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Density Profile of a Plasma Layer Formed by an Electron Beam

Sat, 12/01/2018 - 00:00
Abstract

Results of particle-in-cell simulation of the formation of a near-wall plasma layer produced by an ionization source remote from the chamber walls are verified experimentally. The measured profiles of the density and temperature of the plasma produced by an electron beam in two modes are compared: (i) gas is ionized by a low-density electron beam, while collective interactions are almost absent (beam-induced plasma), and (ii) gas is mainly ionized by plasma electrons heated due to the development of two-stream instability (beam−plasma discharge). The measured spatial profiles of the parameters of the beam-induced plasma are found to qualitatively agree with the model ones.

Spatiotemporal Dynamics of Fast Electron Beams during a Disruption in the T-10 Tokamak

Sat, 12/01/2018 - 00:00
Abstract

Results are presented from experimental studies of fast electrons in modes with disruption instability in the T-10 tokamak. The spatiotemporal dynamics of electron beams is investigated using two sets of cadmium telluride detectors and tomographic recovery of the intensity of suprathermal X-ray emission (Eγ ~ 15–300 keV). Generation of fast electron beams in the central zone of the plasma column in the initial stage of disruption in the T-10 tokamak is established. Analysis of suprathermal X-ray bursts during the development of disruption indicates spatial inhomogeneity of fast electron beams. The characteristic repetition frequency and duration of X-ray bursts are f ~ 2.5–3 kHz and Δt ~ 5 ms, respectively. Analysis of the experimental results shows that oscillations of the X-ray intensity may be related to the filamentation of fast electron beams.

Hall Effect in Laboratory and Space Current Sheets

Sat, 12/01/2018 - 00:00
Abstract

The role of the Hall effect in the formation of thin current sheets (CSs) is analyzed by comparing results of laboratory and satellite experiments. In spite of the strong difference in the plasma parameters, both laboratory and space CSs are characterized by the hot ion component and cold isotropic electron component. Such a relation between the ion and electron components would have to guarantee the predominant contribution of the ion diamagnetic drift current to the total current density. However, the Cluster spacecraft observations of thin CSs in the Earth’s magnetotail have revealed the predominance of a strong electron (rather than ion) current. This may be due to the Hall effect, which causes generation of electric fields, which redistribute the CS currents in favor of the electron currents at the expense of the ion currents. Observations demonstrate that the thinner the CS, the stronger this effect. A similar tendency was also observed in laboratory experiments on CS formation in plasma with ions of different mass, which made it possible to vary the CS relative thickness and monitor the consequences of the Hall effect. Comparison of results of satellite and laboratory experiments indicates that the Hall effect plays an important role in the formation of thin CSs in both magnetospheric and laboratory plasmas.

Charge of a Particle-Absorbing Sphere in Collisionless Plasma by the Results of a Direct Numerical Experiment

Sat, 12/01/2018 - 00:00
Abstract

The process of charging of a spherical body absorbing plasma electrons and ions is modeled by directly solving the Vlasov–Poisson equations. The main goal of the numerical experiment is to determine the sphere charge in a stable steady state established on long times in perturbed plasma. Special attention is paid to the contribution of trapped ions moving in finite orbits to the screening of a charged body. The charge of the trapped particle cloud is determined as a function of the body size in a wide parameter range of an initially unperturbed plasma.

Investigation of the Influence of Injection Parameters on Particles Motion in Electric and Magnetic Fields for Designing Plasma Separation Technique

Sat, 12/01/2018 - 00:00
Abstract

The paper continues studies of the capabilities of plasma treatment of spent nuclear fuel and radioactive waste. The study is devoted to the problem of integration of the plasma source and separator, while the initial conditions of the substance input are considered by taking into account the possibilities of the process implementation. The results of calculations are presented in the one-particle approximation of 3D trajectories of the substance ions simulating the components of spent nuclear fuel. The calculations have been performed for the magnetic field generated by the coils and for the model configurations of the electric field approximated for the experimental capabilities. The electric potential configurations and the initial conditions pertinent to plasma injection along the magnetic field have been proposed, which allow efficiently separating singly charged ions of model substances characterized by masses of 150 and 240 amu, energies in the range of 0.02–20 eV, and an initial angular spread in velocities of 60°. The distance between the separated beams with different masses is found to be 10 cm for the characteristic separator size of 1 m.

Discharge in a Subthreshold Microwave Beam as an Unusual Type of Ionization Wave

Sat, 12/01/2018 - 00:00
Abstract

Ionization−overheating instability of a non-self-sustained discharge in air in a subthreshold microwave field creates a self-sustained discharge with a fine cellular structure, whose UV radiation, in turn, generates a new non-self-sustained discharge on the microwave beam path, where ionization−overheating instability arises again. It is shown that, at microwave intensities in the range of 3–18 kW/cm2, the propagation velocity of the ionization wave, including the region of the self-sustained discharge and the forward region of the non-self-sustained discharge developing in its UV halo, is proportional to the third power of the microwave field strength, while the maximum temperature in the discharge is inversely proportional to the microwave field strength.

Four Tensors Determining Thermal and Electric Conductivities of Degenerate Electrons in Magnetized Plasma

Sat, 12/01/2018 - 00:00
Abstract

A solution to the Boltzmann equation is obtained for a magnetized plasma with strongly degenerate nonrelativistic electrons and nondegenerate nuclei. The components of the diffusion, thermal diffusion, and diffusion thermoeffect tensors in a nonquantizing magnetic field are calculated in the Lorentz approximation without allowance for electron−electron collisions, which is asymptotically accurate for plasma with strongly degenerate electrons. Asymptotically accurate analytical expressions for the electron diffusion, thermal diffusion, and diffusion thermoeffect tensors in the presence of a magnetic field are obtained for the first time. The expressions reveal a considerably more complicated dependence on magnetic field than analogous dependences derived in the previous publications on this subject.

Study of Biological Effects by Pulsed Bremsstrahlung Radiation of Ultrahigh Dose Rate at the Angara-5-1 Facility

Sat, 12/01/2018 - 00:00
Abstract

Radiological methods of treatment of malignant tumors are widespread in medical practice. In addition to the type of radiation and dose and fractionation of the radiation regime, the dose rate is one of the factors that affects the effectiveness of treatment. The therapeutic dose rate lies in the range of tens of mGy/s. At modern high-current accelerators of relativistic electron beams, a significant increase in the dose rate to hundreds of MGy/s is achievable, which is more than 108 times greater than the therapeutic dose rate. It is difficult to predict the nature of processes in tissues and its cells at such radiation intensities. To determine the effect of extreme dose rate on the radiosensitivity of tissues at the Angara-5-1 facility, experiments were conducted to determine the lethal dose (LD50/30) for laboratory mice. Our result on the study of the LD50/30 dose (~100 MGy/s) allows us to make a conclusion about a possible higher lethal dose than the power range of the doses used for medical purposes.

Determination of the Ion Composition of the Outer Ionosphere from the Characteristics of ELF–VLF Waves Detected during the Operation of the Sura Facility

Sat, 12/01/2018 - 00:00
Abstract

The propagation characteristics and physical nature of low-frequency waves observed in the ionosphere during heating of the near-Earth plasma by a powerful short-wavelength radiation are investigated using the singular value decomposition method. The experimental results obtained with the Sura midlatitude heating facility and detected by the onboard equipment of the DEMETER satellite were used. The ion composition of the near-Earth plasma is estimated from the spectral characteristics of the observed radio waves. The method is verified using results of full-scale measurements performed at altitudes corresponding to the Earth’s outer ionosphere.

Preliminary Experimental Results of Pellet Injection on the HL-2A Tokamak

Sat, 12/01/2018 - 00:00
Abstract

A pellet injector was installed on the HL-2A tokamak, and preliminary experiments were performed recently. In this paper, the impact of pellet injection on the scrape-off-layer intermittent transport is described. High-field-side pellet injection experiments are also reported.

Influence of a Nitrogen Admixture on the Value and Radial Profile of the Metastable Argon Atom Density in a DC Glow Discharge in Argon

Sat, 12/01/2018 - 00:00
Abstract

Results of measurements of the value and radial profile of the density of Ar(3P2) metastables in a dc discharge in pure argon and Ar + 0.1%N2 and Ar + 1%N2 mixtures are presented. The electric field strength in the positive column of the discharge was also measured. The experiments were performed in a 2‑cm-radius discharge tube at gas pressures of 1, 7, and 60 Torr and discharge currents in the range of 10–50 mA. It is found that, at a pressure of 60 Torr, a nitrogen admixture to argon leads to a significant decrease in the electric field strength in the diffuse discharge, while at P = 1 Torr, in contrast, the electric field increases substantially. The degree to which the nitrogen admixture affects the density of Ar(3P2) atoms on the discharge tube axis also depends on the gas pressure. At a pressure of 60 Torr, the Ar(3P2) density decreases substantially (by three orders of magnitude for the 1%N2 admixture and 1.5 orders of magnitude for the 0.1%N2 admixture), while at a pressure of 1 Torr, the Ar(3P2) densities in pure argon and in Ar + N2 mixtures differ less than twice. It is also shown that, at all gas pressures under study, a nitrogen admixture to argon leads to the broadening of the radial profile of the Ar(3P2) density. The experiments were accompanied by numerical and theoretical studies. For pure argon, the calculations were performed in a one-dimensional (along the tube radius) discharge model, while for the Ar + 1%N2 mixture, in a zero-dimensional model, which allows one to calculate the plasma parameters on the tube axis. The calculated results were used to qualitatively explain the experimentally observed effects.

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