Том 45, № 11 (2019)

We consider nonplanar electron acoustic shock waves composed of stationary ions, cold and non-extensive hot electrons under multiple temperature electrons model in unmagnetized plasma. In this model Modified Korteweg-de Vries–Burger (KdVB) equation is obtained in the cylindrical/spherical coordinates. On the basis of the solutions of KdVB equation, the variation of shock waves features (amplitude, velocity, and width) with different plasma parameters are analyzed. Dissipation effect is introduced in the model by means of kinematic viscosity term. KdVB equation always leads to monotonic solitons and no oscillatory part or peak may appear. It is observed that the combined effect of particle density \((\alpha )\), nonextensivity parameter q, electron temperature ratio \((\theta )\), and kinetic viscosity \(({{\eta }_{0}})\) significantly changes the properties of shock waves in nonplanar geometry especially in spherical coordinates. Results could be helpful to analyze the soliton features in laboratory and in the space environments.

Results of systematic measurements of the electric and energy characteristics of a dc gas-discharge plasma in contact with a liquid are presented. The discharge was excited in atmospheric-pressure air over aqueous solutions of Zn(NO₃)₂. The experiments were carried out with two types of discharge: with a liquid anode and a liquid cathode. An H-shaped plasma–solution cell was used, whose branches were separated by a membrane to exclude the effect of possible chemical and/or electrochemical processes at the electrode immersed in the solution in one part of the cell on the characteristics of the metal–plasma–solution system in the other part of the cell. The discharge current was varied from 20 to 80 mA, and the zinc nitrate concentration in the solution was varied from 5 to 100 mmol/L. The current–voltage characteristic of the discharge, the electric field strength, and the near-electrode potential drops were measured. Using the geometrical characteristics of the discharge, the current densities are calculated in both the positive column and the region of the contact between the discharge and the solution. From results of spectral measurements, the vibrational temperatures of vibrationally excited nitrogen molecules in the N₂(C³Π_u) state and the temperature of the neutral plasma component are determined. The reduced electric field is calculated in the entire ranges of zinc nitrate concentrations and discharge currents under study. The results obtained for discharges with a liquid cathode and a liquid anode are compared. The results of this work can be used to simulate gas-discharge plasmas over aqueous solutions, in particular, to calculate the densities and fluxes of active particles from the plasma to the solution.

Modulation instability, bright envelope soliton, and rogue waves of ion acoustic waves in dense plasma consisting of ultra-relativistic degenerate electrons and positrons, cold and mobile inertial ions, and negatively charged static dust particles have been investigated using Fried and Ichikawa method. Nonlinear Schrödinger equation has been derived and the growth rate of modulationally unstable ion acoustic wave in such plasma are discussed. It has been found that ion acoustic wave will be always modulationally unstable for all possible values of density of positrons, electrons, and charged dust particle. The solutions of envelope solitons and rogue waves are obtained from the nonlinear Schrödinger equation. The theoretical results have been analyzed numerically and graphically for different values of plasma parameters. It is found that only bright envelope soliton would be excited in the ultra-relativistic degenerate plasma. Our results are new and may be applicable for the study of nonlinear wave processes in relativistic degenerate dense plasmas of astrophysical objects, namely, in white dwarfs and neutron stars.

A three-dimensional multi-component magneto-plasma system consisting of hybrid nonextensive-nonthermal-distributed electrons, positive ions, and immobile negatively charged dust grains is considered to examine the modulational instability (MI) of dust ion-acoustic waves (DIAWs). The reductive perturbation method, which is valid for small but finite amplitude DIAWs, is employed to derive the (3 + 1)-dimensional nonlinear Schrödinger equation (NLSE). The NLSE leads to the MI of DIAWs as well as the formation of dust ion-acoustic rogue waves (DIARWs), which are formed due to the effects of nonlinearity in the propagation of DIAWs. It is found that the basic features (viz. amplitude and width) of the DIAWs and DIARWs (which is formed in the unstable region) are significantly modified by the various plasma parameters such as non-extensive (non-thermal) parameter q(α), number densities of plasma species, threshold modulational obliqueness (\({{\theta }_{c}}\)), etc. The application of the results in space and laboratory magneto-plasma system is briefly discussed.

The current–voltage characteristics (CVCs) and efficiency of electron beam generation in glow discharges in helium and its mixtures with oxygen and nitrogen, as well as in pure oxygen and nitrogen, are studied experimentally. Special attention is paid to creating clean conditions for a discharge operating in helium. It is shown that, under clean conditions and pressures above 10 Torr, the CVC first rapidly grows. Then, the growth slows down and the CVC begins to decrease; however, at voltages above 1.5 kV, it rapidly grows again. These features are explained via changes in the mechanisms of electron emission and electron runaway from the cathode sheath, which lead to a highly efficient (up to 85%) generation of electron beams. In the presence of molecular admixtures, the CVC changes and begins to smoothly grow, the current being substantially higher than in pure helium. In pure oxygen and nitrogen, the CVC also grows smoothly and electron beam generation is highly efficient, but its mechanism is different. In pure helium, electrons are generated primarily due to photoemission, whereas in pure oxygen and nitrogen, electron emission from the cathode is mainly caused by the bombardment by fast heavy particles. In helium mixtures with oxygen and nitrogen, other emission mechanisms can also take place.

The results are presented from measurements of the power absorbed in plasma of the L-2M stellarator in experiments on the electron cyclotron resonance (ECR) plasma heating at the second harmonic of the gyrofrequency. Hydrogen plasma was created and heated in the stellarator vacuum chamber as a result of the resonance absorption of microwave power supplied by the gyrotrons in the pulse-periodic operation regime. The total energy of the toroidal plasma column and the absorbed power were measured using the diamagnetic diagnostics. The shielding effect of the metal vacuum chamber on the diamagnetic signal measurements was taken into account. It was ascertained that, during the axial ECR heating, up to 90% of the inputted gyrotron power is absorbed in the plasma that is consistent with the theoretical estimates.

Switching of a short vacuum gap with an auxiliary discharge over the surface of a dielectric has been studied via high-speed recording of images of discharge plasma radiating in the optical spectral range. Based on the analysis of experimental data, we surmise that the cathode spot and cathode flame radiation play an important role in the formation of the current channel in the discharge.

Conditions of substrate-free syntheses of graphene and graphene-based systems based on conversion of liquid and gaseous carbon precursors in helium, nitrogen, and argon plasma jets generated by a dc plasma torch at a reduced pressure are investigated. Using a number of physical characterization techniques, it is shown that bulk-synthesized (i.e., substrate-free) graphene has the morphology of crumpled paper. By changing the geometry of the flow-through section of the reactor, without using substrates, hydrogenated graphene structures are synthesized. Nitrogen- or copper-doped graphene can be prepared using nitrogen plasma. Thermally stable graphene oxide is obtained by introducing alcohols into argon or helium plasmas. It is concluded that graphene materials can be prepared by the plasma-assisted one-step syntheses.