Sizing of particles levitating in a glow discharge dusty plasma

The dust particle size distribution in a volume glow discharge dusty plasma is studied. Polydisperse quartz particles are used as a dust component. It is found that a dusty plasma forms in a glow discharge not only in a dust trap but also near the wall at the bottom of the discharge chamber. Dust objects in the latter region are large: they contain up to 30000 particles. The size of particles levitating in discharge striations is three times larger than that of particles levitating near the wall. The idea of using the glow discharge dusty plasma for particle-size fractionation of polydisperse powder is put forward.     

Analysis of the size and shape of polydisperse dust particles in a complex glow discharge plasma

The size and shape factor distribution of levitating particles is studied by the method of extraction of dust particles from the discharge chamber. Two dust traps existing in a glow discharge in the strata and above the lower wall of the tube near the bend in the current channel are investigated separately. It is found that the size distribution of polydisperse particles of an arbitrary shape is of the bimodal type due to simultaneous levitation of particles with two shape factors. Polydisperse spherical particles of any size exhibit levitation due to the separation of particles over the wall thickness. For identical parameters of the discharge, the size of the particle in neon is slightly larger than in krypton; the particle size in the trap located in a stratum is substantially larger than the particle size in the trap above the wall of the discharge tube. Precision determination of the shape and size of particles makes it possible to estimate the electric field strength for dust traps. It is shown that the glow discharge can be used as a tool for separation of dust particles in a wide range of their sizes.     

The effect of magnetic field on plasma particles in dusty plasma

In this work, we present the investigation of the formation features and internal structure of dust clouds created in plasma of glow discharge in the external magnetic field corresponding to a range of moderate and strong fields, at which the ion component is magnetized. The analysis of the plasma magnetization in the presence of dust components is carried out. We defined the values of magnetic induction at which the changes in dynamics of plasma particles in magnetic field in light inert gases are expected. The experimental setup was built in two versions. For the purpose of generating of magnetic field, the first setup was equipped with ordinary magnetic coils, and the second one included a superconducting solenoid. The discharge tubes, the main chambers where plasma was ignited and maintained in a glow discharge in lowered pressure, also have certain peculiarities, which we describe below. While using helium as a bulk gas, our study focused only on the dust trap in the region of narrowing discharge current. For neon, we used two traps: the striation trap and one just mentioned above placed in the narrowing of the discharge tube. As a result, the steady dust structures in a glow discharge under the magnetization of ions and electrons were obtained for the first time. Dust structures were rotated and tended to form a dust cluster and shell structure. A number of parameters of magnetization achieved in experiments were calculated.     

Extension of the perturbed plasma region in a low-pressure discharge near the negative probe

It is suggested to consider the region in which the electric field intensity changes its sign as a perturbed plasma region near the negative probe. It is demonstrated that the size of this perturbed region can considerably exceed the probe radius and depends not only on the probe radius but also on the dimensions of the discharge chamber. Within the framework of the considered problem, the chosen criterion is equivalent to another condition that the number of particles in the perturbed zone is equal to the number of particles striking the probe. This contradicts the approach conventionally used in the probe theory according to which particles are accelerated rather than generated in the perturbed region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 10–12, June, 2007.     

Crystalline structures of strongly coupled dusty plasmas in dc glow discharge strata

Strongly coupled dusty plasmas are formed by suspending micron-sized dust particles in strata of a dc glow neon discharge. We have observed for the first time an ordered structure of the negatively charged particles trapped in the strata region. Image analysis reveals the crystalline structure, which is consistent with a large value of the Coulomb coupling parameter.     

Ordered structures in a nonideal dusty glow-discharge plasma

The formation of ordered structures of charged macroparticles in a constant-current neon glow-discharge plasma is investigated. Experiments were performed with two types of particles: thin-walled glass spheres 50–63 μm in diameter and particles of Al₂O₃, 3–5 μm in diameter. Formation of quasicrystalline structures is observed in the standing strata and in an artificially created double electric layer. The formation of extended filamentary structures of macroparticles in the absence of visible stratification of the positive column has been observed for the first time. The influence of the discharge parameters on the formation of the ordered structures and their melting is examined. The form of the interaction potential between the charged macroparticles is considered, as well as changes in the conditions for maintaining the discharge in the presence of high concentrations of dust particles. Zh. éksp. Teor. Fiz. 112, 2030–2044 (December 1997)     

短管螺旋波放电中等离子体参数测量和模式转化研究

对长度为45 cm的短放电管螺旋波放电等离子体进行了Langmuir探针、原子发射光谱以及集成电荷耦合检测器(ICCD)检测诊断,研究螺旋波等离子体的放电特性.Langmuir探针数据显示电子密度在射频功率增加过程中出现两次大幅增长,由此确认了放电模式的转换及螺旋波放电模式的出现.发射光谱测量结果与Langmuir探针测量的电子密度数据一致,发现Ar原子和Ar离子的谱线强度与放电模式变化有着密切相关性.而通过对不同放电模式的ICCD测量,获得射频功率吸收因放电模式转变而变化的方式,认为放电模式转换时电子行为和能量传递方式也发生着变化.     

气压对微束射频容性放电模式调制的研究

微束射频容性放电在纳米晶体颗粒等离子体增强气相合成有着潜在的应用前景.本论文利用ICCD、单反相机、高压探头和电流探头等对微束射频容性放电特性进行了实验诊断研究.结果发现:在纯氩气微束射频放电中,随着气压的增加,放电从辉光放电模式向多通道丝状放电模式转换;在99%氩/1%氢混合气体微束射频放电中,丝状放电模式消失,而是从低气压全空间分布的辉光放电模式,到中等气压向轴心收缩的辉光放电模式,最后到高气压的“环状”辉光放电模式;而在纯氢气微束射频放电中,随着气压的增加,放电模式直接从全空间分布的辉光放电模式向“环状”辉光放电模式转换.最后通过射频电场中电子加热、趋肤效应和气体热传导的共同作用解释了产生不同放电模式的物理机制.     

Analysis of pair correlation functions for macroscopic particles in dusty plasmas: Numerical simulation and experiment

Pair correlation is analyzed for systems of macroscopic particles with various isotropic interaction potentials. Under certain conditions, the behavior of the pair correlation function is determined by an effective order parameter and its decrease toward infinity follows an asymptotic power law. When the effective parameter is smaller than a certain critical value, the decay of pair correlation is much steeper. Experimental results concerning the form of the pair correlation function are presented for liquid-like dust structures localized in the near-electrode plasma sheath of a high-frequency capacitive discharge. An analysis of numerical and experimental results shows that melting dynamics in these systems are analogous to those characteristic of a topological phase transition.     

Optical scanning of dusty 3D-structures formed in a glow discharge

3D-quasi-crystals formed in strata of a glow discharge are scanned in the optical range with the help of a moving laser knife and high-speed videorecording. The spatial positions of dusty grains are determined. The ordering of structures and the type of arrangement of particles are determined from a comparison of pair correlation functions constructed for the structures under study with correlation functions corresponding to ideal crystalline structures. Several types of unit cells are found through the visual collation of separate parts of structures. As compared to data from the literature on experiments in a high-frequency discharge, the structures under study have a clearly pronounced anisotropy.     

Multiple Ring Discharges in a Cylindrical Resonant Cavity

Observations have been made of an rf multiple-ring discharge which is a variant of the well-known single-ring rf discharge described by J. J. Thomson. A set of discretely-located plasma rings (typically 2 to 5 in number) forms coaxially in the central portion of a cylindrical quartz tube on the axis of a cylindrical cavity excited in the TE011 mode. The rings fit closely inside the inner periphery of the quartz tube at pressures ranging from a few Torr to atmospheric, have a minor diameter of order 1/10 the tube diameter, and are spaced at intervals nearly equal to the tube diameter. Excitation of the degenerate companion TM111 mode results in asymmetrical tilting of the rings. Gas flow through the discharge tube results in a continuous unidirectional axial motion of the rings with new rings being abruptly formed toward the input end of the tube and contracting and vanishing abruptly as they move toward the exit end. A theory is developed which accounts for the observed phenomena.     

Microstructure of a liquid two-dimensional dusty plasma under shear

The microstructure of a strongly coupled liquid undergoing a shear flow was studied experimentally. The liquid was a shear melted two-dimensional plasma crystal, i.e., a single-layer suspension of micrometer-size particles in a rf discharge plasma. Trajectories of particles were measured using video microscopy. The resulting microstructure was anisotropic, with compressional and extensional axes at around ±45° to the flow direction. Corresponding ellipticity of the pair correlation function g(r) or static structure factor S(k) gives the (normalized) shear rate of the flow.     

Measurement of temperature of a dusty plasma from its configuration

A new concept called “configurational temperature” is introduced in the context of dusty plasma, where the temperature of the dust particles submerged in the plasma can be measured directly from the positional information of the individual dust particles and the interaction potential between the dust grains. This method does not require the velocity information of individual particles, which is a key parameter to measure the dust temperature in the conventional method. The technique is initially tested using two-dimensional (2D) OpenMP parallel molecular dynamics and Monte Carlo simulation and then compared with the temperature evaluated from experimental data. The experiment have been carried out in the Dusty Plasma Experimental (DPEx) device, where a 2D stationary plasma crystal of melamine formaldehyde particles is formed in the cathode sheath of a DC glow discharge argon plasma. The kinetic temperature of the dust is calculated using the standard particle image velocimetry technique at different pressures. An extended simulation result for the three-dimensional case is also presented, which can be employed for the temperature measurement of a three-dimensional dust crystal in laboratory devices.     

Analysis of dusty plasma in the positive column of glow discharges

The radial distributions of ions,electrons and dust particles in the positive colum of glow discharges are investigated in a tripled-pole diffusion model.The dust particles are mainly trapped in the region around the column axis where the electrostatic potential is the highest.The presence of the dust particles results in the ion density increasing and the electron density decreasiung in the dust-trapped region.The dust-trapped region is wider for a higher dust temperature or a smaller particulate redius.The ions and electrons in the dust-free region away from the column axis are in ambipolar diffusion.     

On the possibility of phase transitions in dusty plasma structures in a glow discharge under the action of a magnetic field

The melting of a dusty plasma structure formed in a stratum of a glow discharge caused by application of a magnetic field and its subsequent increase is studied. The images of two-dimensional cross sections of dusty plasma structures previously obtained by the authors are considered. To identify the phase, the correlation functions are calculated and visual comparison with the space lattices of ideal crystals is performed. It is found that, under the conditions of the previous study of the authors, as the magnetic field increases, the phase state of the dusty structure changes from an anisotropic liquidlike (liquid-ice) state to a disordered one. There is a tendency to a change in the type of the space lattice of the structure.     

Experimental study of heat transfer processes for macroparticles in a dusty plasma

The results of the experimental study of heat transfer processes in liquid dusty structures are reported. The experiments have been carried out for aluminum oxide particles in an rf discharge plasma. The thermal conductivity and thermal diffusivity of the dusty plasma component have been determined by analyzing the steady and unsteady heat transfer processes. The temperature dependence has been obtained for these quantities, which is in qualitative agreement with numerical simulation results for a simple one-atomic fluid.     

Experimental study of nonlinear dust acoustic solitary waves in a dusty plasma

The excitation and propagation of finite-amplitude low-frequency solitary waves are investigated in an argon plasma impregnated with kaolin dust particles. A nonlinear longitudinal dust acoustic solitary wave is excited by pulse modulating the discharge voltage with a negative potential. It is found that the velocity of the solitary wave increases and the width decreases with the increase of the modulating voltage, but the product of the solitary wave amplitude and the square of the width remains nearly constant. The experimental findings are compared with analytic soliton solutions of a model Korteveg-de Vries equation.     

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In order to optimize the process of plasma spheroidization of the lithium micro-powder, a 2-D model of inductively coupled thermal plasma torch is presented. The calculating domain of the electromagnetic field is extended to the air region outside the plasma discharge region and multi-physics coupling calculation was performed by using COMSOL software. The plasma electromagnetic field, temperature and velocity distributions are obtained. The physical mechanism of distributions is analyzed. The simulation found that there are two sets of symmetrical reflux vortices in the coil section of the plasma region. The velocity distribution which locates in the center of coil section and closes to the wall of confinement tube is in disorder. The plasma particles hit the wall along the radial direction. Disorderly flow may cause a rupture area along the circular tube wall. Based on the simulation results, it is proposed that in the process of inductively coupled plasma spheroidization, powder injection port can be moved down to avoid the upper reflux vortex.     

基于COMSOL 的感应耦合等离子体炬多物理场研究

为了优化锂微粉等离子体球化的工艺,对感应耦合等离子体炬进行二维建模,将电磁场计算域扩展到等离子体放电区域之外的空气区域,利用COMSOL 软件进行多物理场模拟。得到了等离子体的电磁场、温度和速度分布,并对分布形成的物理机制进行分析。模拟发现等离子体区域线圈段存在上下两组对称的回流涡,线圈段中部靠近约束管处等离子体速度分布杂乱,有激烈的径向打壁现象,乱流预计会对约束管壁相应位置造成一条环状的破裂效果。基于模拟结果,提出在采用感应耦合等离子体球化锂微粉的工艺工程中,可以将注粉口下移,绕开上回流涡。     

Experimental study of electric discharge treatment of nanodiamond particles in water

A novel type of high voltage pulsed electric discharge in water flow in a Venturi tube is proposed. The influence of the novel discharge on sizes and properties of nanodiamond particles has been studied. Experiments were carried out in water media with purified detonation nanodiamonds made impure by non-diamond carbon material. The ability of high voltage pulsed electric discharge in water to modify nanoparticle conglomerates in water solution and to relieve spherically shaped nanodiamond conglomerates from the initial mixture with non-diamond material can be seen. Prolonged treatment of the suspension made it possible to relieve primary nanodiamond crystals from conglomerates. Formation of ordered and unordered structures from primary (3?C5 nm) nanodiamond crystals has been observed. Study of the electric discharge in water was carried out at the pressure region from atmospheric down to 0.02 atm to reproduce conditions which are typical for the discharge in the Venturi tube in liquid flow and different gap lengths. Two ??types?? of discharge behavior depending on the geometry of the discharge system and other external parameters have been observed. Characteristics that are critical for understanding the behavior of the discharge in the Venturi tube in water flow have been investigated.