Физика плазмы

Abstract

The properties of a plasma sheath are investigated numerically by using a fluid model in which a monoenergetic electron beam is taken into account. To suit the realistic environment, secondary electrons emitted from the wall surface due to collision between the electrons and the wall surface is considered. The result reveals that the effective emission coefficient depends on the emission generated by a monoenergetic electron beam when the temperature of a monoenergetic electron beam is high. The effective emission coefficient of secondary electrons changes monotonically with the increase of emission coefficient generated by beam electrons. Using the Sagdeev pseudopotential method, a modified Bohm criterion can be obtained. It is found that the ion Bohm velocity increases with increasing beam electron energy and emission coefficient generated by a monoenergetic electron beam. Moreover, when the emission coefficient generated by a monoenergetic electron beam is small, the wall potential decreases with increasing beam electron energy and concentration. When the emission coefficient generated by a monoenergetic electron beam is large, the opposite is true. It is also shown that a monoenergetic electron beam can cause an increase in the critical effective secondary electron emission coefficient, and the increase is nonmonotonic.

Abstract

Based on the two-dimensional resistive magnetohydrodynamics (MHD) model, this study explores the impact of shear flow on the resonant magnetic perturbation (RMP)-induced double tearing mode (DTM). The results indicate that the effectiveness of shear flow in suppressing the double tearing mode is primarily dependent on the flow velocity at the outer rational surface. A notable finding is that, with higher flow velocities, the double tearing mode is effectively suppressed. However, in cases of very weak flow velocities, the shear flow still exerts an influence, but with a limited suppression effect. Specifically, it can only inhibit the inner island, while the outer island continues to grow under the influence of the resonant magnetic perturbation.

Abstract

The formation of red plasma diffuse jets (PDJs) initiated by a capacitive discharge in air at pressures of 0.1–10 Torr is studied. Data on the dimensions and properties of plasma diffuse jets including counter-propagating ones with different and identical front polarities are obtained in a pulse-periodic discharge mode in a 15-cm-diameter tube. Photographs of the discharge glow in various modes are presented. It is confirmed that the emission of counter-propagating PDJs is suppressed at the same polarity of voltage pulses. It is shown that during a collision of unipolar PDGs formed from two generators the brightness of the discharge plasma between the ring electrodes increases.

Abstract

The results of studying instabilities in flat aluminum 4-μm-thick foils exploded using the GVP generator with a short circuit current of 10 kA and a current rise time of 350 ns are presented. The dynamics of foil destruction during the explosion was studied using laser probing. During the experiments, it was ascertained that in the presence of the two-dimensional structure of foil, the growth rates of instabilities and their nature depend on the foil orientation relative to the direction of current flow. The conditions are cleared up, under which during the explosion of foils with two-dimensional inherent structures, the development of instabilities is slowed down.

Abstract

In experiments with neutral beam injection on the Globus-M2 spherical tokamak, sequences of long-lasting harmonics of toroidal Alfvén modes were discovered, equidistant from each other in frequency and shifted from zero by a constant value. Using microwave Doppler backscattering diagnostics, the central localization of toroidal modes was determined. In this work, the possibility of “splitting” of toroidal harmonics due to the Doppler shift caused by the toroidal rotation of the plasma is being discussed. It is found that the unshifted frequency of the toroidal Alfvén mode obtained from the spectrum of the magnetic probe signal is in good agreement with the frequency of the mode calculated at the mode location radius, and the toroidal rotation frequency, also determined from the spectrum of the magnetic probe signal, correlates well with the rotation frequency measured using charge exchange spectroscopy diagnostics, but differs by a constant amount. Possible reasons for the discrepancies are being discussed.

Abstract

Electric field on the surface of a metal electrode covered by a continuous dielectric film and immersed in plasma is calculated at negative electrode potential Ψ when parameter eΨ substantially exceeds electron temperature Te \(\left( {\frac{{e\Psi }}{{{{T}_{e}}}} \gg 1} \right)\) . It is established that strong electric field with a magnitude of 1–10 MV/cm can appear inside the film at plasma density of 1012–1013 cm–3 and electron temperature of Te = 10 eV as a result of charging of the outer film 10–1000 nm in thickness surface by a flux of positive ions from plasma. The magnitude of the electric field at film discontinuities is comparable to that inside the film. The magnitude of the electric field at the surface of the dielectric film and at the film-free clean metal surface in plasma is substantially lower than that inside the film. Strong electric fields inside the film and at its discontinuities can lead to electrical breakdown inside the film and at its discontinuities. The electrical breakdown of the dielectric film can initiate the unipolar arcing on metals, drive microplasma discharges and form centers of explosive electron emission on metal surfaces in plasma.

Abstract

It is assumed that the low-frequency noise recorded on the surface of Mars may be associated with a charged dust component in its atmosphere and the occurrence of sound perturbations in such a dust system that modulate the electromagnetic wave from the Sun. It is also shown that it can be associated with plasma-dust processes in meteoroid tails. The mechanism for the excitation of modulational instability of an electromagnetic wave associated with a dust acoustic mode in the Martian atmosphere, namely in dust clouds at an altitude of 60 and 100 km, where the dusty plasma with particles of frozen carbon dioxide is detected, is described. It is shown that the development of modulational instability is due to the influence of high-frequency electromagnetic waves on the dusty plasma in the Martian atmosphere from both natural sources (solar radiation, lightning discharges) and anthropogenic nature (from equipment from space satellites and from stations on the surface of the planet). The parameters of electromagnetic pump waves, at which the active development of modulational instability of electromagnetic waves associated with the dust acoustic mode is expected, and the modulational instability growth rates are found. The development of the modulational instability in the dusty plasma of Martian clouds, in turn, can explain the occurrence of low-frequency noise recorded by equipment on the surface of Mars. The relation between observed radio noise in the range of 3 Hz–3 kHz and plasma-dust processes in the Martian atmosphere, in particular, in dust clouds at 60 and 100 km, as well as in dusty plasma meteroid tails, where the dust concentration is high, is discussed.

Abstract

Physical principles and modern techniques for the formation of spontaneous vacuum ultraviolet radiation are described for three cases: the formation of line spectra of atoms, line spectra of multicharged ions and continuous spectra of excimer molecules. The parameters of the radiation sources are correlated with their applications: real and potential. The following schemes of the formation of vacuum ultraviolet radiation are described: H-type and E-type high frequencies discharges; discharge in a hollow cathode; glow, barrier, and arc discharges; high-voltage nanosecond discharge with a sharply inhomogeneous distribution of electric field strength in a gap; laser, discharge, and hybrid systems for multicharged ions formation; excitation of gas targets under conditions of gyrotron plasma heating. The review covers the state of the art over the last 20 years.

Abstract

The radiation field of a multi-dimensional plasma formation can be substantially asymmetrical. Often, the luminosity of such an object is determined using one-dimensional models with various correction factors to account for the nonsphericity. In this work, a model is presented for the determination of the luminosity of asymmetrical plasma formations based on consistent multi-dimensional radiation–hydrodynamics calculation on the example of the scenarios of supernovae with an equatorial disk. The simulations were compared with the observations of the supernova SN2009ip. The bolometric light curves were determined for the observation of this object in the disk plane and from the pole. A conclusion was made that it is impossible to describe the multi-dimensional structure of the radiation field within the framework of the one-dimensional model with correction factors and that, rather, a full three-dimensional simulation is required.

Abstract

The initiation of a positive corona discharge near a model hydrometeor in air is studied numerically. Hydrometeors in the form of an ellipsoid of revolution and a cylinder with two hemispheres at the ends are considered as models. Threshold characteristics (external electric field strength, particle charge) are obtained for hydrometers of various sizes and shapes at an atmospheric pressure of 0.4−1 atm. Analysis of the results of numerous calculation options shows that the threshold field strength at the top of the hydrometeor is determined by the curvature radius of the surface at this point and air pressure. A universal dependence of the reduced threshold field strength on the product of the curvature radius of the surface and air pressure is obtained. The simulation results indicate the possibility of initiating a corona discharge in a thundercloud from the top of a hydrometeor less than a centimeter long at a subthreshold reduced field strength of 10−15 kV∕(cm atm).

Abstract

The results of preliminary experiments on measuring the spatial asymmetry of plasma flows in the GOL-NB device using movable Mach probe are presented and the diagnostics used is described. In the experiments, the high-field sections were mounted in the configuration with solenoidal magnetic field. The dynamics of plasma flows was recorded which was expected in the trap: the plasma flowed from the plasma gun along the magnetic field, accumulated in the GOL-NB central trap, and then after the plasma gun was switched off, flowed out from the central trap in two directions. At time of transition from the stage of plasma accumulation to the stage of its decay, the direction of plasma flow in the input high-field section was inverted. The balance of particles in the central trap is discussed. Experiments have shown that this technique can be used for studying the effects of improving plasma confinement after switching to the multiple-mirror configuration of high-field sections, in which, according to theory, under optimal conditions, a flow of b-ackscattered particles should arise, which will return them from the multiple-mirror sections to the confinement zone.

Abstract

The paper describes a method for generating aluminum and hydrogen plasma jets. It illustrates the formation mechanism of extended plasma structures produced during the combustion of a high-current vacuum-arc discharge. The current-carrying plasma front is shown to propagate at different velocities for aluminum plasma and hydrogen plasma. The hydrogen plasma has a substantially higher initial velocity (about 30 cm/µs) compared to the aluminum plasma (about 10 cm/µs). It is shown that the bulk velocity of the hydrogen plasma jet is about 9 cm/µs. It was proven by means of spectral diagnostics that the hydrogen plasma jet is indeed composed mainly of hydrogen.

Abstract

A set of magnetic coils used to correct the error fields at the Globus-M2 spherical tokamak, which appear due to the imperfections of the production and assembly of the tokamak magnetic system, is described. The magnetic sensors that are used to monitor the locked helical MHD modes are also described. The results of experiments on detecting the locked modes in the discharges with plasma heating by neutral beam injection are presented. A correlation is found between the appearance of the locked modes accompanied by the loss of fast ions and the confinement of the main plasma.

Abstract

The ion-acoustic instability in the tails of meteoroids as a result of their passage through the Earth’s atmosphere is studied and the conditions under which it develops are given. The development of this instability occurs as a result of the relative motion of the plasma of meteoroid tails and the dusty plasma of the Earth’s ionosphere. Dust, in turn, creates conditions when this instability can develop in a situation of approximately equal ion and electron temperatures, which is observed in the plasma–dust system under consideration. The mechanism of the excitation of ion-sound waves as a result of the development of the ion-acoustic instability in meteoroid tails is shown. The growth rates of the ion-acoustic instability and the characteristic times of its development are found. It is shown that the instability has time to develop during the time of passage of a meteoroid body in the Earth’s atmosphere and the formation of a meteoroid trail, which has values much greater than the time of development of ion-acoustic instability in the system under consideration. The wave vectors and velocities of meteoric bodies, at which the development of the ion-acoustic instability is expected, are found. It is noted that the instability can reach a nonlinear regime at possible large wave amplitudes.

Abstract

We propose a 0-D model describing processes in a system comprising an atmospheric pressure DC discharge and aqueous nickel nitrate solution. The model is represented as two coupled subsystems: plasma and solution. Characteristics of the discharge plasma have been determined by jointly solving the Boltzmann equation for electrons; equations of vibrational kinetics for the ground states of N2, O2, NO, H2, and H2O molecules; and equations of chemical kinetics (328 reactions, 34 components). In doing so, use was made of experimentally determined reduced electric field strength and vibrational and gas temperatures. The kinetics of the processes in the solution included 121 reactions and 34 components. The calculation results agree with experimental data on the vibrational temperatures of N2(X) molecules, the kinetics of the decrease in Ni2+ concentration, and the variation in solution pH. We have determined the degree of Ni2+ conversion and the energy yield of conversion and identified the mechanisms that determine the concentration of the major solution components.

Abstract

The dielectric constant and the dispersion relation of the Ordinary mode (O-mode) instability has been discussed by using the Vlasov kinetic model in the presence of the Heaviside distribution function. The Heaviside distribution includes relativistic effects for both the parallel and perpendicular streaming. It is noted that the relativistic effects play a destabilizing role for the O-mode at higher magnetic field in comparison to the non-relativistic case. The growth rate of O-mode instability is calculated analytically for different values of parallel and perpendicular streaming. It is shown that the parallel streaming destabilizes the O-mode where as the perpendicular streaming provides a stabilizing effect. The results of the non-relativistic O-mode instability have been retrieved under limiting case.