Ion-Focused Propagation of Relativistic Electron Beam in the Self-Generated Plasma in the Atmosphere

It is known that ion-focused regime can effectively suppress the expansion of relativistic electron beam (REB). By using particle in cell-Monte Carlo collision (PIC-MCC) method, the propagation of REBs in neutral gas is numerically investigated. The numerical results demonstrate that the beam body is charge neutralization and a stable IFR can be established. As a result, the beam transverse dimensions and longitudinal velocities keep close to the initial parameters. We also calculated the charge and current neutralization factors of REBs. Combined with envelope equations, we obtained the variations of beam envelopes, which agree well with the PIC simulations. However, both the energy loss and instabilities of REBs may lead to a low transport efficiency during long-range propagation. It has been proved that decreasing the initial pulse length of REBs can avoid the influence of electron avalanche. Using parts of REB pulses to build a long-distance IFR in advance can improve the beam quality of subsequent pulses. Further, a long-distance IFR may contribute to the implementation of long-range propagation of REBs in the space environment.

MATHEMATICAL MODELLING OF RF PLASMA AT LOW PRESSURES FOR CYLINDRIC VACUUM CHAMBER WITH CHARGED SPECIMAN

Для моделирования процессов в ВЧ-плазме пониженного давления с продувом газа разработана гибридная математическая модель при числах Кнудсена - для несущего газа. Модель включает начально-краевую задачу для кинетического уравнения Больцмана, описывающего функцию распределения несущего нейтрального газа, краевые задачи для уравнения неразрывности электронной, ионной и метастабильной компонент, уравнения сохранения энергии электронов, для ВЧ-уравнений Максвелла в форме телеграфных уравнений и уравнения Пуассона для потенциальной составляющей поля. Приводятся результаты расчета электрической напряженности, концентрации электронов, ионов и метастабилей, потенциальной составляющей электромагнитного поля в цилиндрической вакуумной камере. A hybrid mathematical model for the Knudsen numbers - for the carrier gas has been developed to simulate processes in a low pressure RF plasma with gas flow. The model includes an initial boundary value problem for the kinetic Boltzmann equation describing the distribution function of the carrier neutral gas, boundary value problems for the continuity equation of the electronic, ionic and metastable components, the electron energy conservation equations, for Maxwell’s RF equations in the form of telegraphic equations and the Poisson equation for the potential part of field. The results of the calculation of the electric intensity, the concentration of electrons, iones and metastables, the potential component of the electromagnetic field in a cylindrical vacuum chamber are presented.

DSMC simulations of neutral gas flow in the DTT particle exhaust system

Divertor Tokamak Test Facility (DTT) is a new European superconducting tokamak, currently under final design, addressed to investigate alternative power exhaust solutions for DEMO. Although the divertor system is not finalized yet, the machine and port geometry set limitations on the divertor pumping system operational space. A numerical study of neutral gas dynamics in the divertor region is performed based on the DSMC method by applying the DIVGAS code. The study includes both single-null (SN) and double-null (DN) divertor configurations. For both configurations, the SolEdge2D–EIRENE plasma simulations have been performed for a deuterium plasma with neon seeding and the extracted information about the neutral particles on the predefined interfaces is imposed as incoming boundary conditions for DIVGAS simulations. In the SN case, two plasma puffing scenarios and three candidate pumping port arrangements have been considered. The divertor dome influence on the pumped fluxes can reach 50%. An increase of the capture coefficient six times leads to a decrease in the pressure at the pumping openings by a factor of about 4.5–7. The influence of the size of the lower vertical opening has been studied showing that the enlarged vertical port may establish as the main pumping opening. In the DN case, when the pumping is performed from both lower and upper divertor the overall pumped fluxes at the upper divertor are always higher than the corresponding ones for the lower divertor by a factor of 2–2.5, mainly due to the difference in the pumping areas. In both SN and DN cases, the neutrals outflux toward the X-point dominates the particle transport in the private flux region. The operational space provided by this first assessment is relatively stable against modified classical divertor geometries and allows a more thorough assessment of the pumping technology of the DTT fusion device in the future.

Characterisation of supersonic gas jets for different nozzle geometries for laser-plasma acceleration experiments at SPARC_LAB

Plasma-based technology promises a tremendous reduction in size of accelerators used for research, medical, and industrial applications, making it possible to develop tabletop machines accessible for a broader scientific community. The use of high-power laser pulses on gaseous targets is a promising method for the generation of accelerated electron beams at energies on the GeV scale, in extremely small sizes, typically millimetres. The gaseous target in question can be a collimated supersonic gasjet from a nozzle. In this work, a technique for optimising the so generated plasma channel is presented. In detail, a study on the influence of the nozzle throat shape in relation to the uniformity and density of the generated plasma profile is reported. These considerations are discussed first of all from a theoretical point of view, by means of a stationary one-dimensional mathematical model of the neutral gas, thus exploiting the possibility of comparing the properties of the output flow for different nozzle geometries. This is combined with an experimental approach using interferometric longitudinal density measurements of the plasma channel. The latter is generated by a high-power laser pulse focused on a helium gasjet, in the SPARC_LAB laboratories.

Dust-Acoustic Nonlinear Waves in a Nanoparticle Fraction of Ultracold (2K) Multicomponent Dusty Plasma

The nonlinear dust-acoustic instability in the condensed submicron fraction of dust particles in the low-pressure glow discharge at ultra-low temperatures is experimentally and theoretically investigated. The main discharge parameters are estimated on the basisof the dust-acoustic wave analysis. In particular, the temperature and density of ions, as well as the Debye radius, are determined. It is shown that the ion temperature exceeds the temperature of the neutral gas. The drift characteristics of all plasma fractions are estimated. The reasons for the instability excitation are considered.

FEATURES FORMATION ZONE OF WAVES AND BURSTS ON THE SURFACE LADLE BATH

The results studies influence physicochemical properties and thickness cover slag, formed during ladle desulfurization pig iron by blowing a mixture of lime and magnesium, features formation a breaker on the surface bath and the level of metal losses with emissions outside ladle from this zone are presented. The necessity creating conditions for ensuring height breaker, which would not exceed thickness slag layer on the surface bath, has been substantiated. Using results of the high-temperature simulation blowing the cast iron melt with a neutral gas supplied through the nozzles tips stationary and rotating submersible lances, features development of counter waves and metal splashes in the absence and during formation slag cover with thickness of 30—80 mm on the surface bath are determined. The features change in the height and area breakers are determined depending on the gas flow rate for blowing bath and thickness slag. Based on the analysis results low-temperature modeling bath blowing, scientific ideas about the combined effect of the bath blowing intensity, speed of rotation submerged lance and thickness slag layer on the diameter bubbling zone, gas saturation of the bath and features wave formation on its surface in the slag-free zone were further developed (so-called «eye»). The nature relationship between length of the gas jet from lance nozzle, diameter «eye», and geometric parameters breaker has been established. It is shown that dependence profile breaker on speed of rotation lance and thickness slag layer is nonlinear. So, blowing bath through tip of a rotating lance with one nozzle at 240 rpm. with a gas flow rate of 2.2 l/min. creates conditions for raising top breaker to a height that is 33 % higher than the current thickness slag layer and contributes to the intensification formation of waves and bursts on the surface bath. With a decrease in the gas flow rate to 1.0 l/min., Under other unchanged conditions, height breaker is already 2/3 of the height slag layer, and as thickness slag decreases proportionally decreases. The smallest, recorded in the experiments, relative height breaker was 33.3% of the slag thickness at a lance rotation speed in the range of 90—120 rpm. Mathematical models are proposed that are suitable for calculating height breakers depending on the thickness slag layer, speed of rotation lance and intensity of gas injection into the bath. Taking into account established mutually opposite effect thickness of the cover slag layer and speed of rotation submerged lance on the «eye» area and height of the breaker, it is advisable to continue search for ways to improve design tip submerged lance and slag mode of ladle desulfurization.

EUROfusion-theory and advanced simulation coordination (E-TASC): programme and the role of high performance computing

The paper is a written summary of an overview oral presentation given at the 1st Spanish Fusion HPC Workshop that took place on the 27th November 2020 as an online event. Given that over the next few years ITER will move to its operation phase and the European-DEMO design will be significantly advanced, the EUROfusion consortium has initiated a coordination effort in theory and advanced simulation to address some of the challenges of the fusion research in Horizon EUROPE (2021-2027), i.e. the next EU Framework Programme for Research and Technological Development. This initiative has been called E-TASC that stands for EUROfusion-Theory and Advanced Simulation Coordination. The general and guiding principles of E-TASC are summarized in the paper. In addition, an overview of the scientific results obtained in a pilot phase (2019-2020) of E-TASC are provided while highlighting the importance of the required progress in computational methods and HPC techniques. In the initial phase, five pilot theory and simulation tasks were initiated: 1. Towards a validated predictive capability of the L-H transition and pedestal physics; 2. Electron runaway in tokamak disruptions in the presence of massive material injection; 3. Fast code for the calculation of neoclassical toroidal viscosity in stellarators and tokamaks; 4. Development of a neutral gas kinetics modular code; 5. European edge and boundary code for reactor-relevant devices. In this paper we report on recent progress made by each of these projects.

Characterization of Graphenic Carbon Produced by Pulsed Laser Ablation of Sacrificial Carbon Tapes

This paper reports on the pulsed laser deposition of nanocarbon films on metal and dielectric substrates, using high-purity sacrificial carbon tape as a carbon source on a neutral gas background. The films were characterized by X-ray diffraction (XRD), photoelectron (XPS) and Raman spectroscopy. The XRD and Raman structural analyses revealed that the synthesized films have a graphenic nanocrystalline turbostratic structure, with sp2 clusters about 15–18 nm in size, depending on the laser fluence. A significant decrease in the oxygen and hydrogen contents in the films, in comparison with the target material, was established using XPS, as well as a significant decrease in the sp3 carbon content. The deposited films were found to be similar to reduced graphene oxide (rGO) in composition, with a surprisingly low number of defects in the sp2-matrix. The method proposed in the work may have good prospects of application in the production of energy storage and nonvolatile memory devices.