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Spontaneous Break of up–down Symmetry in a Symmetric Double-Null Divertor Configuration

Mon, 07/01/2019 - 00:00
Abstract

A possibility of up–down asymmetry of the profiles of the edge plasma parameters in a symmetric double-null divertor configuration in the case of impurity seeding is found by computational modeling. The physical mechanism of such a symmetry violation, related to radiation-condensation instability, is proposed. This effect can reduce the would-be advantage of the double-null configuration in the power load spreading over a larger area, which is vital for fusion reactor projects.

MHD Waves and Instabilities in Two-Component Anisotropic Plasma

Mon, 07/01/2019 - 00:00
Abstract

Based on the 16-moment MHD transport equations, the propagation of linear waves in an anisotropic homogeneous cosmic plasma is considered. A general dispersion relation is derived with allowance for two plasma components (electrons and protons) and heat flux along the magnetic field. This dispersion relation is a generalization of the previously studied cases of one-component (ion) plasma. The case in which the effects associated with the heat flux are ignored is analyzed in more detail. In the limit of longitudinal propagation, the wave modes fully consistent with the modes known in the low-frequency kinetic theory of collisionless plasma are classified. Firehose and mirror instabilities are analyzed. It is shown that taking into account the electron component modifies the growth rates and thresholds of instabilities.

Specific Features of Charged Particle Confinement in the T-15 Tokamak in the Presence of Magnetic Perturbations

Mon, 07/01/2019 - 00:00
Abstract

Charged particle motion in the regions of magnetic islands and near-separatrix ergodicity of magnetic field lines in the Т-15 tokamak is studied numerically. The trajectories are calculated by integration of exact three-dimensional equations of motion for trapped and passing charged particles starting at different pitch angles. It is shown that the presence of resonant magnetic perturbations qualitatively affects the trajectories of passing particles. In the region of magnetic islands, the orbits of passing particles acquire an island structure, while in the region of the near-separatrix ergodicity of magnetic field lines, the orbits are stochastized. It is shown that magnetic perturbations insignificantly affect trapped particle motion. Even in the absence of magnetic surfaces, the trajectories of trapped particles are regular and their cross sections have the shape of a standard banana orbit. The possibility of charged particles to pass between regions with different magnetic topologies is analyzed.

Radiative and Gravitational Modes and Instabilities in an Inhomogeneous Magneto Dusty Plasma with Charge Variation

Mon, 07/01/2019 - 00:00
Abstract

The Jeans instability of collisional magnetized dusty plasmas is examined incorporating polarization force, charge variation of dust grains, and radiative cooling of ion and electron species. The collisional effects of dust grains with neutrals are considered with sufficient background of neutral particles. A general dispersion relation is obtained using normal mode analysis technique which is found to be modified due to polarization force, dust charge fluctuation, and radiative effects of electrons and ions. The analytical discussion of general dispersion relation is presented in parallel and perpendicular mode of propagation. The Routh Hurwitz criterion is applied to analyze the stability of the considered system. We report the existence of a modified cyclotron mode in parallel propagation and a purely collisional mode in perpendicular propagation along with the gravitational mode. These modes are discussed analytically, as well as numerically, to show the importance of different parameters considering different situations. The implications of the result have been discussed for the molecular clouds.

Laser-Induced Fluorescence Measurements of the Noble-Gas Atom and Ion Densities in a Mirror System

Mon, 07/01/2019 - 00:00
Abstract

The densities of NeI atoms and XeII ions in the PS-1 mirror system were measured by the method of laser-induced fluorescence (LIF). Schemes of levels for LIF measurements of the density are proposed. Models for numerical processing of the obtained LIF signals are developed. The electron temperature and plasma density were also measured using Langmuir probes.

Plasma Production during Implosion of Quasi-Spherical Wire Arrays

Mon, 07/01/2019 - 00:00
Abstract

The motion of the outer boundary of a quasi-spherical tungsten wire array at currents of 2−4 MA was studied experimentally. It is shown that the outer boundary shifts later than it is predicted by simulations by the heterogeneous array model. The instabilities developing at the boundary of the quasi-spherical array are similar to those occurring in a cylindrical array made of the same material. It is found that, in spite of the different time behaviors of the inductances during implosions of quasi-spherical and cylindrical arrays, the energies required to compress these arrays to ~0.4 of the initial radius differ insignificantly.

Effect of Dust Charge Fluctuation on Ion Acoustic Waves in a Plasma with Nonextensive Electrons

Mon, 07/01/2019 - 00:00
Abstract

The linear dispersion for ion acoustic waves (IAWs) including the effects of dust charge fluctuation and electron nonextensivity is derived. It is shown that the presence of nonextensive electrons modifies the charge fluctuation behavior, leading to the dissipative and instability for IAWs. The dependence of the frequency and damping rate of IAWs on the electron nonextensive parameter q is closely to the dust density since it greatly affects the dust charge fluctuation.

Spatial–Temporal Properties of Current-Convective Turbulence in Divertor Plasma under DIII-D-Like Detached Conditions

Mon, 07/01/2019 - 00:00
Abstract

Recent experimental observations of divertor plasma turbulence at the ASDEX Upgrade tokamak have demonstrated the onset of strong fluctuations of plasma radiation intensity in the near X-point region within the inner divertor leg, when the machine was operating with the pronounced asymmetry in detachment of plasma between the inner and outer divertors. As been recently shown, one of the possible mechanisms responsible for these fluctuations can be related to the current-convective instability (CCI). In this contribution, we assess whether the CCI can drive turbulence with spatial and temporal parameters similar to those observed lately in DIII-D experiments. It is demonstrated that the CCI drives saturated plasma fluctuations, dominated by low-frequency modes with toroidal wavelengths on the order of the machine circumference in the inner divertor region—features that are similar to turbulence characteristics found in experiments.

Radiation Spectra of the Plasma Relativistic Microwave Oscillator

Mon, 07/01/2019 - 00:00
Abstract

The spectral composition of high-power microwave radiation of the plasma relativistic oscillator in the range of 1−4 GHz was studied experimentally as a function of the plasma density and the cavity length and configuration. The duration of the generated microwave pulse was in the range of 250−400 ns, and its total energy reached 15 J. The main features of the spectral composition for different cavity configurations are explained.

Measurements of the X-Ray Line Spectrum of a Micropinch Source by a High-Sensitive Track Detector

Mon, 07/01/2019 - 00:00
Abstract

The efficiency of the diffusion chamber as a tool for diagnostics of pulsed high-temperature plasma in a single discharge is demonstrated. Creation of a diffusion chamber operating at a temperature exceeding the temperature of the ambient medium will make it possible to simplify the device design and operation. Based on the experimental data obtained with the help of the diffusion chamber by means of diffraction spectroscopy, a hypothesis is put forward on the advanced development of electron acceleration processes compared to the process of micropinch plasma decay caused by anomalous plasma heating.

Change in the Generation Mode of the Plasma Relativistic Microwave Oscillator

Mon, 07/01/2019 - 00:00
Abstract

Time evolution of the parameters of output radiation of a plasma relativistic microwave oscillator (PRMO) based on the Sinus 550-80 accelerator during a single relativistic electron beam (REB) pulse was studied experimentally. Analysis of the experimental data was accompanied by numerical simulations under conditions close to the experimental ones. It is shown that qualitative changes in the parameters of the PRMO output radiation are associated with a change in the generation mode. The first half of the REB pulse is characterized by the maximum output radiation power and a broadband spectrum consisting of a great number of harmonics. The nonlinear beam–plasma interaction limits the growth of the field amplitude, decreases the number of plasma electrons, and leads to the emergence of longitudinal plasma inhomogeneity. The appearance of an ion background in the drift tube volume and the formation of a radially nonuniform plasma waveguide alter the conditions of beam–plasma interaction in the second half of the pulse. The second half of the REB pulse is characterized by a decrease in the output microwave power and the narrowing of the generation frequency band. Spectral analysis demonstrates preservation of a stable narrow spectral component throughout the microwave pulse.

Discharge in a Subthreshold Microwave Beam as an Effective Means for Mercaptan Decomposition

Sat, 06/01/2019 - 00:00
Abstract

A subthreshold microwave discharge in atmospheric-pressure air is used to process mixtures of mercaptans (thiols) with air and with air and methane. It is found that, at microwave intensities of ~10 kW/cm2 and pulse durations of 1–3 ms, the mercaptan concentration decreases 100-fold in mixtures with air and 20-fold in mixtures with air and methane. It is shown that, at an average specific energy deposition in a single discharge of ~0.4 J/cm3, the specific microwave energy spent on mercaptan decomposition is ~10 J/cm3. It is concluded that the active zone of the discharge with high specific energy deposition occupies less than 5% of the microwave beam volume.

Heating of Plasma Electrons by Laser Radiation under Parametric Resonance Conditions in Strong Magnetic Field

Sat, 06/01/2019 - 00:00
Abstract

Heating of plasma electrons by an extraordinary laser wave at the double upper hybrid frequency in a strong magnetic field is studied numerically by the particle-in-cell method. It is shown that, in this case, significant heating takes place even at considerable detunings from the fundamental parametric resonance. This makes it possible to reduce the resonance magnitude of the magnetic field. The minimum value of the laser field amplitude at which parametric heating of this type can be implemented is found.

Research on Beam-Focusing Characteristics of Krypton Hall Thruster

Sat, 06/01/2019 - 00:00
Abstract

The reserves of krypton are abundant, and it has become an important high specific impulse substitute for Hall thrusters as its physical properties are closest to xenon. However, it has been found that the plasma beam of a Hall thruster has weak plume-focusing characteristics and the half-divergence angle is large when using krypton as a propellant. In this paper, the thrust, specific impulse, efficiency, and plume half-divergence angle of a Hall thruster under different propellant supply flow rates are measured. In addition, the influences of the magnetic field configuration and cathode heating power on the beam-focusing characteristics are researched. The results show that the magnetic field mainly affects the beam-focusing characteristics by the bending direction of the magnetic field lines, thus causing the divergence, focusing, and overfocusing phenomena of the plasma plume. Excessive cathode heating power changes the low-frequency discharge amplitude of the Hall thruster, affects the ionization distribution and the width of the main ionization region, and thereby changes the plume focusing state.

Atmospheric-Pressure Microwave Plasma Torch for CVD Technology of Diamond Synthesis

Sat, 06/01/2019 - 00:00
Abstract

An electrodeless microwave jet plasma source is considered, and its various applications in the technology of chemical vapor deposition of diamond films and dimension increasing of small diamond single crystals synthesized at high pressures and temperatures are discussed. The plasma jet is ignited in an atmospheric-pressure gas (argon) flow with hydrogen and methane additives. The operation of the microwave jet reactor is described, and the plasma characteristics measured using emission spectroscopy are presented. The brightly glowing atmospheric-pressure plasma jet is ignited and stably burns at a microwave power of ≤1 kW supplied from a microwave oven magnetron. The specific microwave power density absorbed by the compact plasma jet (≤104 W/cm3) is comparable with that absorbed by a dc arc. The growth rate of the polycrystalline diamond layer amounts to 40 µm/h. The process of film deposition on the substrate can be controlled by scanning the substrate surface with the jet.

Generation of Terahertz Radiation under Interaction of Counterpropagating Laser Pulses in Underdense Plasma

Sat, 06/01/2019 - 00:00
Abstract

Generation of terahertz waves under interaction of two counterpropagating laser pulses with different frequencies in underdense plasma is analyzed. Spectral, angular, and energy characteristics of terahertz radiation are investigated as functions of the frequency difference of the laser pulses. It is shown that, due to the frequency difference, in addition to the line at the doubled plasma frequency [L.M. Gorbunov, A.A. Frolov, JETP 98, 527 (2004)], a peak near the plasma frequency appears in the emission spectrum. The total energy of the terahertz signal is calculated, and the condition under which emission at the plasma frequency dominates is found. It is shown that the energy of radiation at the plasma frequency reaches its maximum value when the frequency difference of the laser pulses is close to the plasma frequency.

Nonlinear Stabilization of Resistive Instability of a Tubular Charged Particle Beam Moving above a Solid-State Plasma Cylinder

Sat, 06/01/2019 - 00:00
Abstract

Nonlinear stabilization of instability of an infinitely thin tubular electron beam moving along the surface of a solid-state plasma cylinder is analyzed. It is assumed that the electron collision frequency in the plasma cylinder is much higher than the frequency of plasma eigenmodes (oscillations). The beam is assumed to be nonrelativistic, and, thus, the problem is solved in the electrostatic approximation. It is shown that the growth of the wave amplitude is stabilized nonlinearly due to the self-trapping of the beam electrons by the field of the electrostatic wave excited in the beam itself. It is found that the saturation time of instability and the maximum amplitude of the excited wave depend on the radius of the plasma cylinder. It is established that the larger the radius of the plasma cylinder, the later the nonlinear stage of instability begins and the larger the maximum amplitude of the excited wave.

Experiments on Ion Acceleration in the Diode of the KALMAR REB Generator

Sat, 06/01/2019 - 00:00
Abstract

Experiments on ion acceleration in the ambipolar field were carried out at the Kalmar relativistic electron beam generator with a pulsed voltage of ~250 kV, current of ~40 kA, and pulse duration of ~150 ns. Previously developed methods were used to measure the ion fluxes and determine some other properties of the generated ion beams. It was found that aluminum ions (the heaviest particles present in the thin anode film) acquired the maximum energy of 6.1 MeV, while the maximum proton energy was ~800 keV. The energy of ions propagating at an angle of 20° relative to the accelerator axis was approximately 15% lower than the energy of ions propagating along the axis. The total energy of accelerated ions varied from 0.07 to 0.35 J, while the angular divergence of the electron beams was 1.2°–2.2°.

Study of Metal Atom Ionization in a Hollow-Cathode Magnetron

Sat, 06/01/2019 - 00:00
Abstract

The ionization region of a hollow-cathode magnetron discharge was studied experimentally. The discharge was powered by either a dc supply or a high-power pulsed supply. The emission spectra of atoms and ions of the target material and buffer gas were recorded in different discharge regions by using an optical probe. The plasma parameters in the dc and pulsed discharges were determined by means of a Langmuir probe. The goal of this research was to find the position of the ionization region and reveal the mechanism of ionization of sputtered target atoms.

Parameters of the Plasma of a Large-Scale High-Voltage Discharge in Air at Reduced Pressures

Sat, 06/01/2019 - 00:00
Abstract

The Sprite device allows one to ignite an ~1-m-long pulsed high-voltage discharge detached from the metal chamber walls in both a uniform gas and a gas with a large-scale pressure gradient. A microwave interferometer, a magnetic probe, and optical photography are used to diagnose the discharge plasma. It is established that, at pressures of 0.05−1 Torr, the transverse profile of the diffuse discharge glow is close to the current profile. The plasma density in the diffuse discharge reaches 1012 cm–3. The results of interferometric measurements of the plasma density agree well with the values obtained from measurements of the current density in the approximation of a steady-state electron drift. The results obtained are necessary for laboratory modeling of high-altitude discharges in the Earth’s atmosphere.

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