Levitation and dynamics of a collection of dust particles in afully ionized plasma sheath

Examines the dynamics of a collection of charged dust particles in the plasma sheath above a large body in a fully ionized space plasma when the radius of the large body is much larger than the sheath thickness. The dust particles are charged by the plasma, and the forces on the dust particles are assumed to be from the electric field in the sheath and from gravitation only. These forces will often act in opposite directions and may balance, making dust suspension and collection possible. The dust particles are supplied by injection or by electrostatic levitation. The ability of the sheath to collect dust particles, will be optimal for a certain combination of gravitation and plasma and dust particle parameters. In a dense dust sheath, the charges on the dust particles contribute significantly to the total space charge, and collective effects become important. These effects will reduce the magnitude of the sheath electric field strength and the charge on the dust particles. As dust particles are collected, the dust sheath is stretched and the largest dust particles may drop out, because the sheath is no longer able to suspend them. In a tenuous dust sheath, the inner layer, from the surface and about one Debye length thick, will be unstable for dust particle motion, and dust will not collect there. In a dense dust sheath, collective effects will decrease the thickness of this inner dust-free layer, making dust collection closer to the surface possible. By linearization of the force and current equations, the necessary and sufficient conditions for a stable dust sheath are found. The authors consider conditions which resemble those of planetary system bodies, but the results may also be of relevance to some laboratory plasmas     

Nonstationary macroparticle charging in an arc plasma jet

The charging of liquid metal macroparticles in the rarified part of a vacuum arc plasma jet is studied. The sheath in the vicinity of the macroparticle is collisionless and the problem with different Debye length to macroparticle radius ratios is analyzed. Maxwellian velocity distribution functions with different temperatures for the electrons and ions in an arbitrary ratio are allowed in the model. By solving the equation for the electric field together with the equation for ion and electron flux, the charging time and the near electric field of the macroparticles were calculated. The kinetics of the macroparticle charging are controlled by the ion and electron flux to the macroparticle, which depend on the potential distribution in the sheath. The potential falls off slower than 1/r² in the case of the large Debye length to macroparticle radius ratio, and falls off more rapidly than 1/r² in the other case. The charge which accumulates on a macroparticle at distances of about 10 cm from a 100-Å cathode is about 10^-16 C and the charging time is about 10^-5 s. The influence of the plasma drift velocity on the macroparticle charging is small. The model presented here agrees well with an experimental study of macroparticle repulsion from biased substrates     

负离子对等离子体鞘层特性影响的数值研究

构造强电负性等离子体中平板电极处鞘层形成的模型,使用流体动力学方程组研究等离子体鞘层.得到空间电势、空间静电荷分布和鞘层宽度作为距离函数的数值结果.结果表明,强电负性鞘层中,在等离子体区和鞘层区之间几乎没有过渡的预鞘区,在靠近极板附近鞘层里的电子、负离子和正离子的分布形成一个纯正离子鞘区.在靠近鞘边附近,空间静电荷密度分布有一个很尖的峰.发现同电正性情形相比,强电负性鞘层的宽度要窄很多,空间电势下降得快得多.     

Pair-ion plasma generation using fullerenes

We have developed a novel method for generating pure pair plasma which consists of positive- and negative-charged particles with an equal mass. The pair-ion plasma without electrons is generated using fullerene as an ion source through the processes of hollow-electron-beam impact ionization, electron attachment, preferential radial diffusion of ions, and resultant electron separation in an axial magnetic field. Basic characteristics of this plasma are discussed in terms of the differences from ordinary electron-ion plasmas, such as a phenomenon in the absence of sheath and potential structure formation.     

电子温度各向异性对霍尔推力器BN绝缘壁面鞘层特性的影响

建立多价态多组分等离子体一维流体鞘层模型,引入电子温度各向异性系数并考虑出射电子速度分布,研究了电子温度各向异性对霍尔推力器中的BN绝缘壁面鞘层特性和近壁电子流的影响.分析结果表明,相比于纯一价氙等离子体鞘层参数,推力器中的多价态氙等离子体鞘层电势降略有降低,电子壁面损失增加,临界二次电子发射系数减小.推力器中的电子温度各向异性现象可以显著地加大出射电子能量系数,进而降低鞘层电势降,增强电子壁面相互作用.数值结果表明,空间电荷饱和机制下电子温度各向异性对鞘层空间电势分布影响显著. 关键词： 霍尔推力器 电子温度各向异性 空间电荷饱和鞘层     

Vacuum heating evaluation for plasmas of exponentially decreasing density profile

Ultra-short pulse lasers have opened a regime of laser-plasma interaction where plasmas have scale lengths shorter than the laser wavelength and allow the possibility of generating near-solid density plasmas. The interaction of high-intensity laser beams with sharply bounded high-density and small scale length plasmas is considered. Absorption of the laser energy associated with the mechanism of dragging electrons out of the plasma into the vacuum and sending them back into the plasma with the electric field component along the density gradient, so called vacuum heating, is studied. An exponentially decreasing electron density profile is assumed. The vector potential of the electromagnetic field propagating through the plasma is calculated and the behaviour of the electric and magnetic components of the electromagnetic field is studied. The fraction of laser power absorbed in this process is calculated and plotted versus the laser beam incidence angle, illumination energy, and the plasma scale length.     

Contribution to the evaluation of transport coefficients in plasmas containing krypton and sodium

Electrical and thermal conductivities for low-temperature (2000 K to 20000 K) and high-pressure (0.1 MPa to 2.5 MPa) krypton plasma with some sodium (1% and 10%) as additive are presented. The plasma is treated as weakly non-ideal and having attained the local thermodynamical equilibrium states. The problem of the plasma non-ideality according to standard criteria is of the plasma Debye radius, with an improved cut-off length. Use is made of a previously derived modified Debye radius with the Landau length instead of the smallest ionic radii which allowed to regard the plasmas investigated as weakly non-ideal. This alteration in the evaluation of the Debye radius profoundly alters the equilibrium plasma compositions and their transport coefficients.     

Electron-ion Coulomb Bremsstrahlung in nonideal plasmas

The classical electron-ion Coulomb Bremsstrahlung process is investigated in nonideal plasmas. An effective pseudopotential model taking into account the plasma screening and collective effects is applied to describe the electron-ion interaction potential in a classical nonideal plasma. The classical straight-line trajectory method is applied to the motion of the projectile electron in order to visualize the variation of the differential Bremsstrahlung radiation cross-section (DBRCS) as a function of the scaled impact parameter, nonideal plasma parameter, projectile energy, photon energy, and Debye length. The results show that the DBRCS in ideal plasmas described by the Debye-Hückel potential is always greater than that in nonideal plasmas, i.e., the collective effects reduce the DBRCS for both the soft and hard photon cases. For large impact parameters, the DBRCS for the soft photon case is found to be always greater than that for the hard photon case. Received 1st December 1999     

Particle-in-cell modeling of relativistic laser–plasma interaction with the adjustable-damping,direct implicit method

Implicit particle-in-cell codes offer advantages over their explicit counterparts in that they suffer weaker stability constraints on the need to resolve the higher frequency modes of the system. This feature may prove particularly valuable for modeling the interaction of high-intensity laser pulses with overcritical plasmas, in the case where the electrostatic modes in the denser regions are of negligible influence on the physical processes under study. To this goal, we have developed the new two-dimensional electromagnetic code ELIXIRS (standing for ELectromagnetic Implicit X-dimensional Iterative Relativistic Solver) based on the relativistic extension of the so-called Direct Implicit Method [D. Hewett, A.B. Langdon, Electromagnetic direct implicit plasma simulation, J. Comput. Phys. 72 (1987) 121–155]. Dissipation-free propagation of light waves into vacuum is achieved by an adjustable-damping electromagnetic solver. In the high-density case where the Debye length is not resolved, satisfactory energy conservation is ensured by the use of high-order weight factors. In this paper, we first derive the electromagnetic direct implicit method as a simplified Newton scheme. Its linear properties are then investigated through numerically solving the relation dispersions obtained for both light and plasma waves, accounting for finite space and time steps. Finally, our code is successfully benchmarked against explicit particle-in-cell simulations for two kinds of physical problems: plasma expansion into vacuum and relativistic laser–plasma interaction. In both cases, we will demonstrate the robustness of the implicit solver for crude discretizations, as well as the gains in efficiency which can be realized over standard explicit simulations.     

Effects of magnetic field and temperature on the nonrelativistic bremsstrahlung process in magnetized anisotropic plasmas

The magnetic field and thermal effects on the nonrelativistic electron-ion bremsstrahlung process are investigated in magnetized anisotropic plasmas. The effective electron-ion interaction potential is obtained in the presence of an external magnetic field. Using the Born approximation for the initial and final states of the projectile electron, the bremsstrahlung radiation cross section and bremsstrahlung emission rate are obtained as functions of the electron energy, radiation photon energy, magnetic field strength, plasma temperature, and Debye length. It is shown that the effects of the magnetic field enhance the bremsstrahlung radiation cross section for low plasma temperatures and, however, suppress the bremsstrahlung cross section for high plasma temperatures. It is also shown that the magnetic field effects diminish the bremsstrahlung emission rate in magnetized high temperature plasmas.     

The dynamic response of an inhomogeneous plasma diode to an appliedpotential drop

Recent measurements of the response to an applied step voltage in collisionless plasmas have demonstrated the importance of the initial ion density distribution for the resulting time evolution and the formation of electric double layers. The dynamic response of a plasma diode to an applied step voltage is studied by particle-in-cell simulations and an analytical model. It is shown that an ion-density cavity (a local ion-density minimum with a width of many Debye lengths) can support large potential drops for several electron transit times. The potential drop extends over a distance related to the cavity width. When the applied potential drop exceeds a certain critical value, which depends on the cavity depth, the drop instead concentrates in a cathode sheath, which also is the response obtained for homogeneous initial plasma. The existence regions for the two different response in the appropriate parameter plane are found from the simulations and shown to agree with the regions predicted analytically. The analytical potential profiles agree with those simulated     

Debye Length in a Neutral-beam-heated Plasma

The scale length for an effective shielding of a charged particle in a plasma is the Debye length, which is for Maxwellian velocity distributions determined by the ratio of the temperature to the density. However, fusion plasmas heated by neutral-beam injection (NBI) exhibit highly non-Maxwellian slowing-down velocity distributions. In this case, the evaluation of the characteristic shielding distance from these velocity distributions requires the solution of Fokker-Planck-Poisson equations. In this paper, the effect of NBI slowing-down velocity distributions on the shielding distance are discussed. Analytic expressions for the Debye length of a neutral-beam-heated plasma are given. The results are compared to the Maxwellian case.     

Asymptotic theory of boundary layers of weakly ionized thermalplasmas on emitting electrodes

The paper deals with collision-dominated boundary layers of weakly ionized thermal plasmas on emitting electrodes. The case considered is when the dominating ionization mechanism in the plasma is ionization by electron impact, and the dominating recombination mechanism is recombination with an electron as a third body. The ratio of the Debye length to the recombination length is treated as a small parameter, and the method of matched asymptotic expansions is employed. Analytical formulas have been obtained for the distributions of the number densities of ions and electrons and of the electrostatic potential in each asymptotic zone. Formulas have been obtained describing the voltage drop in the boundary layer as a function of the density of the electric current coming from the plasma to the electrode     

Sheath thickness evaluation for collisionless or weakly collisionalbounded plasmas

The widely used Child-Langmuir law for sheath thickness evaluation in semi-infinite collisionless plasmas makes the assumptions of quasi-neutrality (n_e=n_i) and zero electric field intensity E=0 at the sheath edge, as well as applying the Bohm criterion for ions entering the sheath. However, through a whole region fluid model, Poisson's equation has been solved numerically for the steady-state solution through the sheath and presheath without these assumptions. With the sheath edge defined, as in the Child-Langmuir law, at the place where the ion velocity is equal to the Bohm velocity, the sheath thickness of a bounded collisionless or weakly collisional plasma has been found with this model in some cases to be much larger than that obtained with the Child-Langmuir Law. The sheath thickness discrepancy is significant under conditions found in low pressure high density plasma (HDP) tools for plasma processing. Results presented indicate that the sheath thickness is very sensitive to the electric field and space charge density at the sheath edge. The electric field and space charge density can be successfully estimated by an intermediate scale matching method, and are used to derive a modified expression for the potential in the sheath that can be solved numerically for sheath thickness. With these results, the matching problem, arising when sheath and plasma are modeled separately, can be overcome     

Nonthermal effects on the entanglement fidelity for elastic scatterings in Lorentzian plasmas

The nonthermal effects on the entanglement fidelity for the elastic electron-ion scattering are investigated in generalized Lorentzian plasmas. The dynamically screened effective potential and partial wave analysis are employed to obtain the entanglement fidelity in Lorentzian plasmas as a function of the spectral index, collision energy, and plasma parameters. It is shown that the entanglement fidelity increases with decreasing the collision energy, especially, for small Debye radii. It is also shown that the nonthermal effect enhances the entanglement fidelity in Lorentzian plasmas. In addition, it is found that the entanglement fidelity increases with an increase of the plasma temperature.     

Effect of quantization and positron concentration on shielding potentials in electron-positron-ion plasma

The kinetic theory of plasma has been employed to compute the test-charge potential distributions accounting for quantization effects in magnetized electron-positron-ion (EPI) plasmas. In this regard, the degenerate positrons and electrons are assumed to follow the Fermi-Dirac distribution, while inertial ions are modelled by Maxwellian velocity distribution. By solving the Fourier-transformed Vlasov–Poisson equations, a modified dielectric function and electrostatic potential is obtained. By imposing various constraints on the test-charge speed, the potential profile has been analysed in terms of Debye–Hückel (DH), far-field (FF), and wake-field (WF) potentials. It has been found that the amplitude of DH and FF potentials increases by the inclusion of quantization effects, and it becomes the opposite for the WF potential profile. Furthermore, the variation of positron concentration significantly affects the DH, FF, and WF potentials. The present findings are important to understand the shielding phenomenon in degenerate multi-species plasmas.     

The influence of the electron temperature gradient on the space-charge field in a d.c. discharge

The deviations of the space-charge field from its equilibrium value in the d.c. discharge plasma is investigated, taking into account the influence of changed electron temperature. Starting from the plasma-equations, the corresponding steady-state solution in a linear, one-dimensional approximation is found, on the assumption that the ion density disturbance is known and given as a function of the space coordinate, constant in time or only slowly varying with time. As the changed electron temperature influences the electron density, the space-charge field disturbance is no longer restricted (within the limit of the Debye screening length) by the dimension of the positive ion disturbance as is the case of the temperature uninfluenced field. The space charge field now extends widely out of the place of the ion disturbance, the extension being given by the dimension of the electron temperature relaxation length, which in order of magnitude equals the quantityl ₁=2U _e0 /E ₀. A solution in a closed analytical form (25) is found in the case of a quasineutral plasma. Even in this case of zero Debye length, temperature lengthening of the spacecharge field relaxation takes place. The appropriate numerical examples are given for the case of final (non-zero) Debye length.The authors wish to thank J. Václavík and V. Zárybnický for their stimulating remarks and helpful discussion.     

Modal focusing effect of positive and negative ions by a three-dimensional plasma-sheath lens

A complex focusing effect of positive and negative ions caused by the sheath forming to biased electrodes that interface insulators has been found by solving in three dimensions the potential distribution and ion kinetics within the sheath. Thus, intrinsically correlated with the sheath shape, certain electrical charges are focused on the surface, forming well defined patterns named modal lines and modal spots. Their superposition to the previously reported discrete focusing leads to a total flux that represents a "fingerprint" of the entire sheath on the electrode surface. The ion flux pattern is developed experimentally on the surface of square and octagonal electrodes exposed to Ar/SF(6) and CF4 plasmas. Present results are of high potential importance for fundamental studies concerning sheath formation and charge kinetics and also in a wide range of plasma applications.     

Refinement of the 3/2 power law and radial drift theory of the ion current to a small probe or a dust particle in a rarified plasma

The ion current to a probe or a dust particle in a rarified plasma is considered in terms of the plasma sheath theory and the radial drift theory. The effect of the initial directed velocity of ions entering the sheath in accordance with the Bohm criterion on the current-voltage characteristic of the space charge sheath is studied. Computations based on both theories are performed for the extended range of the relative values of probes (0.0001?C20.0) r _p /??_D, where r _p is the size of the probe and ??_D is the electron Debye radius. The two theories are compared in the entire design range of the sizes of probes. Analytical approximations are given for practical application of the results.     

Influence of Plasma Resistance and Fluctuation on Probe Characteristics in Detached Recombining Plasmas

In order to find the causes of the strong anomaly of current‐voltage characteristics of Langmuir probe observed in detached recombining plasmas in a linear divertor plasma simulator, NAGDIS‐II, we have investigated plasma resistance along a magnetic field and potential fluctuations in the detached recombining plasmas. Simple calculation on the ratio between the plasma length, at which plasma resistance and resistance of ion sheath formed around a probe tip become equal, and an electron collection length indicates that the evaluation of electron temperature T_e becomes inaccurate at T_e of less than 0.6 eV when plasma density and neutral pressure are 1.0 × 10¹⁸ m^—3 and 10 mtorr, respectively. The potential fluctuation in detached recombining plasmas was found to be so large compared to T_e/e, which can also modify the probe characteristics.