Code KARAT Research Articles

Выбор формы временной зависимости ускоряющего напряжения для моделирования спектральных характеристик гиротрона мегаваттного уровня мощности

The spectral characteristics of the gyrotron with a megawatt power level in the 170 GHz range were modeled numerically. The calculations were carried out using the large particle method implemented in the KARAT code. The problem of three-dimensional modeling of the excitation of an asymmetric type mode was reduced to a 2.5-dimensional one. For this model, the presence of high-frequency fluctuations at the output of high-voltage power supplies, which determine the initial energy of electrons, was taken into account. The relationship between the shape and width of the radiation spectrum of the simulated gyrotron and the time dependence of the accelerating voltage has been studied. It is shown that the best agreement with experimental data is obtained by combining a relatively slow sawtooth change in particle energy with fast random fluctuations.

Fully Electromagnetic Code KARAT Applied to the Problem of Aneutronic Proton–Boron Fusion

In this paper, the full electromagnetic code KARAT is presented in detail, the scope of which is a computational experiment in applied problems of engineering electrodynamics. The basis of the physical model used is Maxwell’s equations together with boundary conditions for fields, as well as material equations linking currents with field strengths. The Particle in Cell (PiC) method for the kinetic description of plasma is implemented in the code. A unique feature of the code KARAT is the possibility of the self-consistent modeling of inelastic processes, in particular, nuclear reactions, at each time step in the process of electrodynamic calculation. The aneutronic proton–boron nuclear reaction, accompanied by the release of almost only α-particles, is extremely in demand in medicine and, perhaps, in the future, will form the basis for obtaining “clean” nuclear energy. The results of a numerical simulation within the framework of the code KARAT of the key physical processes leading to the proton–boron fusion are presented and discussed both for laser-driven plasma and for a plasma oscillatory confinement scheme.

Observation of quasi-monochromatic resonant Cherenkov diffraction radiation

The first observation and investigation of a new mechanism of resonant Cherenkov diffraction radiation appearing when relativistic 6 MeV electrons move alongside a periodically shaped Teflon target have been presented and analysed. Numerical simulations performed using computer code KARAT are in good agreement with the experimental results. This new mechanism is a promising technique for generation and monochromatisation of THz and sub-THz radiation beams that could be integrated into any short bunch linear accelerator facility including 4th generation light sources providing new opportunities for user community.

Proton-boron fusion in a compact scheme of plasma oscillatory confinement.

We present the results of experiments on the aneutronic fusion of proton-boron (pB) in a single miniature device with electrodynamic (oscillatory) plasma confinement. The device is based on a low energy (∼1-2J) nanosecond vacuum discharge with a virtual cathode, the field of which accelerates protons and boron ions to the energies required for pB synthesis (∼100-300keV) under oscillating ions' head-on collisions. The yields of α particles registered for different conditions of the experiment are presented and discussed in detail. The experiment was preceded by particle-in-cell modeling of main processes accompanying pB reaction within the framework of the full electromagnetic code karat. The summary yield of α particles of about 5×10^{4}/4π was obtained during the pulse-periodic operation of the generator within total 4μs of the high voltage applied, which is ∼10α particles/ns.

Effect of the Erosion of Collector Surface on the Operation of a Pulse-Periodic Plasma Relativistic Microwave Generator

The pulse-periodic operation regime of a plasma relativistic microwave generator is considered. The reasons for the increase of average frequency of the output microwave radiation in a pulse train created in the same conditions of initial plasma generation are analyzed. The effect of solid body erosion that occurs due to the interaction of the relativistic electron beam with the collector surface on the output microwave radiation is studied. The analysis of experimental data is accompanied by numerical simulations by the electromagnetic PIC code KARAT. It is shown that increasing the amount of mater extracted from the surface of the collector during erosion leads not only to an increase of the plasma density, but it can also lead to qualitative changes of the parameters of the output radiation, which are caused by the change of microwave generation regime. In particular, the generation of an ion background that is inhomogeneous in the radial direction caused a change of conditions of plasma–beam interaction during the second half of the pulse and a change of the spectral density of radiation at the main frequency. Numerical simulations were used to demonstrate this for two different admixtures, iron and graphite.

Computer model of a plasma layer formed by an electron beam

The problem of this work is development of scientific foundations of technological plasma processes for defect-free synthesis and processing of nanoscale structures for use in nanoelectronics. The goal of this work is development of a method for numerical calculation of the parameters of the ion flow to the sample for the real geometry of the beam-plasma installation. Results. We have created a numerical model for the development of a beam-plasma discharge by an electron beam in the absence of a buffer plasma and a longitudinal magnetic field. It is shown that the KARAT code allows us to solve the problem of developing beam instability in the absence of a buffer plasma. It was also shown that beam instability develops without a longitudinal magnetic field. The electric field created by the instability does not affect the peripheral plasma. The experimental verification of the numerical modeling results is carried out. The plasma concentration and electron temperature distributions obtained in the model are in qualitative agreement with the experimental ones. Practical significance. The model allows us to select the optimal modes of a plasma-chemical reactor based on a beamplasma discharge for the implementation of processes of defect-free ion-plasma treatment and synthesis of nanoscale structures.

Model of a Plasma Layer Formed by an Electron Beam

Results are presented of the particle-in-cell numerical simulations by the KARAT code of the formation of a plasma–beam discharge in the absence of both the longitudinal magnetic field and the initial plasma. Oscillations of the electric field generated by the plasma–beam instability in the electron beam region almost do not affect the plasma at the periphery of the system. Simulation results are compared to the results obtained earlier by a simplified model and to the results of test experiments. The spatial distributions of the plasma density and electron temperature qualitatively agree with the experimental results.

PiC-code KARAT simulation of coherent Cherenkov radiation from a bunch passing through a hollow conical target

In the report we present simulation results for angular distributions of the radiation as well as spectral distributions obtained by the PiC code KARAT for the conditions of the experiment [1]. We show that for strict satisfaction of the condition θcone=θCR/2 (θcone is an opening cone angle, θCR=arcos 1/βn) is the Cherenkov angle, β =v/c, n is a refractive index) a propagating radiation beam is retained a hollow structure for distances much larger than a radiator length. We simulated “radii” of such a structure at different distances from an exit radiator surface and estimated a divergence of the Cherenkov radiation (CR) beam.

Analysis of the Ion-Optical Systems of Portable Neutron Generators

The results of numerical modeling of the ion-beam dynamics and the dynamics of secondary ions and electrons formed in the ion-optical system of a portable neutron generator are presented. The calculation was performed with the aid of the KARAT code in a two-dimensional flat geometry and axisymmetric geometry. To suppress back electrons, the magnetic field of permanent magnets was used in the flat geometry and a blocking electric field of an additional electrode was used in the axisymmetric geometry. The working configurations of both systems are shown.

Influence of backstreaming ions on spot size of 2 MeV electron beam

AbstractInfluence of backstreaming ions from the target on spot size of focused 2 MeV electron beam was considered. The 2D version of the particle-in-cell code KARAT was used to study the formation of the ions stream and dynamics of the electron beam. It was shown that light species emitted from the target can disrupt the beam. The emission of low-ionized states of tantalum cannot disrupt the beam during about 30 ns or longer. A few non-invasive techniques of mitigation of the beam disruption were considered. Final magnetic lens with fast variation of magnetic field of several hundred Ampere-turns per nanosecond is capable to stabilize initial spot size of the compressed beam at the target.

Optimization analysis of a plasma separator with a vacuum arc based plasma source configuration

In the work, the comprehensive analysis of a plasma separator with a vacuum arc based plasma source is performed. The main purpose is to identify the interdependencies of the initial parameters set and separation rate to be able to optimize the configuration with the possible experimental limitations. The X–Y–Z modeling is performed with KARAT code in a single-particle approximation. Injected particles are single-charged ions with energies from 0 to 20 eV with atomic masses A = 150 and 240 and spreading angle in the range of 0°–30°. Magnetic field is produced by 2 coils of wire, the characteristic field strength in an uniform area is 1–2 kG. Electric field is produced by several electrodes with electric potential up to 1000 V. As a result the correlations were identified, and experimental configuration guidance was developed.

Ion mass separation modeling inside a plasma separator

The results have been obtained in a continuation of the work for ion trajectories calculation in crossed electric and magnetic fields and also in a close alignment with the plasma separation study development. The main task was to calculate trajectories of ions of the substance imitating spent nuclear fuel in order to find a feasible plasma separator configuration. The three-dimensional modeling has been made with KARAT code in a single-particle approximation. The calculations have been performed under the following conditions. Magnetic field is produced by 2 coils of wire, the characteristic field strength in a uniform area is 1.4 kG. Electric field is produced by several electrodes (axial ones, anode shell and capacitor sheets) with electric potential up to 500 V. The characteristic linear size of the cylindrical separator area is ∼ 100 cm. The characteristic size of injection region is ∼ 1 cm. Spatial position of the injection region is inside the separator. The injection direction is along magnetic lines. Injected particles are single-charged ions with energies from 0 to 20 eV with atomic masses A = 150 and 240. Wide spreading angle range was investigated. As a result of simulation a feasible separator configuration was found. This configuration allows to achieve more than 10 cm spatial division distance for the separated ions and is fully compliant with and supplementary to the vacuum arc-based ion source research.

Diffraction Effects in Measurements of Characteristics of High-Power Microwave Pulses with Wide-Aperture Liquid Calorimeters

Numerical nonstationary simulation of diffraction by a quasi-plane three-layer wide-aperture absorbing loads of liquid calorimeters intended for measuring the energy of high-power microwave radiation pulses is performed using the electromagnetic code KARAT with application to the S-band. It is shown that these effects can cause significant distortion of the microwave pulse shape behind the load as well as underestimation of the measured energy by 10–20%.

Dynamics of plasma and ion flux in a vacuum neutron tube

Results of the numerical simulation of the formation of the ion beam in the accelerating gap of a vacuum neutron tube are presented. Calculations are performed with the KARAT code in a two-dimensional nonstationary formulation for plasma formed in arc discharge and inflowing into an accelerating gap with the given time dependences of parameters (density, expansion velocity). The small duration of the vacuum arc leads to a considerable change of parameters of inflowing plasma during the accelerating pulse. Two geometries are considered: the conventional and sectioned diode, in which the total voltage is divided between the anode, intermediate electrode, and cathode.

Efficient 58-GHz relativistic orotron with combined mode selection

This article presents time-dependent numerical modeling of V-band relativistic orotron: a high-power microwave generator with a TM03 oscillation mode in an oversized (D/λ ∼ 2.7) electrodynamic structure and diffractive output in the TM02 mode. Single-mode operation of the oscillator is ensured by the cyclotron selection of the working axisymmetric mode, with simultaneous depression of competing non-axisymmetric modes by means of cutting longitudinal slits in the wall of the slow-wave structure. The simulated output microwave power is 350 MW with a power conversion efficiency of 31% when using a 3.6 kA, 310 keV electron beam transported in the 3.9 T magnetic field. The simulation employed 2.5D axisymmetric and 3D Cartesian versions of the KARAT code.

PiC code KARAT simulations of Coherent THz Smith-Purcell Radiation from diffraction gratings of various profiles

Generation of coherent THz Smith-Purcell radiation by single electron bunch or multi-bunched electron beam was simulated for lamellar, sinusoidal and echelette gratings. The dependences of the CSPR intensity of the corrugation gratings depth were investigated. The angular and spectral characteristics of the CSPR for different profiles of diffraction gratings were obtained. It is shown that in the case of femtosecond multi-bunched electron beam with 10 MeV energy sinusoidal grating with period 292 μm and groove depth 60 μm has the uniform angular distribution with high radiation intensity.

Code KARAT in simulations of power microwave sources including Cherenkov plasma devices, vircators, orotron, E-field sensor, calorimeter etc.

Code KARAT in simulations of power microwave sources including Cherenkov plasma devices, vircators, orotron, E-field sensor, calorimeter etc.

Recent experimental study of DD fusion in the potential well of a virtual cathode at nanosecond vacuum discharge

Processes of nuclear burning of various elements in the scheme of a compact inertial electrostatic confinement implemented on the basis of a nanosecond vacuum discharge (NVD) with low-energy hollow cathode were investigated experimentally earlier. This paper presents the results of a recent series of DD fusion experiments on the newly created experimental set-up NVD-2 combined with x-ray and neutron yield diagnostics. The voltage-current (VA) characteristics of the discharge, and the regimes of generation of x-ray and DD neutrons realized experimentally are presented and discussed. The experimental results are compared with the results of particle-in-cell simulation of the nuclear DD fusion processes in NVD using electrodynamic code KARAT. Recent series of DD fusion experiments have reproducing in TOF scheme some basic features of DD neutrons yield observed earlier. Meanwhile, the analysis of V-A characteristics and anode erosion shows that efficiency of energy deposition at initial stage of discharge is still insufficient, and the ways to optimize the electrophysical processes at NVD-2 are clarified.

Ion mass separation in crossed electric and magnetic fields as a part of the concept of spent nuclear fuel plasma separation

Presented results have been obtained to develop the concept of spent nuclear fuel plasma separation. The main task is to calculate trajectories of ions of the substance imitating spent nuclear fuel in crossed electric and magnetic fields. The 3D calculations have been made by the KARAT code in a single-particle approximation. The calculations have been performed for a number of combinations of azimuthal and axial magnetic fields and different electric fields configurations. Magnetic field is produced by 2 main coils with the characteristic field strength up to 1.4 kG and in several series with additional coil with the characteristic field strength up to 1.6 kG. Electric field is produced by 2 electrodes with electric potential up to 1 kV. The characteristic linear size of the calculation area is 100 cm. The characteristic size of injection region is 1 cm (up to 10 cm along main axis). Spatial position of the injection region and axis of the injection direction are varied. Injected particles are single-charged ions with energies from 0.2 to 3 eV with atomic masses A = 150 and 240, spreading angle is 60°. Calculations have revealed several options to realize a spatial separation of spent nuclear fuel ions.

Mathematical simulation of a low-pressure capacitive RF discharge in an external radial magnetic field using the KARAT code

A high-frequency capacitive discharge is simulated in the geometry of a plasma accelerator with closed electron drift. It has been shown that, in such a discharge, as in a dc discharge, an azimuthal electron drift takes place and a potential drop is formed at the discharge channel exit, which leads to the emergence of an accelerated ion beam from the channel.