Volume 45, Nº 4 (2019)

A 2D full-wave-model is developed and a 2D numerical code is designed to calculate the propagation and absorption of an X-polarized microwave beam in magnetized plasma with allowance for the nonlocal (differential) thermal correction to the plasma permittivity tensor near the electron-cyclotron resonance (ECR) at the second harmonic of the electron gyrofrequency. 2D full-wave numerical simulations of the propagation and absorption of the heating microwave beam in the standard poloidal cross section of the L‑2M stellarator are performed for three ECR heating scenarios differing in the position of the resonance region: central heating, heating on the vacuum magnetic axis, and off-axis heating at the midradius of the plasma column. It is shown that optimal conditions for microwave power deposition in plasma are achieved when the microwave beam is incident normally on the resonance surface, which, in the offered 2D model, takes place under ECR heating on the vacuum magnetic axis.

Carbon materials (C) are revisited to use as a plasma-facing material (PFM) of a fusion reactor. Characteristics of C concerned, such as erosion, redeposition, H retention, and neutron damages, are reinvestigated considering recent observations in laboratories and plasma machines, in particular, JET and JT-60U. It is concluded that C can be a good PFM of a fusion reactor if they are used as armor tiles mechanically fixed to heat sink allowing possible temperature escalation of tile surfaces, which mitigate T retention and neutron irradiation effect. The mechanical fixing ensures easy replacement of the carbon armor tiles by remote handling and periodic break for reactor maintenance gives opportunity for routine replacements of the armor tiles. In addition, routine T removal by isotopic exchange with D discharges and installation of replaceable cooled plates at divertor area to take up redeposited C incorporating with T would significantly reduce T inventory in a rector.

A modified scheme of the RS-20MR generator is presented that can be used not only as a source of high-power pulsed hard X-ray emission (the dose power per 1-L volume in atmospheric air exceeds 10¹⁰ rad/s) but also as a tool to study various processes occurring under the interaction of high-current electron beams with materials: excitation and propagation of shock waves, destruction of solids, etc. Due to specific properties of the sharpening system, the diode voltage pulse has a relatively steep front, which ensures the dominance electrons with energies of about 1 MeV in the beam. For electron beams with currents of I = 20–100 kA and particle energies of E ≥ 0.5 MeV, isochoric energy release can be achieved in the volume of the irradiated sample. This can lead to substantial changes in the formation mechanism of a shock wave and the character of damage it causes. The results of the first experiments of this kind are reported.

The processes of heating and acceleration of argon ions in different charge states in current sheets formed in 2D and 3D magnetic configurations were studied experimentally. It is shown that the temperature of argon ions depends on the ion charge state. The maximum temperatures of Ar II, Ar III, and Ar IV ions are found to be 60, 120, and 200 eV, respectively. The time of energy exchange between ions due to Coulomb collisions is much shorter than the current sheet lifetime, which indicates that ions in different charge states reside in different sheet regions located at different distances from the sheet midplane (y = 0). It is shown that Ar II and Ar III ions are accelerated to energies of \(W_{x}^{{\max }}\) ≈ 200 eV. It is found that the temperature and the energy of directed motion of Ar II, Ar III, and Ar IV ions do not depend on the magnitude of the longitudinal magnetic field. Argon ions Ar IV radiating in the near UV region are detected for the first time in the current sheet plasma.

Results of measurement of the mean neutron yield from plasma focus (PF) chambers filled with deuterium and deuterium–tritium are compared for various Mather-type and spherical PF devices at discharge currents in the range of 200−1000 kA. On the basis of the experimental results, an expression for the ratio of the neutron yields \({{\xi }_{{{\text{exp}}}}} = \left\langle {{{Y}_{{{\text{DT}}}}}} \right\rangle {\text{/}}\left\langle {{{Y}_{{\text{D}}}}} \right\rangle \) in the D + T and D + D reactions as a function of the discharge current is derived, according to which ξ_exp decreases from 150 to 110 (by ≈1.4 times) as the PF discharge current increases from 200 to 1000 kA. Assuming the beam–target mechanism of neutron generation in the PF, the ratio ξ_exp for D + T and D + D reactions is compared with the ratio σ_DT/σ_DD of the cross sections for the corresponding nuclear reactions at different mean energies of accelerated ions. Taking into account the mean energies of accelerated ions (~50–70 keV) determined from the measured space–energy anisotropy of neutron emission, it is suggested that the mean effective energy of accelerated D⁺ and T⁺ ions in the plasma beam formed in the pinch decay stage increases with increasing PF discharge current, which leads to a decrease in the ratio \({{\xi }_{{{\text{exp}}}}} = \left\langle {{{Y}_{{{\text{DT}}}}}} \right\rangle {\text{/}}\left\langle {{{Y}_{{\text{D}}}}} \right\rangle \) with increasing current.

An ultrawideband (UWB) plasma relativistic microwave source based on the amplification of self-noise of a relativistic electron beam in plasma is described. There is a fundamental difference of the described source from the existing high-power UWB radiation sources. The UWB plasma microwave source allows one to obtain higher energy per pulse than existing sources (up to 10 J). In addition, the mean frequency of the microwave signal generated by the UWB plasma microwave source can be tuned from pulse to pulse (e.g., from 2 to 3.5 GHz). It is important that the microwave pulse energy remains unchanged after tuning the frequency.

The role of inelastic collisions of electrons and ions with neutrals during the development of modulational instability involving dust acoustic perturbations in dusty ionospheric plasma, as well as the effect of collisions of electrons, ions, and dust grains with neutrals on the manifestations of modulational interaction in the dusty ionosphere, are estimated. It is shown that, in this case, the influence of collisions of electrons and ions with neutrals is usually less significant than the influence of collisions between dust grains and neutrals. It is demonstrated that the effect of modulational instability on the propagation of electromagnetic waves in the dusty ionospheric plasma is the most significant at altitudes of 100–120 km. The modulational interaction in the dusty ionosphere is important for the explanation of such effects as ground-based observations of low-frequency ionospheric radio noises with frequencies below 50 Hz, generation of infrasonic waves in the ionosphere and the possibility to detect them near the Earth’s surface, enhancement of the green nightglow emission at a wavelength of 557.7 nm from the lower ionosphere layer at altitudes of 110–120 km, and modulational excitation of inhomogeneities in the electron and ion densities in the ionosphere at altitudes of 100–120 km. The absence of observations of low-frequency ionospheric radio noise during such phenomena as noctilucent clouds and polar mesosphere summer echoes caused by the presence of dusty plasma at altitudes of 80–95 km is explained. It is shown that the latter phenomenon is related to the suppression of modulational processes at these altitudes.

Taking into account the vertical current in the Earth’s atmosphere and rejecting the frequently used quasineutral plasma approximation, which is wrong in the given case, the existence in the ionospheric D layer of thin plane regions with a characteristic size of ~1 m in which the plasma density increases abruptly with altitude is shown. Especially strong variations in the plasma density occur in the equatorial ionosphere, where the geomagnetic field is horizontal. The magnitude of the plasma density jump strongly depends on the electric field E at the lower boundary of the layer. In particular, at E = 0.1 V/m and an altitude of 85–90 km in the equatorial ionosphere, the plasma density jumps by a factor of 3.6, while at E = 0.2 V/m, by a factor of 15.5. A mechanism for the emergence of ionospheric precursors of earthquakes associated with the presence of such a jump is offered.

A new magnetic field generation mechanism in radiation-heated plasma is considered. This mechanism is based on the Nernst effect and operates in plasma with oppositely directed temperature and density gradients. The efficiency of the mechanism is determined by the values of these gradients.

Results of comparative experimental studies of the radiation patterns and radiation spectra of the plasma asymmetrical dipole antenna and metal asymmetrical dipole antenna are presented. It is shown that the directions of the main and lateral lobes of the radiation patterns of both antennas are the same; however, the amplitude of the main lobe of the plasma antenna pattern is lower and it has a higher directionality. The recorded spectrum of the continuous-wave signal allows one to estimate power losses for plasma generation in the plasma antenna. The spectrum demonstrates nonlinear amplification of the multiple harmonics of the signal frequency. The possible causes of these nonlinear distortions and the means to decrease them are discussed.

A symmetric radio frequency (RF) (13.56 MHz) electrode discharge system with different geometry has been investigated at low pressure. All electrical properties of symmetric capacitive RF discharge in pure argon (Ar) and pure hydrogen (H₂) have been obtained from the current and voltage waveforms. Here, the changes of voltage and current were investigated according to external electrical discharge parameters. The electron densities of these discharges were measured with a single Langmuir probe. A homogeneous model of capacitive coupled RF (CCRF) discharge was used to determine the electrical properties of a parallel-plate RF discharge system and their dependences on the input RF power and gas pressure .

The mode of a pulsed discharge in a nonuniform electric field is investigated at which bright plasma bunches with a beaded structure are generated in atmospheric-pressure air. Using an ICCD camera, it is found that, at centimeter gap lengths and a voltage pulse duration of ≈300 ns, the beaded structure can be observed with a probability close to 100% within time intervals from a few nanoseconds to several tens of nanoseconds. The beaded structure can also be observed in the time-integrated photographs of the discharge gap, but with a low probability. It is shown that individual beads arise in the point-to-plane gap after the diffuse stage of the discharge and start from the electrode with a small curvature radius. It is established that the spark channel bridges the gap by passing through the formed beads. The glowing beads are again observed in the final stage of the discharge, when the discharge current and, accordingly, the intensity of spark emission decrease.

The paper presents the results of spectroscopic studies of the initial segment of a supersonic plasma jet formed by a pulsed discharge in a capillary made of carbon-containing polymer. Detection of the emission properties of the high-temperature core of the jet (the intensities and contours of the H_α and H_β Balmer lines and the relative intensities of C II ion lines) with high temporal (1–50 μs) and spatial (30–50 μm) resolutions allowed the authors to reveal specific features of the longitudinal distributions of the electron density and temperature caused by the flow nonisobaricity along the initial segment of the supersonic jet.

A multidischarge actuator system (MAS) based on an improved three-electrode scheme with a screening electrode has been studied. The geometrical and physical parameters of the MAS are optimized to increase the velocity and energy efficiency of the generated airflow. The dependences of the airflow velocity at the output of the MAS on the frequency, voltage amplitude, and average supply power are experimentally determined. The maximum velocity is measured as a function of the total thickness of the dielectric substrate.