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Boundary of the Transition to Hollow Dust Structures in a DC Discharge in Neon with Microparticles

Fri, 03/01/2019 - 00:00
Abstract

The boundary (line) of the transition from homogenous dust structures to hollow dust structures in the coordinates gas pressure–discharge current in a glow discharge in neon was found experimentally. The experiments were carried out with spherical particles 2.55 and 4.14 μm in diameter. The transition was simulated using the diffusion–drift model of the positive column of a glow discharge in neon with allowance for the radial temperature gradient. Simulations of the experimental data have shown that the thermophoretic force acting on the microparticles in the dust structure depends on the discharge parameters and the dimensions of the microparticles and the dust structure. The results of this work can be used in dusty plasma technologies.

UV Radiation of High-Voltage Multi-Electrode Surface Discharge in Gaseous Medium

Fri, 03/01/2019 - 00:00
Abstract

UV radiation of high-voltage pulsed multispark surface discharge in atmospheric-pressure air in the spectral range of 200–380 nm was investigated. The discharge was a sequence of microplasma objects with a specific energy deposition of ~1 kJ/cm3 and electron density of 1017 cm–3. Copper and stainless steel were used as the electrode materials. It is found that lines of atoms and ions of the electrode material dominate in the discharge UV spectrum. The UV radiation intensity was measured using the actinometry technique. The UV radiation efficiency relative to the energy deposited in the discharge is found to be ~1%.

Characteristics of X-ray Emission from a Micropinch Discharge at Different Polarities of the Discharge Electrodes

Fri, 03/01/2019 - 00:00
Abstract

The measurement technique and results are presented from studies of the X-ray spectra and X-ray yield from micropinch discharge plasma in a low-inductive vacuum spark device at different polarities of the discharge electrodes. The measurements were performed using the specially designed diagnostic system based on thermoluminescent and scintillation detectors. This made it possible to perform measurements in the photon energy range of 1–300 keV.

Study of the Spectral Composition of X-ray Emission from Different Regions of Micropinch Discharge Plasma

Fri, 03/01/2019 - 00:00
Abstract

The measurement technique and results are presented from studies of the spectral composition of X-ray emission from different regions of micropinch discharge plasma in a low-inductive vacuum spark device. X-ray spectra are measured in the photon energy range of 1–30 keV for the radiation emerging from the plasma spot, near-anode plasma, and anode regions of the micropinch discharge plasma. It is experimentally established that, in the energy range under study, the intensity of X-ray emission from the plasma spot exceeds that from the near-anode and anode regions.

Mechanism of Orientation and Parameters of Lightning in Context of Lightning Protection

Fri, 03/01/2019 - 00:00
Abstract

Details of the process of lightning formation and orientation of the downward leader required to solve applied problems in the field of lightning protection are considered. It is shown that it is necessary to take into account the mechanism of the bipolar leader formation in the thundercloud electric field, according to which the thunderstorm cell cannot be regarded as a conducting charged electrode, the potential carried by the downward leader channel is determined by the start point and path of the lightning, and the effect of the thunderstorm cell charge reveals itself to a much less degree. An algorithm is proposed for calculating the orientation height, charge per unit channel length, and attraction radius of lightning from the value of the return stroke current. It is stated that lightning current measurements currently used at tall structures cannot serve as a basis to estimate the frequency of dangerous lightning strikes to structures of ordinary height. A scheme of the field research of lightning is proposed that allows the required statistics of lightning currents to be accumulated for a foreseeable time period at admissible material costs. The need to study the mechanism of the competing development of counter discharges from ground-based electrodes is proven, and the relevant technique is offered.

Effect of Magnetic Field on the Spectral Characteristics of Thermal Motion of Charged Particles in an Isotropic Trap

Fri, 03/01/2019 - 00:00
Abstract

Results are presented from analytical and numerical studies of the effect of a dc magnetic field on the spectral characteristics of thermal motion of charged particles in an isotropic electrostatic trap. An analytic expression for the spectral density of the shifts of the center of mass of the systems under study is obtained. The analytic expression is verified by numerically simulating ensembles with different numbers of particles interacting via the Coulomb potential in a wide range of parameters.

Modified Canonical Profile Transport Model for Description of On-Axis Electron-Cyclotron Heating of Tokamak Plasma

Fri, 03/01/2019 - 00:00
Abstract

The specific power deposited in plasma under on-axis electron-cyclotron resonance heating (ECRH) is characterized by strong peaking, resulting in large-amplitude sawtooth oscillations. To analyze the global properties of plasma, instantaneous experimental profiles of the electron temperature should be averaged over time and space. In the present work, a modified canonical profile transport model for predictive calculation of such averaged profiles is proposed. As an example, the model is used to determine the parameters of plasma with ECRH in the T-15MD tokamak currently under construction.

Low-Frequency Continuous MHD Spectrum of Toroidally Rotating Tokamak Plasmas with Anisotropic Pressure

Fri, 03/01/2019 - 00:00
Abstract

Continuous-spectrum equations for low-frequency ideal magnetohydrodynamic perturbations in toroidally rotating plasmas with anisotropic pressure in axisymmetric tokamaks are derived in the framework of the Chew–Goldberger–Low model. In the rotating coordinate system, these equations describe the toroidal coupling of Alfvén and slow magnetosonic modes due to the curvature of magnetic field lines, the pressure anisotropy, and the centrifugal and Coriolis effects. The derived general equations are applied to study the spectra of both the zonal flows and the general electromagnetic modes in low-pressure large-aspect-ratio tokamaks. The condition for the instability of zonal flows due to plasma stratification over the poloidal angle on a magnetic surface is obtained. It is shown that a similar instability takes place for the general modes localized in the vicinities of rational magnetic surfaces. Stabilization of this instability by the Alfvén effect for the modes localized far from rational surfaces is shown.

Electric Field and Poloidal Rotation in the Turbulent Edge Plasma of the T-10 Tokamak

Fri, 03/01/2019 - 00:00
Abstract

The dynamics of turbulent edge plasma in the T-10 tokamak is simulated numerically by solving reduced nonlinear MHD equations of Braginskii’s two-fluid hydrodynamics. It is shown that the poloidal plasma velocity is determined by the combined effect of two forces: the turbulent Reynolds force FR and the Stringer–Winsor geodesic force FSW, which is associated with the geodesic acoustic mode of the total plasma pressure \(\left\langle {p{\text{sin}}\theta } \right\rangle \) . It follows from the simulation results that the FR and FSW forces are directed oppositely and partially balance one another. It is shown that, as the electron temperature increases, the resulting balance of these forces changes in such a way that the amplitude of the poloidal ion flow velocity and, accordingly, the electrostatic potential \({{\phi }_{0}}(r,t)\) decrease. As the plasma density increases, the “driving forces” of turbulence (the dn0/dr and dp0/dr gradients) also increase, while dissipation due to the longitudinal current decreases, which results in an increase in the amplitude of turbulent fluctuations and the Reynolds force FR. On one hand, the force FSW increases with increasing plasma density due to an increase in the pressure \(\left\langle {p{\text{sin}}\theta } \right\rangle \) ; however, on the other hand, it decreases in view of the factor 1/n0. As a result, the net force driving poloidal rotation increases, which leads to the growth of the plasma potential. Both under electron-cyclotron resonance heating and in regimes with evolving plasma density, the results of numerical simulations qualitatively agree with experimental data on the electrostatic potential of the T-10 plasma.

Acceleration and Trapping of Ions upon Collision of Ion-Acoustic Solitary Waves in Plasma with Negative Ions

Fri, 03/01/2019 - 00:00
Abstract

The phenomena occurring under head-on collision of ion-acoustic solitary waves in collisionless plasma consisting of positive and negative ions and electrons obeying the Boltzmann distribution are considered. Using particle-in-cell simulations, it is shown that large-amplitude compressive ion-acoustic solitary waves do not preserve their identity after the collision. Their amplitudes decrease and their shapes change. It is shown that the collision is accompanied by the generation of fast positive ions the velocity of which can exceed more than threefold the speed of sound. In addition, the collision is accompanied by the trapping of negative ions by the field of ion-acoustic solitary waves formed after the collision.

Analytical Model of a Surface Barrier Discharge Development

Fri, 03/01/2019 - 00:00
Abstract

The propagation of the front of a single surface dielectric barrier microdischarge is studied using an analytical model based on the charge balance equation. The model allows one to find analytical dependences of the discharge propagation velocity and the length of the discharge zone on the parameters of the dielectric barrier and applied voltage pulse. To solve the problem, the results of numerical simulations of the distributions of the electric field, potential, and electron density along the discharge channel are used. The results obtained with the help of the proposed model agree qualitatively with available experimental data.

Neutral Beam Current Drive in Globus-M Compact Spherical Tokamak

Fri, 03/01/2019 - 00:00
Abstract

The article presents research on neutral beam current drive in Globus-M compact spherical tokamak. The experiments were performed in the plasma current range of 0.17–0.20 MA with a 0.4- or 0.5‑T toroidal magnetic field. The injection impact parameter was 33 cm. The variable parameters included compositions of plasma and the heating beam (hydrogen, deuterium), plasma density, and vertical displacement of the plasma column. The simultaneous increase in the plasma current and drop in the loop voltage were used to determine the neutral beam current drive. The injection of a hydrogen/deuterium beam into deuterium or hydrogen plasma resulted in a significant and reproducible drop in the loop voltage (up to 0.5 V). In order to process the obtained data, the authors developed an ASTRA code-based model, which allows one to calculate the neutral beam-driven current and bootstrap current. The share of noninductive currents as a function of plasma density during the injection of a hydrogen beam (28 keV, 0.5 MW) into the deuterium plasma was calculated. The authors analyze the results of experiments on off-axis beam injection achieved by vertical displacement of the plasma column and the effect of increasing the toroidal magnetic field on the consumption of the poloidal magnetic flux in discharges with atomic beam injection.

MHD Model of Interaction of the QSPA Plasma Flow with the Magnetic Field of a Current-Carrying Ring Conductor

Fri, 02/01/2019 - 00:00
Abstract

Injection of high-speed plasma flows into a region with a magnetic field created by a current-carrying ring conductor is considered. Analysis is performed on the basis of MHD equations expressed through the vector potential of the magnetic field with allowance for the electric conductivity, thermal conductivity, and radiation transport. The results of numerical experiments demonstrate the possibility of using plasma accelerators as injectors for magnetic confinement devices.

Orbital Maneuvers of Earth Observing Satellites Using Electric Propulsion Systems

Fri, 02/01/2019 - 00:00
Abstract

The paper gives examples of orbital maneuver execution of spacecraft (SC) using orbit correction propulsion system (OCPS) based on electric propulsion (EP). The high velocity of the propellant flow achieved in EP allows for orbital maneuvers with significantly lower propellant flow rate than in conventional propulsion systems. The time required to perform the orbital maneuvers using EP is connected with the available on-board power. It is typically much greater than such time needed for conventional jet propulsions. The advantages of EP are realized if mission allows a long-time operation of the propulsion system. Since the early 1970s stationary plasma thrusters (SPTs) developed on the base on the concept proposed by A.I. Morozov are used on the SC Meteor. With the help of OCPS based on SPT EOL-1, SC Meteor was installed on the conventionally synchronous orbit, which provides a fixed grid of tracks with a period of T = 102.31 min. In this case, a complete overview of all daily Earth’s surface with the bandwidth of 2900 km is obtained. In recent decades, there has been a steady trend toward miniaturization of space equipment, which requires the development of acceptable thrusters to meet new requirements. Typical total impulses of thrust required for OCPS are reduced several times. At present, a number of space constellations are being developed in Russia based on small SC with a mass from 60 to 500 kg. VNIIEM Corporation creates a space constellation IONOZOND, designed to monitor the geophysical conditions. The description of the IONOZOND constellation is given and the options for using of various OCPS, in particular, based on SPT, ion and pulsed plasma thrusters are considered. It is shown that their use in small SC can significantly increase the economic efficiency of remote sensing orbital constellations.

Studies of Galatea Multipole Traps at the Russian Technoligical University MIREA

Fri, 02/01/2019 - 00:00
Abstract

Results from studies of plasma production and confinement in Galatea multipole magnetic traps at the Russian Technological University MIREA are presented. The magnetic systems of such traps are considered. It is shown that it is possible to design a system in which plasma bunches and neutral atomic beams are injected along the major radius of the torus. Different methods of plasma production in such traps are studied. It is shown that plasma production by means of an electric discharge is inefficient. The process of loading of the trap with plasma by injecting a plasma bunch is studied in detail. The parameters of the plasma bunch at which it is efficiently captured by the trap are determined. The azimuthal diamagnetic current arising after the plasma bunch is injected into the trap is measured using a Rogowski coil. The interaction of this current with the magnetic field of the trap results in the appearance of the Ampère forces confining the plasma. The plasma temperature in the trap can be determined from the measured value of the diamagnetic current. It is shown that it is possible to design a laboratory prototype of a trap with two levitating coils. The magnetic field and ion temperature in such a trap are estimated to be 0.37 T and >300 eV, respectively.

MHD Stability and Energy Principle for Two-Dimensional Equilibria without Assumption of Nested Magnetic Surfaces

Fri, 02/01/2019 - 00:00
Abstract

Abandoning the assumption of nested magnetic surfaces in tokamak plasma expands the field of research and opens up new approaches for both theoretical and experimental plasma physics. The computer code KINX for calculations of the ideal MHD stability was developed for studies of doublet plasmas with two magnetic axes and using block-structured grids in each subdomain with nested magnetic surfaces. Then, the MHD_NX code on unstructured grids was developed to calculate the stability of two-dimensional equilibria with an arbitrary topology of magnetic surfaces. The study of equilibrium and stability of equilibrium configurations with toroidal current density reversal and axisymmetric n = 0 islands, which are associated with internal transport barrier and low current density at the magnetic axis, as well as with the operation of tokamaks in the alternating current regime, leads to more general issues of MHD stability of two-dimensional solutions of the Grad−Shafranov equations with islands under other types of symmetry—chain of islands in helical symmetry and cylindrically symmetric m = 0 islands in configurations with the longitudinal field reversal. New ideal MHD unstable modes have been discovered for various types of two-dimensional island configurations. The energy principle with MHD-compatible boundary conditions at open magnetic field lines is necessary for the self-consistent stability analysis of divertor configurations in tokamaks with a finite current density at the separatrix, taking into account the plasma outside the separatrix. Several codes have been developed for calculations of plasma equilibrium and stability, taking into account the influence of currents outside the separatrix, which are ready for integration with other codes for edge plasma modeling.

Magnetic Islands and Current Filamentation in Tokamaks

Fri, 02/01/2019 - 00:00
Abstract

It is well known that heat exchange between the hot plasma and solid wall of a magnetic fusion reactor (tokamak or stellarator) depends, to a great extent, on local disturbances of its magnetic configuration, which occur under the action of resonant magnetic perturbations. One possible reason for their development in tokamaks is splitting of an originally axisymmetric current channel into separate filaments. Among negative consequences of such filamentation, there are spatial modulations of the plasma density and temperature, which can considerably increase local heat loads on the tokamak intrachamber elements. This study draws attention to the fact that one of the most dangerous manifestations of current filamentation in tokamaks is the formation of so-called “positive” magnetic islands, which are clearly observed during the development of internal disruptions in the form of “hot spots”—local hot dense helical plasma structures formed in the central region of the plasma column just before the major disruption. An increase in the heating efficiency and improvement of plasma confinement within magnetic islands (which are, in fact, closed magnetic configurations) could explain energy balance in such structures. In this article, stability of such structures and possible sources of initial current disturbances initiating their formation are discussed as illustratively as possible on the basis the initial physical principles, which, in the author’s opinion, can be useful for the experimenters who start solving problems of plasma–wall heat exchange under real tokamak conditions. A number of the well-known experimental observations that can be explained in the framework of this concept are discussed. It is argued that the concept of positive magnetic islands makes it possible to interrelate the sequence of MHD events resulting in the tokamak major disruption.

Experimental Studies of High-Energy Quasi-Steady Plasma Streams Generated by a Magnetoplasma Analogue of the Laval Nozzle in the Compression and Acceleration Regimes

Fri, 02/01/2019 - 00:00
Abstract

Results of many-year-long studies of high-power quasi-stationary acceleration and compression systems whose channel is a magnetoplasma analogue of the Laval nozzle are summarized. Double-stage quasi-stationary plasma accelerators with channels made of rod electrodes or complicated magnetoplasma transformers are described. Results are presented from experimental studies of acceleration and compression systems. It is shown that, in the optimal operating regime corresponding to the proper choice of the initial and boundary conditions, plasmadynamic devices are able to generate plasma streams with parameters close to the theoretical limit for given experimental conditions. A unique set of parameters of the generated streams were achieved: the density of the accelerated stream of up to 1016 cm–3 at a maximum velocity of (4–4.2) × 107 cm/s and the density of the compressed stream of up to 1019 cm–3 at a plasma temperature of 60–100 eV. The total energy content in the accelerated plasma streams reaches 0.9–0.95 MJ at an accelerating channel efficiency of 0.8–0.9. The generation time of accelerated stream amounts to 150–200 particle flight times along the channel of the plasmadynamic device, while the lifetime of the compression region reaches 20–30 particle flight times.

Control of Coherent Structures via External Drive of the Breathing Mode

Fri, 02/01/2019 - 00:00
Abstract

The Hall thruster exhibits two types of large-scale coherent structures: axially propagating breathing mode (m = 0) and azimuthal, with low m, typically m = 1, spoke mode. In our previous work, it was demonstrated that axial breathing mode can be controlled via the external modulations of the anode potential. Two regimes of the thruster response, linear and nonlinear, have been revealed depending on the modulation amplitude. In this work, using the high-speed camera images and developed image-processing technique, we have investigated the response of the azimuthal mode to the external modulations. We have found that, in linear regime, at low modulation voltages, axial and azimuthal structures coexist. At larger amplitudes, in the nonlinear regime, the azimuthal mode is suppressed, and only axial driven mode remains.

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