagnetic field polarization vector (assumed to be linearly polarized) are obtained. It is demonstrated that, for any angles θ, the rate of photon absorption is higher than the rate of induced radiation and, therefore, the Marcuse effect for slow electrons (electromagnetic field amplification) is absent. It is further demonstrated that a slow electron on the average absorbs double ponderomotive energy per collision with an ion (Coulomb center) in Maxwellian plasma. This agrees both with the known results calculation for fast electrons and with the known results of the calculation based on the classical Boltzmann kinetic equation for plasma.