Vol 49, No 11 (2023)

The results on applying the technique of vortex plasma confinement in the GOL-NB facility are presented. The first experiments on optimizing the biasing the in-chamber electrodes demonstrated an improvement in the dynamics of trapping the injected fast hydrogen atoms, as well as a decrease in the fluctuations of local plasma parameters in the central trap and an increase in the plasma decay time. The geometry of in-chamber electrodes arrangement, as well as the polarity and magnitude of the supplied potentials, correspond to the theory of vortex confinement and to those in similar studies at other open traps.

Выполнено 2D- и 3D-моделирование проникновения нейтральных частиц в плазму с параметрами, соответствующими режиму омического нагрева в стеллараторе Л-2М, и проведено сравнение результатов моделирования. Получены радиальные распределения плотности нейтралов в плазменном шнуре и модельные энергетические спектры потока атомов перезарядки, вылетающих из плазмы. Для условий омического нагрева в стеллараторе Л-2М определены значения плотностей плазмы, выше которых необходимо учитывать при моделировании процессы рекомбинации. Сравнение модельных энергетических спектров потока атомов перезарядки, вылетающих из плазмы, с экспериментальными данными позволило построить радиальные распределения плотности нейтралов в абсолютных единицах. При этом расчетное значение плотности нейтралов в центре плазменного шнура составило около 10¹⁵ м^–3, что на четыре порядка меньше плотности заряженных частиц 10¹⁹ м^–3.

In plane current sheets (foils, or liners), which are thin in comparison with their skin depth, an instability may grow because parallel currents in different areas of these sheets are attracted that leads to compression of these areas and partition of the current sheets into threads. This work describes the study of the nonlinear tearing instability evolution for a 1D case, when all the principal quantities (surface current and mass densities, velocity, and the magnetic field component normal to the sheet surface) depend on the coordinate perpendicular to the current density vector only. A 1D system of equations has been obtained, which describes the magnetic field dynamics and the substance motion in current sheets, and a numerical technique based on the Lagrangian discretization of mass has been developed to solve 1D magnetic hydrodynamics problems. It is shown that, if the tearing instability is considered, small perturbations of currents, velocities and mass densities grow in accordance with the earlier found growth rates of small perturbations and later, when the perturbations are no longer small, a nonlinear growth stage comes. In this stage, the perturbations of the surface current density j and the surface mass density μ increase unlimitedly for a finite time period (apparently, according to power laws j t -t -a,~ ( ) sμ t -t -g s, ts is the time, when these quantities become infinite); however the width of the current and mass density peaks tends to zero, so that the total current and total mass in the peaks of the current density and surface mass density decrease and tend to zero.

Spectroscopy methods were used for studying the dynamics of neutral plasma component in current sheets formed in two-dimensional (2D) and three-dimensional (3D) magnetic configurations during discharge in helium. It has been ascertained that when the current sheet is formed in the 2D magnetic field, flows of fast suprathermal helium atoms appear in it, which are directed along the current sheet width (the largest of the sheet transverse dimensions). It is shown that helium atoms can acquire the directed energy Wx due to the resonance charge exchange of accelerated ions in the current sheet plasma. The energy of directed motion of helium atoms can reach Wx ≈ (480 ± 120) eV, which is ~20 times higher than the temperature of helium atoms Ta ≈ (20 ± 2) eV at the same times. During the current sheet formation in the 3D magnetic configuration, fast helium atoms were not observed.

Results of experiments on injection of dense plasma clouds created by a small-scale coaxial generator into vacuum and large-volume background plasma in an ambient magnetic field are presented. The regime of an “infinite” background medium that allows studying the plasma-cloud dynamics on the scale of about one meter in the directions perpendicular and parallel to a quasi-uniform magnetic field is realized on “Krot” plasma device. The dynamics of the diamagnetic cavity appearing upon magnetic-field expulsion by a plasma blob, the electromagnetic noise appearing in the cavity, along with the evolution of plasma-cloud structure during injection and at the stage of its decay, were studied. It is demonstrated that the key properties of the cloud dynamics that are typical of the active space and high-energy laboratory experiments, including complete expulsion of the magnetic field from the cloud and development of the flute instability at its boundary, are reproduced at low injection speed (below 30 km/s) and low plasma energy (on the order of 0.1 J).

The article studies waves in a collisionless magnetoactive plasma with frequencies much lower than the electron cyclotron frequency at an arbitrary ratio between the electron plasma and cyclotron frequencies. A general dispersion relation is obtained for the wave frequency as a function of the wave vector and the ratio of the plasma frequency to the cyclotron frequency. This relation is a bicubic equation with respect to frequency. Based on this equation, the existence domains of various wave modes depending on the specified parameter, their group velocity, polarization, and energy density are studied. Special attention is paid to low-frequency whistler waves in a low-density plasma, since, among the propagating wave modes existing in the specified range of parameters, whistler waves have the highest refractive index and play an important role in theresonant interaction of waves and particles.

Представлены результаты экспериментальных исследований влияния шероховатости поверхности образцов железа на скорость их коррозии в низкотемпературной плазме влажного воздуха атмосферного давления, генерируемой пучком быстрых электронов. Установлено, что процесс плазмохимической коррозии железа, инициированный воздействием плазмы влажного воздуха, имеет две характерные временные стадии: плазменную и постплазменную, которые существенно различаются по длительности, скорости коррозии и конечным продуктам. Показано, что величина шероховатости поверхности не оказывает заметного влияния на скорость коррозии на плазменной стадии, в то время как на постплазменной стадии скорость коррозии заметно увеличивается с ростом величины шероховатости.

The hybrid fluid model for calculating electron transport in air is presented. Within the model, the transport of electrons with energies less than 300 eV is calculated based on the drift-diffusion equation, and in the high-energy range, it is calculated using a set of multigroup equations for the first two moments of the electron distribution function: the balance equations for concentrations and electron flux densities. The hybrid model presented is intended to be used for numerical simulations of electric discharge processes in air proceeding with the participation of runaway electrons. The results of numerical simulations of electron transport in air in homogeneous and inhomogeneous electric fields, which were performed using the hybrid model, were compared with the corresponding results of simulations obtained using the Monte Carlo method.

Представлены результаты исследований образования активных форм кислорода и азота в деионизированной воде Milli-Q® (электрическая проводимость ≤0.1 мкСм/см) под действием многоискрового импульсного разряда с инжекцией газа в межэлектродное пространство. Разряд представляет собой совокупность микроплазменных образований в многофазной среде, для которых был оценен удельный энерговклад. Проанализировано влияние инжектируемых газов (аргон, воздух) на образование плазменного разряда в межэлектродном пространстве и долгоживущих химических соединений: пероксида водорода, нитрит-ионов и нитрат-ионов. Изменение длительности воздействия на воду от 2 до 10 минут приводит к изменению ее химического состава и электропроводящих свойств, но практически не сказывается на характеристиках и длительности пробойной стадии разряда. При этом регистрировалось изменение концентраций пероксида водорода, нитрит-ионов и нитрат-ионов в течение одного часа после завершения плазменного воздействия. Обнаружено распыление электродов из нержавеющей стали, которое составило около 1 мг/минуту и приводило в ряде случаев к выпадению нерастворимого осадка. Полученные данные позволят провести оптимизацию воздействия активированной плазмой жидкости на растения и посадочный материал.

В эксперименте и численном моделировании проводится сравнение интенсивности взаимодействия гелиевой холодной плазменной струи (ХПС) с диэлектрической поверхностью и с кожей животных. ХПС при атмосферном давлении генерируется синусоидальным или положительным импульсным напряжением с различной длительностью импульса в оптимальных режимах. Эффект воздействия оценивается на основе измеренных и рассчитанных токов, интенсивностей линий в спектре ХПС и температурных полей. Измеренные характеристики ХПС показывают, что импульсный характер возбуждения ХПС является предпочтительным по сравнению с синусоидальным режимом. Варьирование длительности импульсов периодического импульсного напряжения позволяет получить максимальные ток и напряженность электрического поля у поверхности в рамках допустимой температуры в зоне контакта ХПС с кожей мышей (<42°C). Показано, что результаты исследования ХПС полученные в физических экспериментах с использованием диэлектрической пластины применимы для воздействия на мышей-опухоленосителей.

The bipolar lightning development model was used to study the dependence of the potential that is transported to the earth by the downward leader channel. It was shown that this parameter strongly depends on the starting position of the lightning and on the trajectories of formation of its bipolar leaders. It was shown that the main reason for the change in potential is not the loss of voltage in the lightning channel with a finite conductivity but its polarization in the electric field of the storm cloud. An estimate was made of the range of potential variation in the channel with ideal conductivity depending on the starting position and trajectory of the lightning at a constant charge in the thunderstorm cell. It was shown that, for the variation of the lighting current within two orders of magnitude, a mere twofold change in the charge of the thunderstorm cell is sufficient. The preferable starting position is found for the lightning whose upward leader can penetrate into the upper layers of the troposphere, turning into a blue jet.

The results of numerical 3D modeling of the development of an electron avalanche initiated by a field emission electron in a small-sized region of an amplified electric field near the microinhomogeneities at the cathode have been presented. The simulation has been carried out for the discharge gaps with an initially homogeneous distribution of the electric field with a reduced intensity significantly lower than that required by the electron runaway criterion. The possibility of the transition of the field emission electrons initiating avalanches and the electrons in these avalanches into runaway regime has been investigated. The microinhomogeneities in the form of a cone, metal droplets, and boundaries between pores or microcraters have been considered. The calculations were carried out for nitrogen in the pressure range from atmospheric to 40 atm. It has been shown that the initial energy obtained near the microinhomogeneity can significantly facilitate the transition of the electron into the runaway mode. And the electron will continue to run away in a discharge gap electric field weak according to the runaway criterion. It has been shown that this effect is especially noticeable at gas pressures above 10 atm. A comparative analysis of the simulation results with the experimental data obtained by us on the switching characteristics of a discharge gap filled with nitrogen when exposed to voltage pulses with subnanosecond fronts of different steepness has been carried out. This made it possible to divide the ranges of experimental conditions into those when only the amplification of the electric field near the microinhomogeneities is sufficient for the runaway of electrons and when the electric field of an avalanche of critical or close to critical size is additionally necessary for the runaway.