Formation Of Plasma Structures Research Articles

Peculiarities of Formation of Plasma Structures in Working Mixtures of Electrochemical HF (DF) Lasers and Methods for Their Study

Peculiarities of Formation of Plasma Structures in Working Mixtures of Electrochemical HF (DF) Lasers and Methods for Their Study

Hydrogen isotope effect on self-organized electron internal transport barrier criticality and role of radial electric field in toroidal plasmas

Self-organized structure formation in magnetically confined plasmas is one of the most attractive subjects in modern experimental physics. Nonequilibrium media are known to often exhibit phenomena that cannot be predicted by superposition of linear theories. One representative example of such phenomena is the hydrogen isotope effect in fusion plasmas, where the larger the mass of the hydrogen isotope fuel is the better the plasma confinement becomes, contrary to what simple scaling models anticipate. In this article, threshold condition of a plasma structure formation is shown to have a strong hydrogen isotope effect. To investigate the underlying mechanism of this isotope effect, the electrostatic potential is directly measured by a heavy ion beam probe. It is elucidated that the core electrostatic potential transition occurs with less input power normalized by plasma density in plasmas with larger isotope mass across the structure formation. This observation is suggestive of the isotope effect in the radial electric field structure formation.

Plasma structure and electron cross-field transport induced by azimuthal manipulation of the radial magnetic field in a Hall thruster E × B discharge

Plasma structure and electron cross field in the z–θ plane of a Hall thruster E×B plasma under an azimuthally inhomogeneous magnetic field are studied by both experimental and numerical approaches. The work is intended to identify a primary role of electron dynamics on the structure formation by manipulating only the strongly magnetized electrons. The plasma potential distribution shows an axial–azimuthal variation; a low magnetic field region results in spatial potential saturation further downstream. The plasma density structure shows a 1D-like azimuthal variation with less axial deformation. A dense region is observed near the location of ∇B>0, where electrons are expected to undergo the ∇B and curvature drift toward the anode where neutrals are introduced. The potential structure is in close correlation to the Hall parameter distribution, indicating that electron dynamics plays a primary role in plasma structure formation, and via multiple consecutive stepwise physical steps, it eventually affects the density structure formation. In the z–θ space, the cross-field transport by E×B and diamagnetic drifts dominantly determines the electron flow and increases the overall axial electron mobility due to the azimuthal inhomogeneity. It is shown that most of the current is carried by the largest structure, but as the macroscopic structure fades out downstream, small structures grow and share the current. By considering the conservation laws, we show that a relation between azimuthal distributions of physical properties is formed to conserve the axial flux by a balance of specific forces, a balance between the resistive force and the magnetic force in the near-anode region and a balance between the electric/pressure force and the magnetic force in the acceleration and plume region, which differs from the Boltzmann relation satisfied in the radial dimension. Based on this principle, with a simplified test case having a uniform plasma density distribution, we show an analytic relation between azimuthal distributions of the magnetic field and the plasma potential and confirm the relation by a 2D hybrid simulation.

Plasma formation and cross-field electron transport induced by azimuthal neutral inhomogeneity in an anode layer Hall thruster

The fluctuation of the azimuthal electric field caused by the electron drift instability or the rotating spokes in the E×B plasma is known to enhance the electron cross field transport. The increased electron current, observed during the operation of a Hall thruster with a nonuniform propellant supply in azimuthal direction, also appears to be related to the azimuthal electric field. In this paper, we experimentally investigate how neutral inhomogeneity in azimuth affects the plasma structure formation, and how this self-organized structure influences the electron cross field transport. We observed an axial-azimuthally varying space potential structure, which results in an alleviated effective axial potential profile and induces the azimuthal electric field. From the distributions of the light emission intensity and plasma density, we show that the azimuthal profile of plasma is skewed in the direction of Ez×Br drift of the magnetized electrons and that the spatial scale of the structure matches the spatial scale of the input neutral variation. The plasma structure reveals that the axial electron drift transport from the induced azimuthal electric field is the most dominant factor due to its equivalent mobility 1/B, which is two orders of magnitude greater than the classical collisional mobility perpendicular to the magnetic field. This indicates that neutrals contribute to the electron cross field transport not only directly through the elastic collisions but also indirectly through their influence on the formation of the plasma structure, which enhances the cross field transport. Lastly, we show that the effective electron mobility rides on 1/16Br line when deviating from the classical mobility line and that the effective Hall parameter is greatly reduced where Eθ develops.

Spatiotemporally Controllable Plasma Lattice Structures in Dielectric Barrier Discharge

A method to generate spatiotemporally controllable plasma lattice structures via dielectric barrier discharge is proposed by utilizing a latticed water electrode. A variety of plasma lattice structures including triangle, hexagon, honeycomb, and complex superlattices are obtained by changing three parameters including the applied voltage, the gas pressure, and the gas compositions. Moreover, these plasma lattices can be quickly reconstructed, allowing for active control both in space and time. Two-dimensional particle-in-cell simulations are carried out to demonstrate the formation of plasma structures under different voltages, which are in good agreement with experimental observations. Our method provides a unique way for the fabrication of controllable plasma lattice structures, which may enable tunable control of microwave radiation for wide applications.

Experiments on the Formation of Galatea–Belt Magnetoplasma Configurations

A brief review is presented of experiments on the formation of Galatea–Belt magnetoplasma configurations carried out by the suggestion of A.I. Morozov at the Plasma Physics Department, Prokhorov General Physics Institute of the Russian Academy of Sciences. The formation of plasma structures consisting of a current sheet and two plasma sheaths around the current conductors immersed in the plasma is demonstrated. It is shown that the change in the direction of the plasma current can lead to the rearrangement of the magnetoplasma configuration. It is found that, for the Galatea–Belt configuration to exist for a long time, it is necessary to provide the optimal relation between the currents flowing in the plasma and conductors.

Increase in the Acceleration Efficiency of Solids in a Hybrid Coaxial Magnetoplasma Accelerator

It is shown that in a hybrid coaxial magnetoplasma accelerator with a channel length of 350 mm and a diameter of 23 mm, the acceleration velocity and the energy conversion efficiency increase as the length of the plasma structure formation channel filled with a gas-generating material decreases from 17 to 9 mm. It is found that it is reasonable to use paraffin as the gas-generating material as it has a less significant deionizing effect on the high-current arc discharge and hence causes a less significant decrease in the discharge current intensity and an increase in conductive and inductive electrodynamic forces.

Effects of radial electric field on suppression of electron-temperature-gradient mode through multiscale nonlinear interactions

Turbulence in fluids and plasmas is ubiquitous in Nature and in the laboratory. Contrary to the importance of the ‘scale-free’ nature of cascade in neutral fluid turbulence, the turbulence in plasma is characterised by dynamics of distinct length scales. The cross-scale interactions can be highly non-symmetric so as to generate the plasma turbulence structures. Here we report that the system of hyper-fine electron-temperature-gradient (ETG) fluctuations and microscopic drift-wave (DW) fluctuations is strongly influenced by the sign of the gradient of the radial electric field through multiscale nonlinear interactions. The selective suppression effects by radial electric field inhomogeneity on DW mode induce a new route to modify ETG mode. This suppression mechanism shows disparity with respect to the sign of the radial electric field inhomogeneity, which can be driven by turbulence, so that it could be a new source for symmetry breaking in the turbulence structure formation in plasmas.

A structural bifurcation of transport in toroidal plasmas44This article is dedicated to the memory of Prof T Ohkawa.

The mechanism of structure formation in plasmas, which is caused by the nonlinear link between the radial electric field, pressure gradient and turbulent fluctuations, is investigated. The suppression of turbulence by the shear and curvature of an inhomogeneous radial electric field is taken into account. The case of L-mode plasmas, where zonal flows are not excited, is studied. The 1D structure is investigated, and the stationary state under a given heat flux (constant in space and time) is analysed. It is shown that the effect of electric field curvature causes structural bifurcation from a state of a constant gradient to a state with a spatially corrugated gradient. The relationship between the period and amplitude of the corrugation is obtained. The electric field shear alone does not lead to such a structural bifurcation.

Properties of Magnetic Field Fluctuations in the Earth’s Magnetotail and Implications for the General Problem of Structure Formation in Hot Plasmas

In this review we discuss the formation of plasma structures in hot plasmas with large β. We use spacecraft observations of magnetic field fluctuations in the Earth magnetotail to reveal the main multiscale plasma properties. Fourier spectra of magnetic field fluctuations observed by various spacecraft in the different domains of the magnetotail at quiet or moderately disturbed times demonstrated a number of practically universal features: (1) the presence of two kinks at low (∼5×10−2 Hz) and high (∼5×10−1 Hz) frequencies, (2) in the frequency interval between the kinks the power law shape with an index ∼2.5, and (3) the significant intensification of fluctuations with the increase of plasma flow velocities. To describe these spectra we consider properties of a principal structure supporting the equilibrium of a hot plasma configuration—the magnetotail current sheet. The observed spatial scales of current sheets are often very small and almost reach the ion Larmor radius. Convection of such mesoscale plasma structures by plasma flows can be responsible for the formation of a certain part of the spectrum of magnetic field fluctuations. Lower- and higher- frequency domains in Fourier spectra can be generated respectively by large-scale MHD oscillations of current sheets and by kinetic small-scale instabilities excited by strong local magnetic field gradients existing in active current sheets.

Meridional electric fields in layer‐type and clump‐type plasma structures in midlatitude sporadic E region: Observations and plausible mechanisms

AbstractThe use of multibased line interferometry technique operated by a VHF coherent scatter radar with relatively broad radar beam pattern makes it possible to reveal zonal drift velocity of 3 m field‐aligned irregularities (FAIs) inside moving layer‐type or clump‐type plasma structures at kilometer scale. We present compelling evidences in this article showing that, no matter what types of the plasma structures, the apparent zonal drift velocity of the 3 m FAIs is always linearly dependent on the zonal trace velocity (or bulk velocity) of the kilometer‐scale plasma structure. This feature seems to imply that the bulk motion of the large‐scale plasma structure may significantly affect the observed 3 m FAI Doppler velocity, which should be removed to obtain true FAI drift velocity. In addition, there is an evident tendency for the 3 m FAIs in the layer‐type plasma structure to drift in the same zonal direction as the moving plasma structure. However, the correlation between the drift direction of the 3 m FAIs and the moving direction of the large‐scale plasma structure is indistinct for the clump‐type plasma structure. The meridional electric fields estimated from the true zonal FAI drift velocities for the layer‐type plasma structures moving in the west (east) direction point in the northward/upward (southward/downward) direction. Statistical results show that the mean value of the meridional electric field inside the layer‐type plasma structure is approximately 2.7–2.8 mV/m, about one order of magnitude larger than that for the clump‐type plasma structure. Mechanisms that are, respectively, responsible for the generations of the layer‐type and clump‐type plasma structures are discussed in this article. It is believed that neutral wind shear is very likely the main cause of the layer‐type plasma structure formation, while the clump‐type plasma structure may be associated with propagating gravity wave.

Investigation of the processes of periodic plasma structure formation in transverse nanosecond discharge with a slotted cathode

A periodic standing-like plasmic striation in inert gases has been observed and investigated under transversal nanosecond electric discharges. The pressure limits relevant for formation of a periodic plasma structure have been established, and the critical values for the voltage amplitude and discharge current have been found corresponding to the upper boundary of the range in which the plasma structure is formed. The external magnetic field effect on the parameters of the periodic plasma structure has been studied. It is stated that the plasma structure is formed at the stage of steady-state discharge and that the lifetime of the periodic structure considerably increases when the transversal magnetic field is applied.

Periodic plasma structures in nanosecond discharge with a slit cathode

The spatial structure of optical emission from a transverse nanosecond pulsed electric discharge with a hollow (slit) cathode has been experimentally studied. A regular periodic plasma structure has been observed during nanosecond pulsed discharge in helium at medium pressures. Conditions of the plasma structure formation with respect to the discharge voltage and current are determined. It is shown that the observed plasma structure differs from the known types of striations.

Verification of Polarization Reversal of Electromagnetic Waves with Electron Cyclotron Frequency Controlling Plasma-Structure Formation

Selectively launched electromagnetic waves with left- and right-handed polarizations for m = 0, +1, and -1 modes are investigated in terms of polarization reversal around an electron cyclotron resonance (ECR) region in inhomogeneously magnetized plasmas, where m is an azimuthal mode number. It is observed for the first time that a left-handed polarized wave for m = 0 mode is absorbed near the ECR point as a result of the polarization reversal in the axial direction. Dispersion analysis in bounded plasmas can explain this quantitatively. For m = +1 and -1 modes, on the other hand, polarization reversal occurs along the radial axis, i.e., wave polarization for m = +1 mode is right-handed (left-handed for m = -1 mode) around the central area and left-handed (right-handed for m = -1 mode) around the peripheral area of the cross section of the plasma column. Furthermore, we investigate plasma-potential structures formed by ECR of high-power m = +1 and -1 waves and, for the first time, demonstrate control of the plasma-potential structure by changing the azimuthal mode, i.e., a positive potential hill is formed near the ECR point around the central and peripheral areas for m = +1 and -1 modes, respectively.

Electrode microwave discharge and plasma self-organization

The processes of plasma self-organization and formation of plasma structures at reduced pressures is considered with respect to the electrode microwave (2.45 GHz) discharge (EMD). The term EMD is attributed to discharge maintained near the ending of powered electrode/antenna if plasma region is less than characteristic dimensions of discharge chamber. EMD is generated at gas pressures ranged between 0.5 and 400 Torr, plasma gases are Ne, Ar, H2, N2, O2, air and mixtures containing CH4 C2H2. Electrodes/antennas of different shapes were used to ignite the discharge: solid and tube cylindrical antennas with diameter of 1-6 mm, needle, spiral and trident antennas etc. Plasma absorbed power is less than 30 W. Phenomenology of EMD is presented. Results of emission spectroscopy and probe measurements together with results of EMD modelling are described. The concept of ‘‘selfsustained - non-selfsustained’’ mechanism of EMD is concluded from analysis of publications on microwave discharges and results of study of EMD. Areas of possible EMD application are discussed.

Theoretical Progress of Zonal Flow

An overview of the theory of zonal flows is presented. The contents include description of the mechanism by which zonal flows are generated, there relation between turbulence and zonal flows, and transport in drift wave-zonal flow turbulence. The physics of zonal flows is a prototypical example of a paradigm shift from the linear-local-deterministic view to the nonlinear-nonlocal-statistical view of structure formation in plasmas. Research regarding the turbulence-zonal flow system in laboratory plasma will deepen our understanding of this aspect of nature.

Propagation of a high-pressure microwave discharge in prebreakdown fields: Formation of plasma structures

A 3D model is developed, which describes the propagation of an initiated high-pressure microwave discharge in a linearly polarized wave beam. The equations for the electric field amplitude are solved self-consistently with the set of equations of plasmochemical kinetics. Plasma structures of the experimentally observed “snake” and “chain” types are obtained using this model. The calculation results enable one to determine the conditions of formation of one or another spatial structure, as well as the parameters of the plasmoids being formed. The results of calculation of the discharge propagation toward the radiation source are compared with the available experimental data.

Nonlocal Transport Phenomena and Various Structure Formations in Plasmas. Nonlocality and Structure Formation in Cosmic Plasmas.

Nonlocal Transport Phenomena and Various Structure Formations in Plasmas. Nonlocality and Structure Formation in Cosmic Plasmas.

Evolution of voids in dusty plasmas

It is shown that formation of dust voids is a general phenomenon in dusty plasmas. Homogeneous dusty plasmas are universally unstable for formation of dust voids and dust structures. Both collision-dominated and collision-less voids can be formed. They are distinguished by the ratio of the void size to the ion-neutral collision mean free path being either large or small, while the mean free path for dust-ion collisions in both cases is small or comparable with the void size. The boundary which separates the dust region from the dust-free region is always sharp as compared to the void size for low ratio of dust temperature Td to ZdTe (Te is the electron temperature and Zd is the dust charge in units of electron charge). It is shown that stationary voids are stable for large deviations from equilibrium size. A review is given on theoretical and numerical investigations of both collision-dominated and collision-less voids together with a comparison with experimental results and observations in space. Investigation is performed of voids in presence of floating potential boundaries. Both flat and spherical voids are considered. Dusty plasma experiments in micro-gravity are computer modeled and necessary conditions for the dusty plasma structure formation are found. A condition is that the structure should be separated from the floating potential wall by a void. Presence of a void region between the dust structure and the wall is shown to be necessary for structures to be formed. These voids are different both from the voids in absence of boundaries and from collisional sheaths in absence of dust. Properties of these voids are investigated. Computer experiments for micro-gravity dusty plasmas shows the possibility of creation of almost homogeneous self-confined dust structures surrounded by voids with properties determined by the dust structure.

Plasma structure formation in a free-localized high-pressure microwave discharge

The phenomenon of plasma self-organization in a free-localized high-pressure microwave discharge has been investigated on the basis of the mechanism of ionization-field (IF) instability. Linear and weakly nonlinear stages of discharge evolution have been studied. It has been shown that IF instability at the nonlinear stage, due to the interaction of the harmonics with different wavevectors, can lead to the formation of a rich spectrum of plasma structures, from simple plane plasma autowaves to various 3D spatial structures (helical filaments and three-dimensional Bernard cells).