Radiation reflection by a plasma with electron temperature anisotropy

The reflection of a test electromagnetic wave normally impinging on a plasma surface is investigated within the formalism of the surface impedance. The plasma is assumed to possess an anisotropic two-temperature bi-Maxwellian electron velocity distribution function. The linearly polarized impinging wave during reflection transforms into an elliptically polarized one, the degree of ellipticity depending on the electron temperature anisotropy. Polarization modifications of the reflected wave are particularly important in the conditions of the anomalous skin-effect, when the influence of the wave magnetic field on the electron kinetics in the skin layer is strong. Relations are reported connecting the reflected wave basic parameters to those of the reflecting plasma surface, making possible, through the experimental determination of the reflected wave characteristics, to find the plasma electron concentration and the two effective temperatures. Received 21 May 2002 / Received in final form 21 August 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: zarcone@unipa.it     

垂直入射电磁波在大气压非平衡等离子体层中的传播特性

用数值模拟的方法对大气压非平衡等离子体薄层中，不同的电子密度分布对微波反射、吸收和透射的影响进行了研究。所采用的理论分析方法是分层模型和镶嵌不变原理。计算中考虑了微波在子层间的多次反射和吸收。数值结果表明，对于电磁波的吸收来说，等离子体中具有二次分布的电子密度，其效果要高于线性分布10%左右；而对于反射来说，线性分布效率更高。功率反射系数随波长的增大而增大，功率吸收系数A也不是单调的，当电子密度不变时，A存在一个峰值，随着电磁波波长的增加而增加，达到最大值后，缓慢降低。     

飞秒光丝阵列对10 GHz电磁波的吸收特性

为了研究飞秒光丝阵列对10 GHz电磁波的吸收特性,建立了飞秒光丝阵列吸收电磁波的有限元模型,研究了光丝内电子温度、电子数密度、光丝直径和电磁波的极化等参数对吸收系数的影响。研究结果表明:当电磁波偏振方向与光丝轴向垂直时,阵列对电磁波是透明的;增加光丝内电子数密度或提高电子温度,吸收系数先增大后减小;当光丝直径与电磁波趋肤深度相等时,吸收系数达到最大值。对于S极化电磁波,当光丝直径为50 μm时,吸收系数随入射角的增大而变大;当光丝直径为100~200 μm时,在入射角较小时,吸收系数随入射角的增大而变大;在入射角较大时会出现吸收峰值,最高可达0.45,且光丝直径越大,吸收峰值对应的入射角就越小;对于P极化电磁波,吸收系数随入射角增大而降低。     

Effect of electron correlation on superconducting pairing symmetry

The role of electron correlation on different pairing symmetries are discussed in details where the electron correlation has been treated within the slave boson formalism. It is shown that for a pure s or pure d wave pairing symmetry, the electronic correlation suppresses the s wave gap magnitude (as well as the ) at a faster rate than that for the d wave gap. On the other hand, a complex order parameter of the form () shows anomalous temperature dependence. For example, if the temperature () at which the d wave component of the complex order parameter vanishes happens to be larger than that for the s wave component (), then the growth of the d wave component is arrested with the onset of the s wave component of the order parameter. In this mixed phase however, we find that the suppression in different components of the gap as well as the corresponding due to coulomb correlation are very sensitive to the relative pairing strengths of s and d channels as well as the underlying lattice. Interestingly enough, in such a scenario (for a case of )the gap magnitude of the d wave component increases with electron correlation but not for certain values of electron correlation. However, this never happens in case of the s wave component. We also calculate the temperature dependence of the superconducting gap along both the high symmetry directions ( and ) in a mixed symmetry pairing state and the thermal variation of the gap anisotropy [0pt][0pt] with electron correlation. The results are discussed with reference to experimental observations. Received: 26 August 1997 / Revised: 31 December 1997 / Accepted: 28 January 1998     

Double- and multi-photon pair production and electron-positron entanglement

We investigate entanglement of electrons and positrons produced via absorption by a vacuum of two or several photons from two external electromagnetic waves. The waves are modelled by finite-length focused pulses. Structures of the arising electron and positron wave packets are investigated in the momentum and coordinate representations. Conditional and unconditional widths of these wave packets, as well as the Schmidt number K are found, and they are used to evaluate the degree of entanglement. The conditions are found when entanglement is large. It is shown that the highest entanglement can be reached at nonrelativistic energies of electrons and positrons. Possibilities of observing the entanglement effects in experiments on pair production are discussed.     

Effects of size and surface anisotropy on thermal magnetization and hysteresis in the magnetic clusters

Based on Monte Carlo simulation, the spin configurations, thermal magnetization and hysteresis loops of the clusters coated by the surface shell with radial anisotropy are studied. Interestingly, a new multidomain containing a few of subdomains whose easy directions are along those of the configurational anisotropy, a magnetization curve in steps and a first order phase transition from the single domain to the multidomain in the thermal and field magnetization processes, are found, which is as a result of the interplay of the configurational anisotropy, the size effect, the surface anisotropy, the applied field and the thermal fluctuation. In this first order transition, we find a critical temperature, a critical surface anisotropy and a critical size. The simulated temperature dependence of the coercivity of the cluster with the surface anisotropy can be fitted by H_c (T)=H_c (0)(1-C_αT^α) with low value of α, which explains well the experimental results of the nanoparticles. Moreover, it is found that the hysteresis loops and coercivity are strongly affected by the cluster size and the thickness of the surface layer.     

CdSe quantum dot formation induced by amorphous Se

The mechanism allowing the transition from a two-dimensional strained layer of CdSe on ZnSe to self-assembled islands induced by the use of amorphous selenium is still not fully understood. For a better understanding, atomic force microscopy and transmission electron microscopy studies were performed on CdSe films with a thickness close to that for quantum dot formation. Below this thickness, the sample surface results in undulations along the [1 1 0] crystal direction, while few quantum dots are situated in the wave valleys. Plan view transmission electron microscopy studies reveal a strong anisotropy of the islands and show that the Se desorption conditions are crucial.     

The measurements of anisotropy of ultrasound propagation and magnetic susceptibility in viscous ferrofluid

Results of experimental study of anisotropy of ultrasound propagation and anisotropy of magnetic susceptibility in a ferrofluid of considerable viscosity are reported. The dependence of the propagation velocity and absorption coefficient of ultrasonic wave on the angle between the direction of measurement and that of the magnetic field provides important information on the ferrofluid structure in a magnetic field. The results show that the ultrasonic absorption coefficient of a wave propagating in a viscous ferrofluid in a DC magnetic field depends mainly on the rotational degree of freedom. Measurements of the anisotropy of the magnetic susceptibility of a ferrofluid bring the information on the magnetic moment of the magnetic particles and their mean radius.     

Electron mobility in a modulation doped AlGaN/GaN quantum well

We consider a two dimensional electron gas confined to a modulation doped AlGaN/GaN quantum well and study the dependence of low field mobility on various parameters such as composition, well width, remote impurity and interface roughness as a function of temperature. GaN is assumed to be in the zincblende structure. Acoustic and optical phonon, ionized remote impurity and interface roughness scatterings are taken into account in mobility calculations. The scattering rates are calculated using the self-consistently calculated wave functions obtained from the numerical solution of Poisson and Schr?dinger equations. Also found from the self-consistent solutions are the potential profile at the junction, the energy levels in the well and electron concentrations in each level. Ensemble Monte Carlo method is used to find the drift velocities of the two dimensional electrons along the interface under an applied field. The mobility of two dimensional electrons is obtained from the drift velocity of electrons. It is found that while remote impurity scattering is very effective for small values of spacer layer and doping concentrations, increasing Al concentration reduces the mobility of electrons. The effect of surface roughness, on the other hand, on mobility is almost independent of well width. The results of our simulations are compatible with the existing experimental data.     

Surface polaritons in a thin layer of an anisotropic superconductor

An expression for the permittivity tensor of a superconductor is derived within the framework of a two-fluid model of the electron subsystem. For the temperature range T ? T _c , the dispersion relations for surface polaritons that propagate in the direction of the three principal crystallographic axes of a thin superconducting layer along its two interfaces with isotropic dielectrics are found. A significant effect of the superconductor anisotropy on the dispersion relations is revealed. The polarization structure of the wave field inside and outside the superconducting layer is determined.     

Anomalous absorption of intense electromagnetic radiation in a non-uniform overdense plasma

In this work, we present numerical computation of anomalous absorption of a powerful electromagnetic wave incident normally, on an isothermal plasma with a linear density gradient. We consider first, the “intensity” dependence of anomalous absorption treating two important cases: one that of a typical high temperature, high density fusion plasma occurring in a laser-pellet application and the other that of an ionospheric (F-layer) plasma. It is found that for low incident powers, reflection is almost constant, corresponding essentially, to classical electronion collisions. As we increase the incident power, however, we find that the reflection starts dropping and the absorption takes over. This result is interpreted in terms of the instability generated anomalous absorption of the incident wave. Next, we consider the “wavelength” dependence of anomalous absorption, wherein we observe that the effect of anomalous absorption is increasing for longer wavelengths of the incident laser radiation.     

Dynamic anisotropy and heterogeneity of polystyrene thin films as studied by inelastic neutron scattering

We studied the dynamic anisotropy and heterogeniety of polystyrene thin films in glassy state with inelastic neutorn scattering. Adjusting the scattering vector to the directions parallel and perpendicular to the film surface, we observed the elastic scattering intensities as a function of temperature. It was found for the 200 A film that the elastic intesity decreased with increasing temperature more rapidly in the perpendicular direction than in the pararell direction, showing the higher mobility in the perpendicular direction. However, such dynamical anisotropy was not observed in the 1000 A film. The decrease in the mobility was observed with the film thickness in both the directions. These results were explained in terms of an interface hard layer. We also evelauated the dynamical heterogeniety from the non-Gaussian parameter A0, which increased with decreasing the film thickness, showing the increase in the dynamical heterogeneity. Assuming a simple bi-layer model consisting of the interface hard layer and the bulk-like layer, we analyzed the thickness dependence of the non-Gaussian parameter A0 and the mean square displacement (u2) to find that the hard layer has a thickness of approximately 130 A and a mean square displacement of approximately 0.018 A2 at 230 K.     

Spin wave excitations in Fe films grown on GaAs(1 1 3)A substrates

The spin wave excitation and its size effect has been studied in Al-capped Fe films grown on low-symmetry GaAs(1 1 3)A substrates. The temperature dependence of saturation magnetization follows an effective Bloch's law as long as magnetization remains larger than about 70% of its saturation value. A significant increase of the spin wave parameter B is found in Al-capped ultrathin Fe films grown on GaAs(1 1 3)A compared to bulk Fe, Fe films on GaAs(0 0 1) and other systems. This is explained as a result of the reduction in uniaxial magnetic anisotropy observed in this orientation for the same thickness range. However, this observed uniaxial magnetic anisotropy is found to be a likely reason for stabilizing the ferromagnetism.     

On the origin of solar wind. Alfvén waves induced jump of coronal temperature

Absorption of Alfvén waves is considered to be the main mechanism of heating in the solar corona. It is concluded that the sharp increase of the plasma temperature by two orders of magnitude is related to a self-induced opacity with respect to Alfvén waves. The maximal frequency for propagation of Alfvén waves is determined by the strongly temperature dependent kinematic viscosity. In such a way the temperature jump is due to absorption of high frequency Alfvén waves in a narrow layer above the solar surface. It is calculated the power per unit area dissipated in this layer due to damping of Alfvén waves that blows up the plasma and gives birth to the solar wind. A model short wave-length (WKB) evaluation takes into account the 1/f² frequency dependence of the transversal magnetic field and velocity spectral densities. Such spectral densities agree with old magnetometric data taken by Voyager 1 and recent theoretical calculations in the framework of Langevin-Burgers MHD. The presented theory predicts existence of intensive high frequency MHD Alfvén waves in the cold layer beneath the corona. It is briefly discussed how this statement can be checked experimentally. It is demonstrated that the magnitude of the Alfvén waves generating random noise and the solar wind velocity can be expressed only in terms of satellite experimental data. It is advocated that investigation of properties of the solar surface as a random driver by optical methods is an important task for future solar physics.     

Electron-Cyclotron Resonance Heating in Tandem Mirrors

Using numerical ray-tracing techniques, we study the propagation and absorption profiles of electromagnetic waves launched in the end cells of three different tandem mirrots: Phaedrus, an overdense low-temperature tandem mirror with plugs; TASKA, a conceptual tandem mirror with plugs and thermal barrier; and TMX-U, a tandem mirror with a significant hot-electron population. In particular, the effects of weakly relativistic thermal anisotropy on the absorption profile are examined. In general, at sufficiently low densities and temperatures, the X mode can access the plasma and achieve significant heating of the electrons. As the electron temperature increases, the X mode gets quickly absorbed at the edge and only the O mode achieves significant penetration and heating. For sufflciently large launching angles, the presence of thermal anisotropy can actually shift the region of maximum absorption towards the electron-cyclotron resonance layer. Regions of whistler instability appear along rays launched nearly along the machine axis, when the thermal-anisotropy ratio, temperature, and density reach sufficiently high values.     

低混杂波驱动电流对等离子体电导率的影响

本文考虑了波驱动通量对磁场位形以及电子密度和温度的空间分布的依赖,应用伴随关系,计算了低混杂波驱动的等离子体电导率的空间分布。结果表明,注入功率谱对电导率空间分布的影响相当敏感,等离子体环外侧的电导率略高于内侧的电导率。但是不论在外侧或内侧,随着离子电荷数增大,波驱动的等离子体电导率降低,而它与斯必泽-亥姆电导率之比却增大。     

Effects of Spatial Variation of Thermal Electrons on Whistler-Mode Waves in Magnetosphere

A ray-tracing method is developed to evaluate the wave growth/damping and specifically propagation trajectories of the magnetospherically reflected Whistler-mode waves. The methodology is valid for weak wave growth/damping when plasma is comprised of a cold electron population and a hot electron population, together with background neutralizing ions, e.g. protons. The effect of anisotropic thermal electrons on the propagation of Whistler-mode waves is studied in detail. Numerical results are obtained for a realistic spatial variation model of plasma population, including the cold electron density distribution, and the thermal electron density and temperature distribution. It is found that, analogous to the case of the typical cold plasma approximation, the overall ray path of Whistler-mode waves is insensitive to the thermal electron density and temperature anisotropy, and the ray path reflects where wave frequency is below or comparable to the local lower hybrid resonance frequency flhr. However, the wave growth is expected to be influenced by the thermal electron population. The results present a first detailed verification for the validity of the typical cold plasma approximation for the propagation of Whistler-mode waves and may account for the observation that the Whistler-mode waves tend to propagate on a particular magnetic shell L where the wave frequency is comparable to fthe.     

Self-organization of a plasma due to 3D evolution of the Weibel instability

The nonlinear evolution of the thermal Weibel instability is studied by using three-dimensional particle-in-cell simulations. After a fast saturation due to a reduction in the temperature anisotropy, the instability evolves to a quasistationary state which includes a single mode long wavelength helical magnetic field and a finite degree of temperature anisotropy. The nonlinear stability of this state is explained by periodic variations of the temperature anisotropy axis. At long time scales the magnetic field, wave number, and temperature anisotropy slowly evolve to the decreasing magnitudes.     

非磁化等离子体中的电子碰撞频率

电子碰撞频率是非磁化等离子体的一个重要参数,它对等离子体与电磁波相互作用的性质具有较大影响。基于产生等离子体的气压条件不同,其电子碰撞频率具有较大变化范围。研究了它对非磁化等离子体吸波能力的影响,得出了当电子碰撞频率与等离子体频率和入射波频率相当时,等离子体对电磁波具有较大吸收的结论,给出了对应于最大吸收的最佳碰撞频率值。分析了通过选择放电气体来调整等离子体的电子碰撞频率。