Volume 45, Nº 6 (2019)

A subthreshold microwave discharge in atmospheric-pressure air is used to process mixtures of mercaptans (thiols) with air and with air and methane. It is found that, at microwave intensities of ~10 kW/cm² and pulse durations of 1–3 ms, the mercaptan concentration decreases 100-fold in mixtures with air and 20-fold in mixtures with air and methane. It is shown that, at an average specific energy deposition in a single discharge of ~0.4 J/cm³, the specific microwave energy spent on mercaptan decomposition is ~10 J/cm³. It is concluded that the active zone of the discharge with high specific energy deposition occupies less than 5% of the microwave beam volume.

The reserves of krypton are abundant, and it has become an important high specific impulse substitute for Hall thrusters as its physical properties are closest to xenon. However, it has been found that the plasma beam of a Hall thruster has weak plume-focusing characteristics and the half-divergence angle is large when using krypton as a propellant. In this paper, the thrust, specific impulse, efficiency, and plume half-divergence angle of a Hall thruster under different propellant supply flow rates are measured. In addition, the influences of the magnetic field configuration and cathode heating power on the beam-focusing characteristics are researched. The results show that the magnetic field mainly affects the beam-focusing characteristics by the bending direction of the magnetic field lines, thus causing the divergence, focusing, and overfocusing phenomena of the plasma plume. Excessive cathode heating power changes the low-frequency discharge amplitude of the Hall thruster, affects the ionization distribution and the width of the main ionization region, and thereby changes the plume focusing state.

An electrodeless microwave jet plasma source is considered, and its various applications in the technology of chemical vapor deposition of diamond films and dimension increasing of small diamond single crystals synthesized at high pressures and temperatures are discussed. The plasma jet is ignited in an atmospheric-pressure gas (argon) flow with hydrogen and methane additives. The operation of the microwave jet reactor is described, and the plasma characteristics measured using emission spectroscopy are presented. The brightly glowing atmospheric-pressure plasma jet is ignited and stably burns at a microwave power of ≤1 kW supplied from a microwave oven magnetron. The specific microwave power density absorbed by the compact plasma jet (≤10⁴ W/cm³) is comparable with that absorbed by a dc arc. The growth rate of the polycrystalline diamond layer amounts to 40 µm/h. The process of film deposition on the substrate can be controlled by scanning the substrate surface with the jet.

Heating of plasma electrons by an extraordinary laser wave at the double upper hybrid frequency in a strong magnetic field is studied numerically by the particle-in-cell method. It is shown that, in this case, significant heating takes place even at considerable detunings from the fundamental parametric resonance. This makes it possible to reduce the resonance magnitude of the magnetic field. The minimum value of the laser field amplitude at which parametric heating of this type can be implemented is found.

Nonlinear stabilization of instability of an infinitely thin tubular electron beam moving along the surface of a solid-state plasma cylinder is analyzed. It is assumed that the electron collision frequency in the plasma cylinder is much higher than the frequency of plasma eigenmodes (oscillations). The beam is assumed to be nonrelativistic, and, thus, the problem is solved in the electrostatic approximation. It is shown that the growth of the wave amplitude is stabilized nonlinearly due to the self-trapping of the beam electrons by the field of the electrostatic wave excited in the beam itself. It is found that the saturation time of instability and the maximum amplitude of the excited wave depend on the radius of the plasma cylinder. It is established that the larger the radius of the plasma cylinder, the later the nonlinear stage of instability begins and the larger the maximum amplitude of the excited wave.

Generation of terahertz waves under interaction of two counterpropagating laser pulses with different frequencies in underdense plasma is analyzed. Spectral, angular, and energy characteristics of terahertz radiation are investigated as functions of the frequency difference of the laser pulses. It is shown that, due to the frequency difference, in addition to the line at the doubled plasma frequency [L.M. Gorbunov, A.A. Frolov, JETP 98, 527 (2004)], a peak near the plasma frequency appears in the emission spectrum. The total energy of the terahertz signal is calculated, and the condition under which emission at the plasma frequency dominates is found. It is shown that the energy of radiation at the plasma frequency reaches its maximum value when the frequency difference of the laser pulses is close to the plasma frequency.

The ionization region of a hollow-cathode magnetron discharge was studied experimentally. The discharge was powered by either a dc supply or a high-power pulsed supply. The emission spectra of atoms and ions of the target material and buffer gas were recorded in different discharge regions by using an optical probe. The plasma parameters in the dc and pulsed discharges were determined by means of a Langmuir probe. The goal of this research was to find the position of the ionization region and reveal the mechanism of ionization of sputtered target atoms.