Dusty plasma in the stratified glow discharge in moderate magnetic field

This article presents the experimental study of dust structures formed in striations of glow discharge in an external magnetic field of up to 10⁴ G, which was created using a cryomagnet. A magnetic field classification based on probe theory is described. In moderate magnetic field, we obtained stable dust structures. The corresponding experimental setup and the experiment itself are described in detail. Special attention was paid to the influence of magnetic field on the change of the diameter and inter‐particle distance of dusty cloud. And, we also determined the dependence of angular velocity of dust rotation on magnetic induction.     

Dusty plasma structures in magnetic fields in a dc discharge

The formation of dusty plasma structures has been experimentally investigated in a cylindrical dc discharge in axial magnetic fields up to 2500 G. The rotation of the dusty plasma structures about the discharge symmetry axis with a frequency depending on the magnetic field has been observed. When the field increases to 700 G, the displacement of dust particles from the axial region of the discharge to the periphery, along with the continuation of the rotation, has been observed. The kinetic temperatures of the dust particles, the diffusion coefficients, and the effective nonideality parameter have been determined for various magnetic fields. The explanation of the features in the behavior of the dust particles in the discharge in the magnetic field has been proposed on the basis of the analysis of ambipolar diffusion in the magnetized plasma. The maximum magnetic field at which the levitation of the dust particles in the discharge is possible has been estimated.     

The onset of rotational motion of dusty plasma structures in strata of a glow discharge in a magnetic field

An interpretation is given to the previously observed action of a magnetic field on the state of a dusty plasma structure in strata of a glow discharge. The conditions of previous experiments are analyzed, in which a nonuniform rotation and a change in the degree of order of a dusty plasma structure (the translational order), as well as a correlation between them, were revealed. Based on this analysis and on data in the literature on dusty plasmas in a magnetic field, a hypothesis is made that the reason for the rotation of the structure is the ion drag force. Additional experiments on the observation of the onset of rotational motion of a structure in “weak” and “strong” magnetic fields are conducted. It is shown that rotation reversal (and rearrangement of the order of the structure) is caused by changes in the direction of ionic flows—from internal regions of the structure to its periphery and vice versa—in the weak and strong magnetic fields. The results obtained agree qualitatively with the hypothesis adopted, as well as with the data of the two-dimensional theory of strata.     

Mechanism of diffusion of positively charged dust particles in a photoemission cell under microgravity conditions

The decay of a dusty plasma in a photoemission cell under microgravity conditions is investigated on the basis of the method of nonlocal moments. It is founds that plasma decay in space experiments occurs in accordance with the mechanism of free electron diffusion followed by dust particle drift. An analytic solution is found for the evolution of radial distributions of the dust particle concentration and the electric field under the experimental conditions. The effect of abnormally high temperatures of dust particles is considered. The effect of axial magnetic fields on the decay of dusty plasma is investigated. It is shown that the plasma decay in a magnetic field is governed by the ambipolar diffusion mechanism, the decay being prolonged up to 10³ s in a magnetic field on the order of 10³–10⁴ G in strength.     

The effect of a longitudinal magnetic field on the plasma-dust structures in strata in a glow discharge

The plasma-dust structures in strata in a glow discharge exposed to a longitudinal magnetic field are studied in detail. In a weakly ordered structure, the angular velocity has a vertical gradient. A reversal of rotation of the structure in a magnetic field corresponding to the magnetization of electrons is found. With the help of the pair distribution function of particles, changes in the degree of order of the structure in the magnetic field are revealed. These changes correlate with changes in the angular velocity of rotation. To explain this effect, it is assumed that the dust structure is subjected to the action of ions in crossed electric and magnetic fields.     

Ion drag as a mechanism of plasma dust structure rotation in a strata in a magnetic field

In experiments on complex plasmas, afixed strata region in which the levitation of dust structures is observed is investigated using the method of probing by calibrated dust particles of different sizes in an applied magnetic field under elevated pressures. The measured azimuthal velocity of the probing particles corresponds to the action of the ion drag force for 4 μm-size particles and to the entrainment by the rotating gas owing to the electron vortex flow inside the strata for 1 μm-size particles. Extrapolation to pressures and magnetic fields in which the rotation inversion of dust structures is observed in experiments shows that the ion drag is the dominating force causing rotation with a negative projection of the angular velocity onto the magnetic induction.     

Rotational properties of annulus dusty plasma in a strong magnetic field

The collective dynamics of an annulus dusty plasma formed between a co-centric conducting (non-conducting) disk and ring configuration is studied in a strongly magnetized radiofrequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In the absence of the magnetic field, the dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in the anticlockwise direction with increasing magnetic field (B > 0.02 T ), and the constant value of the angular frequency at various strengths of the magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B > 0.6 T ) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B > 1 T ) lowers the angular frequency. Low value of the angular frequency is expected by reducing the width of the annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of a magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of a magnetic field.     

Charge and structures of dust particles in a gas discharge at cryogenic temperatures

A dusty plasma in a dc gas discharge is considered at low (cryogenic) temperatures of the gas. The formation of dusty plasma structures consisting of monodisperse poly(styrene) particles (d = 5.44 μm) in a dc glow discharge is experimentally investigated at cryogenic temperatures in the range from 4.2 to 77 K, and the results obtained are presented. The ion velocity distribution function and the charging of dust particles at cryogenic temperatures are calculated using the molecular dynamics method. The primary attention is focused on the correct inclusion of ion-atom collisions in the analysis. This is essential to the understanding of the main mechanisms of the experimentally observed increase in the density of dust particles with decreasing temperature of the gas in the discharge.     

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.     

Langmuir Probe Diagnostics of a Low Temperature Non-Isothermal Plasma in a Weak Magnetic Field

This article presents measurements by a cylindrical Langmuir probe in the plasma of a DC cylindrical magnetron discharge át the pressure 1.5 Pa that aim at the experimental assessment of the influence of a weak magnetic field to the estimation of the electron density when using conventional methods of probe data interpretation. The probe data was obtained under the presence of a weak magnetic field in the range 1.10^?2?5.10^?2 T. The influence of the magnetic field on the electron probe current is experimentally assessed for two cylindrical probes with different radii, 50 μm and 21 μm. This assessment is based on comparison of the values of the electron density estimated from the electron current part with the values of the positive ion density estimated from the positive ion current part of the probe characteristic respectively by assuming that at the magnetic field strengths used in the present study the probe positive ion currents are possible to be assumed as uninfluenced by the magnetic field. For interpretation of the probe positive ion current two theories are used and compared to each other: the radial motion model by Allen, Boyd and Reynolds [10] and Chen [11] and the model that accounts for the collisions of positive ions with neutrals in the probe space charge sheath that we call Chen-Talbot model [8]. At lower magnetic field 3 · 10^?2 T the positive ion density values interpreted by using the Chen-Talbot model [8] are in better agreement with the values of electron density compared to those obtained by using the theory [10,11]; therefore the model [8] is used for calculation of the positive ion density from the probe data at higher magnetic fields. The comparison of the positive ion and electron density values calculated from the same probe data at higher magnetic fields shows that up to the magnetic field strength 4 . 10^?2 T with the probe 100 μm and up to 5 . 10^?2 T with the probe 42 μm in diameter respectively the decrease of the magnitude of the electron current at the space potential due to the magnetic field does not exceed the error limits that are usual for Langmuir probe measurements (absolute error ±20%).     

Collective Phenomena in Strongly Coupled Dissipative Systems of Charged Dust: from Ground to Microgravity Experiments

In present work the formation of dusty plasma structures in cryogenic glow dc discharge was investigated. The ordered structures from large number (～10⁴) of charged diamagnetic dust particles in a cusp magnetic trap have been also studied in microgravity conditions. The super high charging (up to 5·10⁷e) of dust macroparticles under direct stimulation by an electron beam is experimentally performed and investigated. The results of the investigation of Brownian motion for strongly coupled dust particles in plasma are presented. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Modulation of Void Motion Behavior in a Magnetized Dusty Plasma

Based on fluid equations,we show a time-dependent self-consistent nonlinear model for void formation in magnetized dusty plasmas.The cylindrical configuration is applied to better illustrate the effects of the static magnetic field,considering the azimuthal motion of the dusts.The nonlinear evolution of the dust void and the rotation of the dust particles are then investigated numerically.The results show that,similar to the unmagnetized one-dimensional model,the radial ion drag plays a crucial role in the evolution of the void.Moreover,the dust rotation is driven by the azimuthal ion drag force exerting on the dust.As the azimuthal component of ion velocity increases linearly with the strength of the magnetic field,the azimuthal component of dust velocity increases synchronously.Moreover,the angular velocity gradients of the dust rotation show a sheared dust flow around the void.     

Ordered dust structures in glow discharge plasmas

Ordering of dust grains suspended in glow discharge plasmas into quasi-steady liquid-or crystallike structures in an external field is considered. The self-consistent electric field generated by free electrons, ions, and dust grains is found. An estimate is obtained for the confining potential required to hold dust grains in the direction perpendicular to the discharge axis. It is shown that the potential energy of interaction between ordered dust particles has the form characteristic of ionic crystals. Critical parameters are estimated for a liquidlike dust structure. The correlation function calculated for a dusty plasma by using this approach is compared with a measured one.     

Influence of external magnetic field on dust acoustic waves in a capacitive RF discharge

This paper reports experiments on self-excited dust acoustic waves (DAWs) and its propagation characteristics in a magnetized rf discharge plasma. The DAWs are spontaneously excited in dusty plasma after adding more particles in the confining potential well and found to propagate in the direction of streaming ions. The spontaneous excitation of such low-frequency modes is possible due to the instabilities associated with streaming ions through the dust grain medium. The background E-field and neutral pressure determine the stability of excited DAWs. The characteristics of DAWs strongly depend on the strength of external magnetic field. The magnetic field of strength B < 0.05 T only modifies the characteristics of propagating waves in dusty plasma at moderate power and pressure, P = 3.5 W and p = 27 Pa, respectively. It is found that DAWs start to be damped with increasing the magnetic field beyond B > 0.05 T and get completely damped at higher magnetic field B ∼ 0.13 T. After lowering the power and pressure to 3 W and 23 Pa respectively, the excited DAWs in the absence of B are slightly unstable. In this case, the magnetic field only stabilizes and modifies the propagation characteristics of DAWs while the strength of B is increased up to 0.1 T or even higher. The modification of the sheath electric field where particles are confined in the presence of the external magnetic field is the main cause of the modification and damping of the DAWs in a magnetized rf discharge plasma.     

On the hypothesis of gas rotation in the magnetomechanical effect

This study continues a cycle of works published in Optika i Spektroskopiya (Optics and Spectroscopy) on the magnetomechanical effect in a gas discharge. It is devoted to the hypothesis of the appearance of a torque acting on an object placed in a gas discharge in a magnetic field and arising due to the momentum transfer from the rotating neutral gas. The velocities of gas rotation required for the formation of the observed moment of forces are estimated. Measurements of the velocities performed using laser Doppler anemometry are analyzed. The method of observing dust structures in a magnetic field to reveal gas rotation is justified. Corresponding experiments with the use of the method of optical visualization for the observation of plasma-dust structures are conducted. The experimental dependence ω(r) obtained for a liquidlike structure is not consistent with the hypothesis of the transfer of rotation from the gas. Additional observations of dust plasma upon tilting of the discharge tube in the gravitational field are conducted. It is found that, in this case, the structure shifts from the central region of the stratum to the periphery and the rotation ceases. Neither the analysis nor the experiments performed in this study are consistent with the hypothesis of gas rotation in the magnetomechanical effect. The results of the experiments with dust structures are of independent interest for the physics of dust plasma.     

Triangular antiferromagnets with a layered structure in a uniform field

We study the magnetic states and phase transitions in layered triangular antiferromagnets and show that in compounds of the VBr₂ (or VCl₂) type the quantum effects alter the structure of the ground state and initiate a series of transitions as the magnetic field strength is increased. We establish that planar structures with different spin configurations are realized when the magnetic field strength is far from the saturation value, while a nonplanar structure of the umbrella type is realized in fields close to the saturation value. Finally, we build the phase diagram of the ground state and indicate a finite range of field strengths where a collinear phase is possible, too. Zh. éksp. Teor. Fiz. 111, 627–643 (February 1997)     

Effect of a variable magnetic field on the instability of a stratified rotating fluid layer in porous medium

The instability of a stratified rotating fluid layer through porous medium in the presence of an inhomogeneous magnetic field is investigated. For exponentially varying density and magnetic field variations, an eigenvalue solution has been obtained. The dispersion relation is obtained and discussed for both the stable and unstable stratifications separately. It is found, for non-porous medium, that for the stable mode of disturbance, the system is always stable, and for the unstable mode of disturbance, it is stabilized only under a certain condition for the Alfvèn velocity, rotation and the stratification parameter. In the latter case, both rotation and magnetic field are found to have a stabilizing effect on the growth rate. In the presence of porous medium, it is found, for real growth rate n, that the inhomogeneous magnetic field has always a stabilizing effect on the considered system. It is found also, for complex growth rate n, that the system is stable for the stable stratification case, while it is stable or unstable for the unstable case under a certain wavenumbers range depending on the Alfvèn velocity and the stratification parameter. The presence of the magnetic field is found to stabilize a certain wavenumbers band, whereas the system was unstable for all wavenumbers in the absence of the magnetic field. Also, the presence of porous medium is found to hide the stabilizing effect played by rotation on the considered system for non-porous medium, i.e., rotation does not have any significant effect on the stability criterion in this case.     

Magnetohydrodynamics of a viscous spherical layer rotating in a strong potential field

An analytic solution is given for classical magnetohydrodynamic (MHD) problem of almost rigid-body rotation of a viscous, conducting spherical layer of liquid in an axisymmetric potential magnetic field. Large-scale flows bounded by rigid spheres are described for the first time in a new approximation. Two problems are solved: (1) in which both spheres are insulators and (2) in which the outer sphere is an insulator and the inner sphere a conductor. Axially symmetric flows and azimuthal magnetic fields are maintained by a slightly faster rotation of the inner sphere. The primary regeneration takes place in the boundary and shear MHD layers. The shear layers, described here for the first time, smooth out the large gradients at the boundaries of the MHD structures encompassed by them. There is essentially no azimuthal magnetic field inside these original structures, which are bounded by potential contours tangent to the spheres. An applied constant magnetic field creates a rigid MHD structure outside an axial cylinder tangent to the inner sphere. Inside the cylinder the rotation is faster and the meridional flux depends on height. A magnetic dipole forms a structure tangent to the outer equator. Outside the structure, the rotation is also rigid-body when both spheres are insulators. When a conducting sphere is present, the liquid rotates differentially everywhere, while near the axis and inside the MHD structure, it rotates even faster than the inner sphere. The last example of a general solution is a quadrupole magnetic field. In this case, two equatorially symmetric MHD structures are formed which rotate together with the inner sphere. Outside the structures, as in the most general case, the rotation is differential, the azimuthal magnetic field falls off as the first power of the applied field, and the meridional flux falls off as the square of the field in the first problem, and as the cube in the second. Zh. éksp. Teor. Fiz. 112, 2056–2078 (December 1997)     

Faraday effect in Tb3Ga5O12 in a rapidly increasing ultrastrong magnetic field

The Faraday effect is measured in paramagnetic terbium gallate garnet Tb₃Ga₅O₁₂ at a wavelength λ=0.63 μm at 6 K in pulsed magnetic fields up to 75 T increasing at a rate of 10⁷ T/s for field orientation along the crystallographic direction 〈110〉. The experimental data are compared with the results of theoretical calculations taking into account the crystal fields acting on the Tb³⁺ ion and various contributions to the Faraday rotation. Since the measurements in pulsed fields are carried out in the adiabatic regime, the dependence of the sample temperature on the magnetic field acting during a current pulse is obtained from the comparison of the experimental dependence of Faraday rotation with the theoretically calculated dependences of the Faraday effect under isothermal conditions at various temperatures. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 11, 2002, pp. 2013–2017. Original Russian Text Copyright ? 2002 by Levitin, Zvezdin, Ortenberg, Platonov, Plis, Popov, Puhlmann, Tatsenko.