Plasma Physics Reports最新文献

The distributions of the current and energy densities of low-energy (up to 30 keV), high-current (up to 20 kA) electron beam of microsecond duration have been studied with the use of thermal imaging and wide-band oscilloscope. It was shown that energy density distribution is quite uniform (inside the circle of 2.5 cm in diameter that is closer to the outer diameter of cathode emitting part) at the guide magnetic field compared or somewhat higher in induction to the beam self-magnetic field. In the case of low guide magnetic field or its absence, the beam focuses and its energy density distribution becomes sharp non-uniform. It was also shown that even low magnetic field (about 25 mT) stabilizes the beam position in cross section. Any micro- non-uniformities of millimeter scale were not observed in the energy density distributions.

Abstract

Results of computer simulation of the structure of the electromagnetic field of a microwave discharge in a quartz bulb placed in a cylindrical resonator the plasma of which is confined by a magnetic trap are presented. The cold plasma approximation is used. The cylindrical resonator is excited through a narrow slot in the lateral wall. It is shown that the traditional model of the electron cyclotron resonance in crossed fields in the discharge under study is applicable at low electron densities. An increase in the density is accompanied by the formation of a wave propagating in the azimuthal direction from the excitation region. With a further increase in the electron density, the absorption coefficient of the wave decreases and the angular distribution of the field has the form of a standing wave.

Abstract

The paper presents the results of studies of the formation of reactive oxygen and nitrogen species in Milli-Q® deionized water (electrical conductivity of ≤0.1 µS/cm) under the influence of a multi-spark pulse discharge with gas injection into the interelectrode space. The discharge is a set of microplasma formations in a multiphase medium, for which the specific energy input is estimated. The influence of injected gases (argon, air) on the formation of a plasma discharge in the interelectrode space and long-lived chemical compounds: hydrogen peroxide, nitrite ions and nitrate ions is analyzed. The variation of the exposure duration to water from 2 to 10 min leads to a change in its chemical composition and electrical conductivity properties, but has virtually no effect on the characteristics and duration of the breakdown stage of the discharge. At the same time, changes in the concentrations of hydrogen peroxide, nitrite ions and nitrate ions are recorded within 1 h after the termination of the plasma exposure. Sputtering of stainless steel electrodes is detected, which is about 1 mg/min and leads in some cases to the formation of an insoluble precipitate. The data obtained allow the optimization of the effect of plasma-activated liquid on plants and planting material.

Abstract

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).

Abstract

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.

Abstract

Recently, a new mechanism of the lightning discharge initiation in a thundercloud has been discussed. It is based on a noise-induced kinetic transition, which consists of an increase in the concentration of atmospheric ions in the intracloud environment under the impact of the stochastic electric field of charged and polarized hydrometeors (drops, snowflakes, graupel, hail). The source of noise is electric field bursts that occur during collisions or near collisions of hydrometeors and are accompanied by the streamerless corona discharge ignition. The key point in this scenario is the relay process, in which new ionization centers, when the corona discharge is ignited, arise against the background of spots of negative ion charge remaining from the spreading old centers. This leads to a gradual increase in the concentration of negative ions, which could serve as a source of free electrons with a new increase in the electric field. In this work, we theoretically study the possibility of electron detachment from negative ions formed in a streamerless negative corona near hydrometeors under thundercloud conditions. It is shown that in this case the dominant negative ions near corona hydrometeors are ({text{O}}_{2}^{ - }{{({{{text{H}}}_{2}}{text{O}})}_{k}}) and ({text{O}}_{4}^{ - }) cluster ions. It follows from the calculations that when a high electric field is applied, electrons are released not via direct detachment from cluster ions, but in multi-steps in a sequence reverse to that observed without a field during the formation of cluster ions. As a result, to detach electrons from ions like ({text{O}}_{2}^{ - }{{({{{text{H}}}_{2}}{text{O}})}_{k}}), fairly moderate pre-breakdown reduced electric fields are required at a level of 65 Td.

Abstract

It is shown that the pressure and temperature of the radiating arc plasma can be determined from the measured values of the voltage on the plasma column, the discharge current and the photocurrent caused by the radiation flux of the entire volume of plasma to the photodetector. For the case of axially symmetric homogeneous arc plasma in a state of the local thermodynamic equilibrium, equations are formulated that connect the plasma parameters with the measurement results. The equation for the photocurrent is obtained from the solution of the radiation transfer equation in the arc plasma of the arbitrary optical density. The cases of electrode surfaces reflecting and absorbing electromagnetic radiation are considered. It is shown that the problem of determining the parameters of the arc plasma is reduced to solving a system of two non-linear equations with respect to pressure and temperature. The described method is used to determine plasma parameters of a high-current vacuum arc at the stage of the anode activity. The stability of the method with respect to the errors of the initial data is shown by the example of the vacuum arc plasma.

Abstract

The intensity of the interaction between a helium cold atmospheric plasma jet (CAPJ) and a dielectric surface or animal skin is compared in experiment and numerical simulation. A cold plasma jet at atmospheric pressure is generated by a sinusoidal or positive pulsed voltage with different pulse durations in optimal modes. The impact effect is estimated on the basis of measured and calculated currents, line intensities in the CAPJ spectrum and temperature fields. The measured CAPJ characteristics show that the pulsed voltage for the CAPJ excitation is preferable compared to the sinusoidal mode. Varying the pulse duration of the periodic pulsed voltage makes it possible to obtain the maximum current and electric field strength at the surface within the permissible temperature in the contact area of the CAPJ with the skin of mice (<42°C). It is shown that the results of the CAPJ study obtained in physical experiments using a dielectric plate are applicable for tumor-bearing mice treatment.

Atomic beam injection is one of the main methods of plasma heating in thermonuclear facilities. An injector of high-energy hydrogen atoms for plasma heating based on the acceleration and neutralization of negative hydrogen ions is being developed at the Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences. The injector uses a surface plasma source, in which a plasma flow is created using a radio-frequency driver: an induction radio-frequency (RF) discharge, ignited inside a cylindrical ceramic chamber when an RF voltage is applied to an external antenna. As a part of this work, a new version of the RF driver is being developed. A protective screen is used to prevent overheating and erosion of the ceramic wall of the driver. The operation of RF driver with different protective screens is studied. These screens reduce the efficiency of RF power transmission into the discharge, but make it possible the operation of the ion source in multi-second or steady-state pulses.

A new approach to the problem of the passage of the extraordinary wave through the region of the electron–cyclotron resonance in inhomogeneous plasma was considered in the framework of the full set of Maxwell’s equations, taking into account the effects of spatial dispersion and resonance dissipation. For the model one-dimensional geometry, field distributions were calculated in the vicinity of the cyclotron resonance at the second harmonic for the normal incidence of the extraordinary wave and the reflection, transmission, and absorption coefficients were found depending on the parameters of the resonance region. As a result, the fine effect of the reflection of electromagnetic radiation from the cyclotron resonance region in transparent plasma was described and the results were compared with the observations of this effect in the experiments on the microwave heating of the plasma at the L-2M stellarator.