Vol 44, No 6 (2018)

In net-free current plasmas that are confined in the L-2M stellarator, an electromagnetic mode having thresholds in the plasma density and pressure was observed. The characteristic frequency of the unstable mode was 70–90 kHz, and the direction of its rotation pointed toward the direction of ion diamagnetic drift. The instability was observed at n(0) > 1.5 × 10¹³ cm^-3 and 〈β〉 > 0.14%, where n(0) is the plasma density averaged over the central chord and 〈β〉 is volume-averaged ratio of the gas kinetic pressure to the magnetic pressure. This phenomenon cannot be described within the theory of resistive interchange modes. Possible reasons for the occurrence of the observed electromagnetic mode are discussed using analytical estimations.

The process of collision of ion-acoustic solitary waves in a collisionless plasma with cold ions and Boltzmann electrons is studied using numerical simulations. It is shown that solitary waves with sufficiently large amplitudes do not preserve their identity after the collision. Their amplitudes decrease, and the shapes change. It is found that the collision is accompanied by the generation of fast ions with velocities exceeding threefold the ion sound speed.

The energy of directed plasma flows accelerated in a current sheet is studied experimentally as a function of the current flowing through the sheet. It is found that the plasma flow energy rapidly increases with increasing current amplitude, nearly according to a power law with an exponent of 1.8–1.9. Using relations for the neutral current sheet and the concept of plasma acceleration under action of the Ampère forces, the energy of directed plasma flows is estimated as a function of the total current flowing through the sheet. It is shown that, as the current rises, the ion energy grows due to an increase in both the Ampère forces and the width of the current sheet within which plasma is accelerated.

Electron dynamics and acceleration in an electromagnetic field configuration modeling the current sheet configuration of the Earth’s magnetotail region is investigated. A focus is made on the role of the dawn−dusk magnetic field component B_y in the convection electron heating by an electric field E_y. For numerical integration of a large number of test particle trajectories over long time intervals, the equations of motion written in the guiding center approximation are used. It is shown that the presence of a B_y ≠ 0 magnetic field significantly changes the electron heating and allows electrons with small pitch angles to gain energy much more efficiently than the equatorial electrons. As a result, the convection heating in the current sheet with B_y ≠ 0 leads to the formation of an accelerated anisotropic population of particles with energies higher than a few hundred electronvolts. The obtained results and spacecraft observations in the Earth’s magnetotail are compared, and possible limitations in the proposed model approaches are discussed.

Erosion of the leading edge a low-energy high-current electron beam injected into a low-pressure neutral gas under conditions of virtual cathode formation in the absence of an external magnetic field is studied theoretically. Beam losses are calculated as functions of the pressure and sort of gas, beam electron energy, and system geometry. The dependence of the duration of the leading edge erosion on the system parameters is analyzed.

The development of a discharge in a point−plane gap filled with a saline solution with a salt content of 3% was studied experimentally. The duration of the voltage pulse applied to the gap was about 2 ms. Data are presented on the formation dynamics of gas microcavities at near-threshold voltages at which gas-discharge plasma appears in some microcavities. The cavities are conglomerates of microbubbles with a typical size of ≈100 μm. At the threshold voltage (≈750 V), the active electrode is covered with a gas layer and the gap voltage is in fact applied to this layer, which leads to the development of discharges in individual microbubbles. In this case, the discharge operates in the form of short current pulses. The number of microcavities filled with plasma increases as the voltage grows above the threshold value. At the plasma boundary, new microbubbles are formed, in which discharges are ignited. As a result, the plasma front propagates from the active electrode into the gap with a characteristic velocity of 10³ cm/s.

This work continues the series of studies of pulsed high-voltage discharges propagating over liquid surfaces in the presence of barriers. In this work, the influence of the barrier position relative to the air electrode on the discharge voltage in the stage of the completed discharge is analyzed. The effect of the water depth and barrier submersion depth on the propagation of a pulsed discharge is studied.

Results are presented from the experimental study of conditions for the formation of a plasma ring by a microwave discharge in a narrow coaxial cavity in an axisymmetric magnetic field the magnitude of which is below the electron-cyclotron resonance value. It is established that the necessary condition for this process is the presence of an electrostatic wave propagating in the azimuthal direction, the circumference of the plasma ring being a multiple of the wave half-length.

A decrease in the amplitude of the current of a vacuum spark discharge over a dielectric surface with increasing discharge gap length is established. It is shown that, in the presence of a longitudinal magnetic field, the leading edge of the discharge voltage pulse is extended, whereas the clearly pronounced current pulse transforms into a train of alternating current oscillations. The discharge is found to decelerate when the spark is preceded by a low-current discharge.

A new form of discharge excited by a microwave beam in a high-pressure (up to atmospheric and higher) gas in free space and in a closed chamber is discussed. For the first time, the discharge was implemented by means of a gyrotron with a pulse power of 200 ≤ P ≤ 600 kW, a pulse duration of 0.5 ≤ τ ≤ 20 ms, and a wavelength of λ = 0.4 cm. Under deeply subthreshold conditions in atmospheric-pressure air, a plasma column with a length of L = 50 cm was generated by a microwave beam formed with the help of a quasi-optical transmission line. With the use of the MIG-3 gyrotron complex with the above parameters, generation of a plasma column with a length of several meters is possible in principle. The parameters and structure of the formation of the plasma investigated make it possible to class it as a self-non-self-sustained (SNSS) discharge, discovered and described for the first time at the Prokhorov General Physics Institute, Russian Academy of Sciences. One of the important applications of this type of discharge is plasmachemical cleaning of the urban air environment of hazardous contaminants.