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

The processes on the cathode surface prior to the establishment of a diffuse mode of vacuum arc discharge on gadolinium cathode was investigated. Using high-speed video recordings and voltage waveforms, the breakdown process and the ejection of cathode material droplets were examined. The temporal dependences of cathode temperature and discharge voltage after breakdown were analyzed. Characteristic times for the transition to the steady-state diffuse discharge mode were determined. It was found that during breakdown and after, when discharge is unstable, droplets and voltage spikes can occur. Discharge stabilization can be achieved by increasing the cathode temperature, which leads to the transition to a mode with a voltage of about 5 V and the absence of voltage spikes and droplets. These results are significant for the development of a plasma source for the plasma separation of spent nuclear fuel.

Numerical kinetic modeling of spherically expanding plasma with parameters characteristic of the plasma of the cathode spot of a low-current vacuum arc is performed. It is shown that under such conditions a group of electrons arises in the plasma, the energy of which practically does not change during expansion. As a result, at some distance from the cathode, the electron velocity distribution function in the direction of plasma expansion differs significantly from the drifted Maxwell distribution by the presence of a group of suprathermal electrons. The distribution function for velocities perpendicular to the direction of expansion remains Maxwellian.

Accelerators that can be used for small Cubesat-class spacecraft are currently gaining popularity. One of these accelerator types is an ablative pulsed plasma accelerator, which allows spacecraft to rotate at small angles, adjust the orbit, etc. The paper presents spectral characteristics studies of plasma radiation produced by ablative pulsed plasma discharge in vacuum. A coaxial ablative pulsed plasma source is a system of coaxial electrodes with POM-C dielectric located between them. Operation is based on a high-current pulsed discharge, energy invested in the discharge (5.6 J) is spent on dielectric ablation, dielectric mass ionization, acceleration and radiation. Radiation was recorded in the frontal and lateral directions through a quartz window using Solar S100 (200–1100 nm, resolution δ ~ 1 nm) spectrometer. During spectral analysis, emitting substances were identified, including atoms, ions and molecules, evaporating from the dielectric surface (C, O, H), electrodes (Cu) or being in the residual atmosphere (N), discharge dynamics were studied using a G5-54 pulse generator.

The dependences of the breakdown voltage U_br on the parameter p × d—the product of the working gas pressure p and the interelectrode distance d (Paschen curve) in helium (p × d = 1.1–300 Torr cm), neon (p × d = 0.25–30 Torr cm) and argon (p × d = 0.09–570 Torr cm) were investigated. It is shown that the right branch of the Paschen curve (p × d) > (p × d)_min for all investigated gases demonstrates lower breakdown voltage values than the known results, which is associated with a change in the emission properties of cold cathodes operating in pure gases with the effect of impurities influence minimization on current generation processes.

The results of using two generators with an operating voltage of about 50 kV for high voltage electric pulse crushing of quartz raw are presented. A comparison with previously obtained results of quartz raw crushing on a generator with an operating voltage of 250 kV is performed. Reducing the operating voltage of the generator allows improving its operating qualities and simplifying the generator design due to the exclusion of a step-up pulse transformer from the circuit. It is shown that while maintaining the energy level in the high-voltage storage and reducing the operating voltage from 250 to 50 kV, the crushing efficiency remains at the same level. An increase in the stored energy while other parameters constant does not lead to an increase in the crushing efficiency, since energy consumption and the proportion of overgrinding fraction increase.

We examined the influence of pulsed ultraviolet (UV) laser radiation on the microstructure, phase composition, and functional characteristics of surface layers of Ti–6Al–4V alloy. Laser treatment leads to significant changes in the phase composition of the alloy surface, where newly formed oxide phases (TiO₂, TiO) are detected in addition to the main initial α- and β-phases of Ti. It is shown that UV laser treatment leads to an increase of nanohardness by 25–30%, an increase of roughness, and a significant increase in hydrophilicity. In the initial state, the average value of wetting contact angle is 80°, and it decreases to 9°–13° after UV laser treatment. It is shown that the observed changes are due to the saturation of surface layer with oxygen, the concentration of which increses to 12–20%. This saturation leads to the formation of oxide phases and with an increase in free surface energy. Futhermore, it alters the ratio of the surface energy components, resulting in a decrease in a dispersity and a significant increase in the polar component. Based on the research conducted, it is concluded that UV laser treatment is an effective method to change the surface structure of Ti–6Al–4V alloy, its morphology and properties.

We presented the results of numerical simulation of plasma formation in a traditional discharge scheme for vacuum arc plasma-assisted deposition and compares them with experimental measurements. It was found that the average electron temperature is 1.00 eV without arc-activated evaporators. In the case of all plasma sources operating, the average electron temperature can achieve 1.35 eV, while plasma concentration is at the level of 10¹⁶–10¹⁷ 1/m³. In addition, the coefficient of unevenness of plasma concentration is 82% when metal plasma evaporators are not activated, and it decreases to 125% when they are active. A decrease in this coefficient was also observed as the radius decreased due to a reduction in the influence of gas and metal plasma generators. The agreement between the results of computer simulation and experiment provides grounds for using the hydrodynamic representation when solving the problem of reducing the degree of plasma distribution unevenness in large-sized discharge systems in the hollow anode mode (more than 0.1 m³) and optimizing installations for generating gas-metal beam-plasma formations.

We present the results of probe measurements of plasma parameters and ion mass-to-charge state of a DC magnetron sputtering system (MSS) in gas and vacuum (gasless) operating modes at operating pressures from 2 down to 0.05 mTorr. It is shown that, in comparison with the gas operating mode, the discharge plasma of the vacuum magnetron is characterized by higher values of electron temperature and plasma potential, along with the generation of multiply charged metal ions of the target material. This mode is also characterized by a higher degree of ionization of sputtered material and enhanced ion current density onto the substrate. The revealed differences may be useful for improving quality and properties of formed coatings.

The work is devoted to the study of the structure of plasma jets formed by a high-current vacuum arc discharge. The experiments were conducted on the high-current generator IMRI-5 with a current amplitude of 250 KA. The plasma jet was formed while burning a vacuum arc discharge between coaxial aluminum electrodes with initiation on the surface of a capropolon dielectric. The following were registered in the experiments: a current flowing along the plasma jet at different distances from the plasma gun cathode; visible image of a plasma jet at different moments in time with an exposure of 10 ns; the propagation speed of the plasma jet. Numerical simulation of the evolution of the plasma jet was carried out. The results of the simulation are compared with the experiments.

High-entropy borides (HEB) are a new class of unique materials characterized by outstanding properties. In this paper, a comprehensive study of the oxidation properties of a high-entropy boride of the composition (Ti_0.2Zr_0.2Nb_0.2Hf_0.2Ta_0.2)B₂ is carried out for the first time. HEB was synthesized by a vacuum-free electric arc method by a direct current arc discharge of 100 A for 90 s. The oxidation stability of the obtained powder (Ti_0.2Zr_0.2Nb_0.2Hf_0.2Ta_0.2)B₂ was studied by two approaches. In the first case, it was found that the (Ti_0.2Zr_0.2Nb_0.2Hf_0.2Ta_0.2)B₂ sample is characterized by an oxidation temperature of ~800°С based on the assessment of phase transformations in situ during heating of the powder in the temperature range from 0 to 1200°С in air. In the second case, it was determined that the (Ti_0.2Zr_0.2Nb_0.2Hf_0.2Ta_0.2)B₂ powder can withstand temperatures up to ~700°С according to the results of the analysis of the composition of HEB during oxidation in an atmospheric furnace. The formation of oxides is confirmed by the results of scanning electron microscopy. The characteristics of HEB obtained in the work are consistent with the currently known literature data.