Vol 44, No 11 (2018)

Prototypes of lithium emitters and collectors designed on the basis of capillary-porous systems (CPS) are being tried-out at the T-11M tokamak within the concept of continuous lithium circulation, previously proposed for stationary fusion neutron sources (FNSs). One of the goals of the T-11M tokamak research program are to implement the closure of the lithium circulation circuit and to develop the technology of recuperation (extraction and removal from the tokamak chamber) of hydrogen isotopes by using special lithium CPS-based collectors in order to prevent hydrogen isotope accumulation on the inner surface of the discharge chamber. In order to determine the optimal temperature regime for the future FNS operation, the effect of the temperature of the collector surface on its ability to capture lithium and hydrogen isotopes was studied. It is ascertained that the efficiency of capturing lithium and hydrogen isotopes by metal (12Cr18Ni10Ti) collectors in the operating regimes of the T-11M tokamak depends on the temperature of their collecting surfaces; namely, it remains almost constant in the temperature range from–196 to 50°С and then gradually decreases as the temperature rises to 300–400°С. In this case, the amount of the collected lithium is reduced by no more than sixfold, whereas the amount of collected hydrogen isotopes decreases by more than two orders of magnitude. Thus, the wall of a tokamak reactor chamber, which will be heated to 400°С but still coated with residual lithium (or its chemical compounds), will serve as a “mirror” for the incident hydrogen isotopes. It is found that the liquid lithium surface of the CPS-based collector can efficiently capture hydrogen isotopes falling onto it. As the lithium CPS-based collector is heated from 100 to 240°С, the amount of captured deuterium is reduced only twofold. This means that, in a steady-state mode, if, e.g., an MHD pump is used to transport liquid lithium from the collector back to the emitter, then, together with lithium, the captured hydrogen isotopes can also be transported into the recuperation zone.

A discharge operating in a 80-cm-long discharge tube with an inner diameter of 15 mm, filled with a 3 : 1 neon–argon mixture at a pressure of 1 Torr, was investigated experimentally. Square voltage pulses with a period of 1 s were supplied to one of the tube electrodes, the second electrode being ungrounded. The initial stage of breakdown—the primary breakdown between the high-voltage (active) electrode and the tube wall, accompanied by the propagation of the prebreakdown ionization wave—was the same as in the conventional scheme with a grounded low-voltage electrode. Since the discharge gap was not closed, the discharge was not ignited. An essentially new effect was observed after the end of the voltage pulse. After a certain time interval, voltage spikes of opposite polarity, the amplitude and shape of which were close to those observed during the primary breakdown, appeared in the voltage and current waveforms of the active electrode. Simultaneously, a radiation pulse from the region adjacent to the active electrode was observed and an ionization wave began to propagate toward the second electrode. This work is dedicated to investigating this effect (which was named “reverse breakdown”) and analyzing its mechanism. A conclusion is made on the similarity of this phenomenon to the processes occurring in atmospheric-pressure dielectric barrier discharges.

Numerical simulation of a specific technical RF inductively coupled argon plasma with three coils, discharge current in the range of J_coil = 100–250 A, and generator frequency 3 MHz is presented. The temperature, pressure, and velocity fields are obtained under different discharge currents and different flow rates of central gas. A reversed flow (vortex) is found between the injected cool gas and high-temperature plasma-forming gas. The formation mechanisms of such a vortex and the influence of the discharge current and flow rate of central gas on the vortex structure and intensity are studied. Special attention is paid to investigating two different kinds of vortex flow patterns—Benard and toroidal. A critical flow rate of central gas above which the flow pattern would transform from Benard to toroidal is determined and approximated as a function of the discharge current by theoretical calculations and numerical simulations. The maximum negative velocities along the axis in the vortex zone are also determined under different discharge currents and different flow rates of central gas.

The effect of long-wavelength magnetic field disturbances typical of the Earth’s auroral region on the generation of auroral kilometric radiation in a narrow three-dimensional plasma cavity in which a weakly relativistic electron flow propagates against the background of cold low-density plasma is analyzed. The dynamics of the propagation and amplification of fluctuation waves with initial group velocities directed toward the higher magnetic field is considered in the geometrical optics approximation. Analysis of wave trajectories shows that the wave amplification coefficients depend on the magnetic field gradient in the reflection region. If the wave reflection point lies in the region where the gradient of the disturbed magnetic field is less than that of the undisturbed dipole field, then the wave amplification coefficients exceed those of waves propagating in the undisturbed field, and vice versa. Thus, the shape of the spectrum of generated waves changes in the presence of long-wavelength disturbances of the dipole magnetic field in such a way that segments with different curvatures can form in the spectrum.

An alternative mechanism is proposed for the generation of harmonics of the electron plasma frequency due to the development of explosive instability in a system of interpenetrating electron and proton flows in the solar atmosphere. The efficiency of the new mechanism in comparison with the earlier discussed mechanisms involving multistage processes of nonlinear interaction of waves in plasma is determined. It is shown that the development of explosive instability can lead to the excitation of the second and third harmonics of the plasma frequency with comparable amplitudes.

It is shown that fluctuating charge spots and local electric fields of up to 10⁶ V/m and higher can form on a dielectric surface exposed to charged particle flows. A stochastic differential equation describing the dynamics of these fluctuations is derived.

The nonlinear characteristics of dust-electron-acoustic (DEA) waves in a dusty electronegative magnetoplasma system consisting of nonextensive hot electrons, inertial cold electrons, positively charged static ions, and negatively charged immobile dust grains has been investigated. In this observation, the well-known reductive perturbation technique is employed to determine different types of nonlinear dynamical equations, namely, magnetized Korteweg–de Vries (KdV), magnetized modified KdV (mKdV), and magnetized Gardner equations. The stationary solitary wave and double layer solution of these three equations, which describe the characteristics of solitary waves and double layers of DEA waves, are obtained and numerically analyzed. It is noticed that various plasma parameters (viz., hot electron nonextensivity, positive ion-to-cold electron number density ratio, dust-to-cold electron number density ratio, etc.) significantly affect the basic properties of DEA solitary waves (DEASWs) and Gardner solitons (GSs). The prodigious results found from this theoretical investigation may be useful for researchers to investigate the nonlinear structures in various space and laboratory plasmas.

Ignition of hydrocarbon–oxygen mixtures by means of a nanosecond surface dielectric barrier discharge (NSDBD) was studied experimentally. The propagation velocity of the flame wave and the ignition delay time in mixtures of oxygen with methane, ethane, ethylene, and dimethyl ether were measured using a high-speed camera. The experiments were carried out at room temperature and gas mixture pressures in the range of 0.75–1.25 atm. It is shown that, for all hydrocarbons under study, the flame velocity decreases with reducing pressure and stoichiometric ratio, as well as when the mixture is diluted with molecular nitrogen. Theoretical analysis of the processes in the NSDBD plasma and measurements of the flame velocity in hydrocarbon-containing mixtures without plasma agree qualitatively with the measurement results, except for the increasing dependence of the flame velocity on the pressure, which is decreasing in experiments without a discharge plasma.

Using equations for small plasma oscillations in an elliptic current-carrying cylinder, it is demonstrated that, in a configuration with magnetic surfaces and geodesic curvature, excitation of a longitudinal displacement with a poloidal mode number determined by the geodesic curvature is accompanied by the appearance of a zero angular harmonic in the transverse displacement.