Hybrid MHD/PIC simulation of a metallic gas-puff z pinch implosion

We present the hybrid MHD/PIC simulations of the time evolution of a bismuth metallic gas-puff z pinch in external axial magnetic field (AMF). Recent experiments with IMRI-5 generator (450 kA, 450 ns) [1] show the certain effect of an axial magnetic field on radiation energy produced during z pinch implosion. In order to perform the numerical simulation of gas puff z pinch a hybrid model was developed. The hybrid model treats the electrons as a massless fluid and ions as macroparticles. The macroparticle dynamic is calculated with the use of PIC method. Ion-ion Coulomb collision is considered with the use of MC method. The radiation transfer of bismuth plasma was accounted in the framework of P1 method. The interelectrode gap pumping by plasma of 8 μs 80 kA bismuth arc with the following plasma implosion by 450 kA / 450 ns current pulse in different external AMF was modelled. The obtained results are in a reasonable agreement with the experimental results. © Published under licence by IOP Publishing Ltd.The work was supported by Russian Science Foundation (project No. 16-19-10142)

Computer simulation of high current vacuum arc with developed anode spot

A self-consistent simulation of anode heating by 4.5 kA vacuum arc during the half-wave of 50 Hz current with 1 m/s contact opening was carried out. The calculations were done in the framework of hybrid high current vacuum arc model, which treats ions and atoms as macroparticles with the help of particle-in-cell methods, but electron subsystem is treated as massless fluid with quasineutrality assumed. The occurrence of an anode plasma plume (similar to that found in experiments) was obtained as a result of modeling. It is shown that the energy flux of line radiation from the interelectrode plasma to the anode is a critical reason of the appearance and maintenance of the anode plume. © 2019 IOP Publishing Ltd. All rights reserved.Russian Foundation for Basic Research, RFBRRussian Academy of Sciences, RAS: 1118-2-2-16This work was supported in part by RFBR (grants nos: 17-02-00346, 18-08-00547, 19-08-00783, 19-58-53006), by RAS Program (project no. 11) and UB RAS Program (project no. 18-2-2-16)

Modeling of plasma jet of vacuum arc with copper-chromium cathode under action of strong axial magnetic field

This paper deals with the computer simulation of a vacuum arc with composite cathode under action of external axial magnetic using hybrid model. The described hybrid model treats the electrons as a massless fluid and ions as macroparticles. It is shown that the average charge state of ions in a jet of a vacuum arc increases with increasing magnetic field. The calculation results are consistent with experimental data. © 2019 IOP Publishing Ltd. All rights reserved.Russian Science Foundation, RSF: 18-19-00069This work was supported by Russian Science Foundation (project No. 18-19-00069)

On Hybrid Type of Cathode Attachment in High Current Vacuum Arcs

This paper discusses the issues of a possible change of the type of cathode attachment of high-current vacuum arcs (HCVA) with an average cathode current density of more than 105 A/cm2. This type of HCVA is used as pumping plasma gun in experiments with plasma puff z-pinches. These experiments showed that the measured linear mass of the HCVA plasma jet is much higher (by a factor of 10 or more) than the expected mass, which can be obtained from the assumption that cathode attachment occurs only through a multitude of cathode spots emitting supersonic plasma jets. It is shown that in HCVA of the type under consideration, at some time instant there are two types of cathode attachments - cathode spots and thermionic erosion attachment (TEA). It can be said that HCVA of this type have a hybrid cathodic attachment. Unlike cathode spots, TEA produces a subsonic plasma flow, which contributes to an increase in the linear mass of the HCVA plasma jet. © 2021 Institute of Physics Publishing. All rights reserved

Ultrafast spectroscopy of CdS/CdSe quantum dots

© 2017, Allerton Press, Inc. Results from the nonresonance spectroscopy of CdS/CdSe quantum dots (composites of CdSe–CdS nanoparticles (core–shell)) are presented. The nonlinear optical properties of CdS/CdSe QDs in PMMA are studied with fs pulses at 1053 nm using the transient lens technique. QDs generate rapidly oscillating signals with amplitude rise times of around 200 fs and decay times of around 500 fs, while pure PMMA polymer only generates an oscillating signal whose envelope coincides with its autocorrelation function

A transverse current rectification in graphene superlattice

A model for energy spectrum of superlattice on the base of graphene placed on the striped dielectric substrate is proposed. A direct current component which appears in that structure perpendicularly to pulling electric field under the influence of elliptically polarized electromagnetic wave was derived. A transverse current density dependence on pulling field magnitude and on magnitude of component of elliptically polarized wave directed along the axis of a superlattice is analyzed.Comment: 12 pages, 6 figure

Сравнительный анализ характеристик бандажей колёсных пар, закалённых с использованием лазерного и плазменного источника тепла

Lateral wear of the locomotive wheel flange is one of the main types of wear that occurs during rolling stock operation. An important characteristic of a rail wheel rim is its wear resistance, which directly depends on the carbon content in steel. The carbon content in wheel steel of group 2 in the amount of 0,55–0,65 is due to the fact that at its lower concentration, the proportion of grain boundary ferrite increases, which leads to a decrease in the contact strength of wheels, and a higher one leads to a tendency to brittle fracture.The increased carbon content makes it possible to harden the surface of steel. Plasma and laser hardening technologies can be used to reduce tyre flange wear and increase the service life of locomotive wheelsets.The objective of this work is to determine advantages and disadvantages of technologies for laser and plasma hardening of working surfaces of tyres of wheelsets of railway rolling stock.The comparative analysis related to microstructure and microhardness of wheelsets tyres of the 2TE25KM diesel locomotive which are made of wheel steel of group 2 according to GOST 398-2010 state standard and hardened respectively by plasma and laser hardening.The tasks set were solved using theoretical and experimental research methods. The preparation and study of hardened samples was carried out using the equipment of the testing laboratory of LLC Scientific and Technical Association «IRE-Polus».A study referred to hardened zones in various areas and sections of the tyre. Tribological tests concerned wear resistance of specimens hardened with a high-power fiber laser. The main advantages and disadvantages of laser and plasma hardening processes are revealed.The conducted studies allow making a conclusion that the use of laser hardening technology for hardening wheelsets tyres, as an alternative to the plasma hardening process, is highly promising.Боковой износ гребня колёсной пары локомотива является одним из основных видов износа, возникающего при эксплуатации подвижного состава. Важной характеристикой обода железнодорожного колеса является его износостойкость, которая напрямую зависит от содержания углерода в стали. Содержание углерода в колёсной стали 2 в количестве 0,55–0,65 связано с тем, что при его меньшей концентрации увеличивается доля зернограничного феррита, что приводит к снижению контактной прочности колёс, а при большей – приводит к склонности к хрупкому разрушению.Повышенное содержание углерода позволяет осуществить закалку поверхности стали. Для уменьшения износа гребня бандажа и увеличения ресурса колёсных пар локомотивов возможно применение технологий плазменного и лазерного упрочнения.Целью данной работы является определение преимуществ и недостатков технологий лазерного и плазменного упрочнения рабочих поверхностей бандажей колёсных пар железнодорожного подвижного состава.В работе проводится сравнительный анализ микроструктуры и микротвёрдости упрочнённых методом плазменной и лазерной закалки бандажей колёсных пар тепловоза 2ТЭ25КМ, выполненных из колёсной стали 2 ГОСТ 398-2010.Поставленные задачи решались с использованием теоретических и экспериментальных методов исследования.Подготовка и исследование закалённых образцов выполнено на оборудовании испытательной лаборатории ООО НТО «ИРЭ-Полюс».Проведено исследование зон упрочнения в различных областях и участках бандажа. Проведены трибологические испытания на износостойкость образцов, закалённых с использованием высокомощного волоконного лазера. Выявлены основные преимущества и недостатки процессов лазерной и плазменной закалки.Проведённые исследования позволяют сделать вывод о высокой перспективности применения технологии лазерной закалки для упрочнения бандажей колёсных пар как альтернативы процессу плазменного упрочнения

On possible reasons of positive near-anode voltage drop in high-current vacuum arc

In this paper we consider some reasons for the occurrence of a positive near-anode potential drop during current constriction in high-current vacuum arc. It is shown that if the radius of constriction is much larger than the electron mean free path, then a positive voltage drop in the near-anode plasma sheath does not occur, but an additional drop may occur in the near-anode plasma layer due to the anomalous resistance. If the current constriction radius is much less than the electron mean free path, then a positive voltage drop in the sheath arises under the usual condition, and the kinetic instabilities in the near-anode plasma do not develop. © 2019 IOP Publishing Ltd. All rights reserved.Russian Foundation for Basic Research, RFBR: 17-02-00346, 18-08-00547Russian Academy of Sciences, RASNational Natural Science Foundation of China, NSFC: 5191153011318-2-2-16This work was supported in part by RFBR(grant Nos. 17-02-00346, 18-08-00547, 19-08-00783, 19-58-53006), by RAS Program (project No. 11) and UB RAS Program (project No. 18-2-2-16), as well as by National Natural Science Foundation of China (project No. 51911530113)

Formation of double shell during implosion of plasma metal puff Z-pinches

This work presents the results of experimental and theoretical research of impact of tailored density profile and application of external axial magnetic field on initial spatial distribution of the plasma density in the plasma metal puff Z-pinch and on its implosion dynamics. It has been discovered that upon implosion of the plasma metal puff Z-pinch some stripes interpreted as the system of two coaxial shells appear on the optical images. With the help of numerical simulation, the formation of the plasma liner consisting of a mixture of carbon and bismuth ions and formed by the expansion of the plasma jet of the arc burning on the bismuth electrode has been considered in this work. It has been shown that the lightweight carbon ions facilitate formation of the density distribution smoothly decreasing with the increase in radius, that, in turn, leads to suppression of the Rayleigh-Taylor instability in the current sheath upon further implosion. It has also been demonstrated that availability of the two types of ions in plasma considerably different in mass leads to formation (in the compression phase) of a double shell with externally located heavy ions. It has also been revealed that the application of the external axial magnetic field leads to reduction in the plasma metal puff Z-pinch initial diameter. © 2020 Author(s).This work was funded by Russian Science Foundation. Project No. 19–19-00127