Bulletin of the Russian Academy of Sciences: Physics最新文献

We presented the results of simulating the growth of a plasma column in a barrier discharge ignited in a long dielectric tube at a low-pressure of a molecular gas (nitrogen/air at a pressure of 1 Torr). The spatial images of the plasma structure and the electric field that occur in the pulse-periodic mode of a barrier discharge in gas with external ring electrodes (pulse duration of 1 μs, voltage amplitude of 5 kV, pulse repetition frequency of up to 200 kHz) are presented. The plasma diffuse channel as a negative streamer is formed between the positively charged plasma of the column and the end wall of the discharge tube. It is shown that near the head of the growing diffuse channel, the electric fields are sufficient to form a flow of runaway electrons.

The single crystals of CaF₂–SrF₂–BaF₂–CeF₃ solid solutions with different concentrations of cerium were grown by Bridgman technique in vacuum with CF₄ fluorinating atmosphere. The recorded X‑ray powder diffraction (XRD) patterns correspond to single-phase samples with a fluorite crystal structure. The composition of the samples was confirmed by energy dispersive analysis (EDX). Five excitation bands around 210, 245, 256, 298 and 314 nm were observed according to spectral-luminescent data and had corresponding luminescence and absorption bands. Luminescence bands with maxima at 313 and 333 nm are related to impurity center with the symmetry C_4v, and those which around 340 and 370 nm are typical for the Ce³⁺ center of trigonal symmetry C_3v. From the recorded absorption spectra it follows that cerium ions concentration decreases along the crystals with maximum value at the beginning of crystallization part.

We report on the synthesis of Nd³⁺ (1 mol %):YF₃ phosphors obtained via co-precipitation from aqueous solutions. Annealing in air of the synthesized Nd³⁺ (1 mol %):YF samples resulted in the additional formation of an oxygen-vacancy complex (O_F–O_V complex). The luminescence intensity of the O_F–O_V complex increased by a factor of 12.5 relative to the non-annealed sample. The impact of oxygen-vacancy defects (O_F–O_V complex) on the luminescent properties of samples that doped with Nd³⁺ ions in temperature range from 80 to 320 K was studied. It was suggested that the thermal trapping of charge carriers by the defective O_F–O_V complex at low temperatures (≈80 K) leads to an increase in the intensity of defect luminescence compared with the luminescence of Nd³⁺ ions and vice versa. It was found that the energy transfer between the O_F–O_V complex and Nd³⁺ ions is unlikely.

The results on cathode material erosion under action of atmospheric-pressure glow discharge in argon flow powered by unipolar microsecond voltage pulses following with a pulse repetition rate up to 100 kHz are presented. An amplitude of the current pulses was from 500 to 800 mA. The presence of erosion products from the cathode material in the plasma was determined through the detection of ionic and atomic metal lines in its emission spectrum, as well as through analyzing powder samples collected during the operation of the discharge system. It has been shown that, depending on the method used to create the temperature conditions required for cathode erosion, different nanoparticles are formed. These nanoparticles differ in their morphology and chemical composition.

We presented results of an experimental study of a continuous magnetron discharge in vacuum (gasless) mode with a flat target. The base pressure in the vacuum chamber during the discharge operation was 3 × 10^–5 Torr at a magnetron current of 10 A. We developed techniques for measuring the mass-to-charge composition of plasma ions and the electron temperature under conditions of intense target sputtering. Measurement of target temperature was carried out. We developed techniques for comparison of ion current density and degree of ionization of the target material near the substrate in gas and vacuum modes.

Ground water was treated with a discharge in bubbles fed with voltage pulses with a rise time varied from 51 to 1400 ns. The resulting water–salt solutions (plasma-activated water—PAW) were used to germinate soft spring wheat (Triticum aestivum L.) of the “Iren” variety. As a result of observing plants for 5 days, it was found that PAW obtained in different ways had different effects on their development. Compared to the control sample, the greatest moisture uptake and growth of the plant root system is ensured by treatment with 51 ns voltage pulses, while the use of microsecond pulses leads to a decrease in these parameters. It is assumed that the use of pulses with a shorter rise time leads to the formation of chemically active species in PAW. The experiments performed demonstrate the importance of taking into account the electrophysical parameters of devices when developing technologies for the production and use of PAW in plasma agriculture.

It was shown that Pt–Pd PAPLAL nanoparticles catalyze the decomposition of hydrogen peroxide in water. Addition of Pt–Pd nanoparticles at a concentration of 8.3 μL/mL for 30 min reduced the relative amount of hydrogen peroxide by 44.7%, and the higher the concentration of nanoparticles, the faster the decomposition occurs. The observed catalytic activity makes PAPLAL particles promising in the treatment of skin diseases caused by oxidative stress (dermatitis, vitiligo).

A numerical simulation of a multiwave Cherenkov generator with a diffraction reflector in the subterahertz (112–114 GHz) frequency range has been performed using a 2.5D hybrid electromagnetic PIC code. A tubular beam with an electron energy of 180–490 keV and a current of 1–30 kA was injected into an electrodynamic system with a diameter of 36.4 mm. Two regimes were found in numerical experiments. With electron energy near 280 keV, a high (88%) efficiency of bringing radiation power forward was obtained. With electron energy near 250 keV, a power self-modulation regime was found. In the multiwave Cherenkov generator, for the first time, a transition was found from stable generation at a power of 6.8 MW to stable generation at a power of 800 MW through self-modulation and chaotic power oscillations with an increase in beam current. In this case, the ratio of the beam current to the starting generation current varied from 1.3 to 16.7.

Crystals of the UV lasers active media Ce³⁺:LiCaAlF₆, which are characterized by the formation of several types of Ce³⁺ ion centers, have been studied. The presence of nonequivalent impurity centers was confirmed by detection of zero-phonon lines for two centers in low-temperature luminescence spectra, as well as from EPR spectra for various magnetic field orientations relative to the crystallographic axes of the samples. We have shown that for Ce³⁺:LiCaAlF₆ crystals, with an increase in the concentration of Ce³⁺ ions in the melt , crystallization occurs in such a way that the concentration of impurity centers of lower symmetry increases to a greater extent than centers of higher symmetry . Moreover, laser oscillation belongs to precisely this type of center.

We consider the cascade acceleration of subnanosecond electron beams. A brief review is given of experiments with coaxial diodes in which the electron emission and acceleration processes occur in two successive gaps. An experiment on the emission of runaway electrons in a gas diode of this configuration gave rise to a study of magnetically insulated explosive emission vacuum diodes. It has been demonstrated that in this type of diode the spread in energies of electrons emitted within the rise time of the accelerating pulse can be significantly reduced. A numerical simulation has shown that the energy of the electrons of a high-current beam increases if they are accelerated in the successive gaps, formed by a drift tube and powered with precisely timed voltage pulses, generated by independent RADAN-type sources.