Neoclassical radial electric field and transport with finite orbits

Neoclassical transport in a toroidal plasma with finite ion orbits is studied, including for the first time the self-consistent radial electric field. Using a low-noise deltaf particle simulation, we demonstrate that a deep electric-field well develops in a region with a steep density gradient, because of the self-collision-driven ion flux. We find that the electric field agrees with the standard neoclassical expression, when the toroidal rotation is zero, even for a steep density gradient. Ion thermal transport is modified by the electric-field well in a way which is consistent with the orbit squeezing effect, but smoothed by the finite orbits.     

A self-consistent kinetic modeling of a 1-D, bounded, plasma in equilibrium

A self-consistent kinetic treatment is presented here, where the Boltzmann equation is solved for a particle conserving Krook collision operator. The resulting equations have been implemented numerically. The treatment solves for the entire quasineutral column, making no assumptions about λ_mfp/L, where λ_mfp is the ion-neutral collision mean free path and L the size of the device. Coulomb collisions are neglected in favour of collisions with neutrals, and the particle source is modeled as a uniform Maxwellian. Electrons are treated as an inertialess but collisional fluid. The ion distribution function for the trapped and the transiting orbits is obtained. Interesting findings include the anomalous heating of ions as they approach the presheath, the development of strongly non-maxwellian features near the last λ_mfp, and strong modifications of the sheath criterion.     

On the interaction of microparticles with ion flux in gas discharge plasma

The charging and interaction of microparticles in the gas discharge near-electrode region, where the role of the external electric field and ion drift is significant, are considered. It is shown that the ion focusing, responsible for, as is generally accepted, the formation of vertical linear dust structures, is strongly suppressed under conditions typical of experiments with dusty plasma (i.e., the gas discharge at pressures of 10–100 Pa). The contribution of ions trapped by an ion microparticle to the dusty particle charge is estimated. It is shown that trapped ions can appreciably reduce the microparticle charge.     

钡和铯释放的电离层扰动效应对比

碱金属或碱土金属在电离层释放后,迅速在太阳辐射作用下发生光电离,产生正离子和电子,形成人工等离子体云团.本文基于三维双成分流体模型,考虑释放区域水平风场的影响,探讨了钡和铯在电离层释放后的时空演化规律,并对钡和铯的电离层扰动效应进行了对比.模拟结果表明,不考虑中性风场时,生成的等离子体云团逐渐沿磁场被拉伸成椭球形结构,同时,膨胀的等离子体云会推开背景氧离子,在释放中心形成氧离子密度空洞,并在两侧产生两个对称的密度尖峰;水平风场的存在会使得生成的离子云逆风侧的密度梯度变陡,释放物质对背景氧离子的扰动也更大;对比钡与铯的释放结果发现,由于铯的扩散系数较小,钡云的膨胀更为迅速,Ba+云团的覆盖区域更广;而由于光电离率较大,释放相同质量下铯的离子产率更高;此外,Cs+的扫雪机效应比Ba+扫雪机更强,氧离子密度空穴和凸起处的扰动也更大.     

Atomic structure and transition properties of H-like Al in hot and dense plasmas

The atomic structure and transition properties of H-like Al embedded in hot and dense plasmas are investigated using modified GRASP2 K code. The plasma screening effect on the nucleus is described using the self-consistent field ion sphere model. The effective nuclear potential decreases much more quickly with increasing average free electron density,but increases slightly with increasing electron temperature. The variations of the transition energies, transition probabilities,and oscillator strengths with the free electron density and electron temperature are the same as that of the effective nuclear potential. The results reported in this work agree well with other available theoretical results and are useful for plasma diagnostics.     

Interaction between glow discharge plasma and dust particles

The effect of dust particle concentration on gas discharge plasma parameters was studied through development of a self-consistent kinetic model which is based on solving the Boltzmann equation for the electron distribution function. It was shown that an increase in the Havnes parameter causes an increase in the average electric field and ion density, as well as a decrease in the charge of dust particles and electron density in a dust particle cloud. Self-consistent simulations for a wide range of plasma and dust particle parameters produced several scaling laws: these are laws for dust particle potential and electric field as a function of dust particle concentration and radius, and the discharge current density. The simulation results demonstrate that the process of self-consistent accommodation of parameters of dust particles and plasma in condition of particle concentration growth causes a growth in the number of high-energy electrons in plasma, but not to depletion of electron distribution function.     

New interior solution describing relativistic fluid sphere

Nonlinear low-frequency electrostatic waves in a magnetized, three-component plasma consisting of hot electrons, hot positrons and warm ions have been investigated. The electrons and positrons are assumed to have Boltzmann density distributions while the motion of the ions are governed by fluid equations. The system is closed with the Poisson equation. This set of equations is numerically solved for the electric field. The effects of the driving electric field, ion temperature, positron density, ion drift, Mach number and propagation angle are investigated. It is shown that depending on the driving electric field, ion temperature, positron density, ion drift, Mach number and propagation angle, the numerical solutions exhibit waveforms that are sinusoidal, sawtooth and spiky. The introduction of the Poisson equation increased the Mach number required to generate the waveforms but the driving electric field E ₀ was reduced. The results are compared with satellite observations.     

Unstable twisted modes in interpenetrating space plasmas containing superthermal species

The electrostatic twisted modes with orbital angular momentum and associated kinetic instability are studied in a permeating space plasma containing streaming particle species. The plasma containing superthermal electrons and ions is modeled by using a non-gyrotropic Kappa distribution function which penetrates through a relatively slow moving (static) plasma and gives rise to dispersion, damping and growth of ion-acoustic mode under various conditions. Using the Vlasov-Poisson model, the solutions of twisted modes are defined by Laguerre-Gaussian mode functions, which decompose the plasma distribution function and electric field into components characterized by the axial and azimuthal wave numbers. The dielectric constant is derived and analyzed for threshold condition of wave dispersion and instability in the presence of helical electric field with illustrations. The wave excitations due to penetration of solar wind into cometary clouds or interstellar electron-ion plasmas is examined.     

Surface states and the charge of a dust particle in a plasma

We investigate electron and ion surface states of a negatively charged dust particle in a gas discharge and identify the charge of the particle with the electron surface density bound in the polarization-induced short-range part of the particle potential. On that scale, ions do not affect the charge. They are trapped in the shallow states of the Coulomb tail of the potential and act only as screening charges. Using orbital-motion limited electron charging fluxes and the particle temperature as an adjustable parameter, we obtain excellent agreement with experimental data.     

Effect of deviations from isothermality of ions on the propagation of localized dust-acoustic waves

The effect of deviations from isothermality of ions on arbitrary amplitude dust-acoustic solitary structures is studied in an unmagnetized dusty plasma which consists of a negative charged dust fluid, free electrons and hot ions obeying a trapped distribution. For the finite deviation from isothermality of ions, the basic properties of large amplitude solitary waves are studied by employing pseudo-potential approach. It is shown that the effect of such ion behavior changes the maximum values of the Mach number and the amplitude for which solitary wave can exist. For the case that the deviation from isothermality due to nonlinear resonant particle effects is small, calculations by reductive perturbation method leads to a generalized Korteweg-de Vries equation with mixed nonlinearity. The latter admits a stationary dust-acoustic solitary solution with similar width and qualitatively different amplitude in comparison to the case that deviations from isothermality are finite. Furthermore, effects of the equilibrium free electron density and such trapped ions on the amplitude of solitary structures imply a non-uniform transition from the Boltzmann ion distribution to the trapped ion one.     

Current Structures with Magnetic Shear in Space Plasma

Observations of current structures with magnetic shear by the Cluster satellite quartet in the magnetotail and the Wind satellite in the solar wind have been reported. In current structures with magnetic shear, the following structural features have been identified: (i) thickening of a current layer, (ii) plasma density distribution asymmetric with respect to the layer plane, and (iii) formation of an asymmetric current density profile. The kinetic features of the dynamics of ions in current layers with initial shear deformation have been considered. A mechanism has been proposed for the formation of self-consistent current sheets with a nonzero shear magnetic field component.     

Formation of Suprathermal Particle Fluxes in a Strongly Turbulent Plasma

We consider the problem on the formation of suprathermal particle fluxes by electrostatic structures in strongly turbulent cosmic plasmas. It is shown that regions with a strong plasma turbulence can be large accelerators of charged particles. We give solutions of the stationary kinetic equation in a turbulent layer for different acceleration regimes and estimate the efficiency of diffusion over the longitudinal and transverse velocities of particles with respect to the magnetic field. The transverse diffusion in velocity space is more efficient for ions and leads to strong isotropization of ion fluxes. Electrons move almost along the magnetic field. We reveal the conditions under which the regular force in a nonuniform magnetic field influences the stochastic-acceleration process. The average energy of axial motion of the particles and the particle fluxes at large distances from the injection region are estimated. Ions and electrons can be accelerated up to comparable energies. We analyze the characteristic features of the motion of the relativistic-particle beams. It is shown that strong plasma turbulence can form particle beams with specific energies. The proposed mechanism is useful for explanation of the properties of energetic particles in cosmic plasmas with magnetic-field-aligned currents, e.g., in high-latitude regions of planetary magnetospheres, force-free configurations of the solar corona, and the solar wind.     

Hot Ion Plasma Heating Experiments in SUMMA

Initial empirical results are presented for the hot-ion plasma heating experiments conducted in the new SUMMA (Superconducting Magnetic Mirror Apparatus) at NASA Lewis Research Center. A discharge was formed by applying a radially inward DC electric field near the mirror throats. Data were obtained at midplane magnetic flux densities from 1.0 to 3.5 tesla. Charge-exchange neutral particle energy analyzer data were reduced to ion temperatures using a plasma model that included a Maxwellian energy distribution super-imposed on an azimuthal drift, finite ion orbits, and radial variations in density and electric field. Using this plasma model, the highest ion temperatures computed were 5 keV, 1.2 keV, and 1 keV for He+, H2+, and H+, respectively. These were obtained at a mid-plane magnetic flux density of 1.6 T. Ion temperature was found to scale roughly as (P/B)n, where P/B is the ratio of power input to magnetic flux density and n is about 1 for hydrogen and 2 for helium. Optical spectroscopy line-broadening measurements yielded ion temperatures about 15 percent higher than the charge-exchange neutral particle analyzer results for hydrogen and about 50 percent higher for helium. Spectroscopically obtained electron temperatures ranged from 3 to 30 eV.     

Generation of Extra-Ordinary Mode Radiation by an Electrostatic Pump in a Two-Electron-Temperature Plasma

Coherent wave-wave coupling can produce radiation with a high efficiency. Recently, there has been a great deal of interest in the study of electro-magnetic wave generation in magnetized plasmas. We have investigated theoretically the effect of finite ion temperature on the parametric instability of an electro-static upperhybrid pump into an X-mode nonthermal radiation and low frequency ion waves in a two electron temperature plasma. The latter may include the lower-hybrid, the electron-acoustic and the ion-cyclotron waves. The loss cone distribution existing permanently at low altitudes acts as a free energy source generating the upper-hybrid waves. The upper-hybrid waves can also be present because of a linear instability produced by runaway electrons. Nonlinear dispersion relation and the growth rates are derived for each case using the hydrodynamical model. We find extra numerical factor arising due to the ions of finite temperature in the growth rate expression. This study may be useful in magnetosphere, auroral ionosphere, solar wind, solar radio bursts, and laboratory plasmas where ion has finite temperature and electrons have two distinct energy distributions.     

Numerical modeling of low-pressure RF plasmas

A particle-in-cell simulation is used to model the plasma generated in a parallel plate RF reactor at low pressure. Nonperiodic boundary conditions are used, and the electric field and particle motion are obtained by finite-difference methods leading to the self-consistent creation of sheaths on the boundaries. Model cross sections are used to describe collisions between particles. Ionization is included, and the plasma is maintained by fast electrons generated in the RF sheaths. Most of the power dissipation is due to the acceleration of ions in the time-average sheath fields. At high applied voltage, the power dissipation is described well by the power law P∝V^5/2. Simple scaling laws for the density and plasma potential are obtained. The effect of ion mass and charge-exchange colisions on the ion energy spectrum collected by the electrodes is examined. The ion loss rate drops in the presence of charge-exchange collisions, and this leads to an increase in the density. The collisions also markedly alter the ion energy distribution function     

Probe Measurements in Electronegative Plasmas: Modeling the Perturbative Effects of the Probe‐Holder

A basic property of an electronegative plasma is its separation into two distinct regions: an ion‐ion region far from boundaries, where the densities of positive and negative ions are higher then electron density, and a near‐boundary electron‐ion region, where negative ions have practically negligible density. This is due to the influence of the ambipolar electric field, which depends on electron (not negative ion) plasma parameters. This electric field “holds off” negative ions from the boundary, as the ions have lower mobility and temperature compared to the electrons. Therefore, negative ions will be repelled by any object inserted into the plasma. This can lead to errors in measurements of negative ion and electron parameters by any invasive method. Numerical modeling of electric probes in an argon‐oxygen plasma clearly demonstrates possible errors of direct measurements of negative ion probe current. This can also affect results from the photo‐detachment method and direct measurements of negative ion energy distribution (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modulational instability of relativistic ion-acoustic waves in aplasma with trapped electrons

The association between the modified Korteweg-de Vries solitary wave and the modulationally unstable envelope solitary wave in a weakly relativistic unmagnetized plasma with trapped electrons is discussed. The effect of trapped electrons modifies the nonlinearity of the nonlinear Schrodinger equation and gives rise to the propagation of the modulationally unstable ion-acoustic solitary wave. The amplitude of the envelope solitary wave increases while the number of trapped electrons decreases. The velocity of the solitary wave decreases with increasing ionic temperature and increasing particle velocities. The ion oscillation mode, which satisfies the nonlinear dispersion relation, is also derived. The theory is applied to explain space observations of the solar energetic flows in interplanetary space and of the energetic particle events in the Earth's magnetosphere     

Analytical models of laser-trigged ion acceleration

We review the recent advances in constructing analytical solutions of self-consistent Vlasov-Poisson equations in plasma. These solutions describe different physical phenomena, such as the ion acceleration in the adiabatic expansion of a plasma bunch and the Coulomb explosion of cluster plasma. The particle distribution function, mean velocity, and density distribution, as well as the energy spectra for accelerated ions, are discussed.     

Wave-trapped particle interaction in a weak transverse magnetic field

Damping of an electrostatic plasma wave loaded with a small density bunches of trapped electrons and propagating across a weak magnetic field is studied. To describe the time evolution of the wave, simple algebraic equations are derived under some restrictions on the parameters of the physical system. It is shown that the nonlinear frequency shift of the wave due to the presence of the trapped particles plays an important role and must be taken into account in the self-consistent treatment of the wave–particle interaction.