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.

A review of theoretical and experimental studies of secondary electron emission microwave discharge (multipactor) on a dielectric is presented. The coefficient of microwave power absorption by a single-surface multipactor on a dielectric is found as a function of the incident microwave power and secondary electron emission properties of the dielectric. Results of experimental studies of a single-surface multipactor on a lithium fluoride (LiF) single crystal in a rectangular waveguide are presented and compared with theoretical results. The dependence of the micowave power absorbed by a single-surface multipactor on the incidence angle of microwave radiation is studied numerically and analytically. The influence of the metal waveguide walls on a single-surface multipactor in a parallel-plate waveguide is analyzed. It is shown that, at a sufficiently high microwave intensity, a combined single + double-surface multipactor develops in the region adjacent to the dielectric window.

Results from solving a full-wave two-dimensional model problem on the propagation and absorption of a microwave beam in plasma for the magnetic configuration of the L-2M stellarator are presented. The coefficients of transmission and reflection are calculated. The distributions of the microwave power absorbed in plasma are obtained. It is found that, under conditions typical of L2-M experiments on ECR plasma heating at the second harmonic of the electron gyrofrequency, an appreciable fraction (about 10%) of the incident microwave power can be deflected downward from the plasma axis, not reaching the absorption region. The fraction of the downward-deflected microwave power is shown to increase considerably at central plasma densities close to the cutoff density (ne ≈ (0.8–0.9)ncut). The simulation results are compared with results of calculations by the ray-tracing method.

Results are presented from experimental and theoretical studies of microplasma discharges excited by a plasma flow on constructional metals partially covered with a dielectric (oxide) film. Tribological tests of metal samples treated by microplasma discharges show a significant increase in their wear resistance due to the formation of a strong microrelief on the sample surface.

Compact expressions are derived for the nonlocal permittivity tensor of weakly relativistic plasma in a 2D nonuniform magnetic field near the resonances at the second harmonic of the electron cyclotron frequency for an extraordinary wave and at the first harmonic for an ordinary wave. It is shown that the wave equation with allowance for the obtained thermal correction to the permittivity tensor in the form of a differential operator in transverse (with respect to the external magnetic field) coordinates possesses an integral in the form of the energy conservation law.

An original 2D3V (two-dimensional in coordinate space and three-dimensional in velocity space) particle-in-cell code has been developed for simulation of multipactor discharge on a dielectric in a parallelplate metal waveguide with allowance for secondary electron emission (SEE) from the dielectric surface and waveguide walls, finite temperature of secondary electrons, electron space charge, and elastic and inelastic scattering of electrons from the dielectric and metal surfaces. The code allows one to simulate all stages of the multipactor discharge, from the onset of the electron avalanche to saturation. It is shown that the threshold for the excitation of a single-surface multipactor on a dielectric placed in a low-profile waveguide with absorbing walls increases as compared to that in the case of an unbounded dielectric surface due to escape of electrons onto the waveguide walls. It is found that, depending on the microwave field amplitude and the SEE characteristics of the waveguide walls, the multipactor may operate in two modes. In the first mode, which takes place at relatively low microwave amplitudes, a single-surface multipactor develops only on the dielectric, the surface of which acquires a positively potential with respect to the waveguide walls. In the second mode, which occurs at sufficiently high microwave intensities, a single-surface multipactor on the dielectric and a two-surface multipactor between the waveguide walls operate simultaneously. In this case, both the dielectric surface and the interwall space acquire a negative potential. It is shown that electron scattering from the dielectric surface and waveguide walls results in the appearance of high-energy tails in the electron distribution function.

Results are presented from experimental and analytical studies of the processes resulting in the excitation of microplasma discharges (MPDs) on a metal surface partially covered with a thin dielectric film under the action of an external plasma flow in vacuum. It is shown experimentally that MPDs are excited at the interface between the open metal surface and the region covered by the dielectric film. The probability of MPD excitation is investigated as a function of the thickness of the dielectric film deposited on the metal. It is found that, for a film thickness of 1 μm, the probability of MPD excitation is close to unity. As the film thickness decreases below ~10 nm or increases above ~10 μm, the probability of MPD excitation is reduced by more than two orders of magnitude. A two-dimensional kinetic numerical code is developed that allows one to model the processes of Debye sheath formation and generation of a strong electric field near the edge of a finite-thickness dielectric film on a metal surface in a plasma flow for different configurations of the film edge. It is shown that the maximum value of the tangential component of the electric field is reached at the film edge and amounts to E max ≈ |φ0|/2d (where φ0 < 0 is the electric potential applied to the metal and d is the film thickness), which for typical conditions of experiments on the excitation of MPDs on metal surfaces (φ0 ≈–400 V, d ≈ 1 μm) yields E max ≈ 2 MV/cm. The results of kinetic simulations confirm the qualitative idea about the mechanism of the formation of a strong electric field resulting in the excitation of MPDs at the edge of a dielectric film on a metal surface in a plasma flow and agree with experimental data.

Reflection and backscattering of high-power (400 kW) gyrotron radiation creating and heating plasma at the second harmonic of the electronic cyclotron frequency in the L-2M stellarator have been investigated experimentally. The effect of the displacement of the gyroresonance region from the axis of the plasma column under doubling of the plasma density on the processes of reflection and backscattering of microwave radiation has been examined. A near doubling of short-wavelength ( k ⊥ ≈ 30 cm –1 ) turbulent density fluctuations squared is observed. The change in the energy confinement time under variations of plasma parameters and characteristics of short-wavelength turbulence is discussed. A discrepancy between the measured values of the reflection coefficient from the electron cyclotron resonance heating region and predictions of the one-dimensional model is revealed.

Excitation of microwave discharges by pulsed microwave radiation (≤2 MW, 1.95 GHz, 1-10 μs) on dielectric surfaces in vacuum (10-6 torr) was studied experimentally. Different stages of a surface microwave discharge were observed: secondary-electron-emission microwave discharge (multipactor), surface microwave breakdown (filamentary microwave discharge), and plasma-flare microwave discharge. It is found that, in the stage of microwave breakdown (which lasts for ~0.1 μs), ≥70% of the incident microwave power is absorbed by a dense plasma filament with a diameter of ~100 μm, an electron density of up to 2 × 1018 cm-3, and an electron temperature of about 2 eV. Strong interaction of the dense plasma of the filamentary microwave discharge with the dielectric leads to local destruction of the dielectric surface in the form of a long thin erosion track (l ≈ 6 cm, d ~ 100 μm). The coefficient of microwave power absorption by a plasma filament in a rectangular waveguide is calculated using an electrodynamic model that does not involve expansion in the waveguide modes and takes into account reflections of the scattered wave from the waveguide walls by introducing an array of filament mirror images. It is shown that the coefficient of microwave power absorption by the filamentary discharge plasma can reach 70% and more, which agrees well with the experimental data.

Reflection of the heating extraordinary microwave incident obliquely onto the surface of the electron cyclotron resonance (ECR) at the second harmonic of the electron gyrofrequency in the 3D magnetic configuration of the L-2M stellarator was studied experimentally. The plasma was heated using two gyrotrons with a total power of 600–700 kW, the specific heating power being 2.4–2.8 MW/m3. The displacement of the ECR region in the course of heating was monitored by measuring the phase of the reflected extraordinary wave. It is found that the growth of the plasma density is accompanied by the displacement of the ECR heating region from the center of the plasma column toward its periphery. The coefficient of reflection of the heating microwave beam from the ECR region was measured. The spectra of short-wavelength (k s ≈ 30 cm−1) plasma density fluctuations were explored by analyzing backscattered microwave radiation. A tenfold increase in the energy of short-wavelength density fluctuations and the growth of the spectral density of fluctuations in the frequency range of 0.3–1.5 MHz were observed.

In experiments on electron cyclotron resonance (ECR) heating of plasma at the second harmonic of the electron gyrofrequency in the L-2M stellarator, the effect of partial reflection of high-power gyrotron radiation from the ECR heating region located in the center of the plasma column was revealed. The reflection coefficient is found to be on the order of 10 −3 . The coefficient of reflection of an extraordinary wave from the second-harmonic ECR region is calculated in the one-dimensional full-wave model. The calculated and measured values of the reflection coefficient are found to coincide in order of magnitude.

Multipactor discharge on a dielectric is studied numerically and analytically for different inclination angles α of the microwave electric field with respect to the dielectric surface. The power absorbed in the discharge is calculated, and analytic estimates for the average current density of secondary electrons and the average energy of electrons bombarding the dielectric surface are obtained as functions of the angle α and the electron oscillation energy in the microwave field. It is found that the dependence of the absorbed power on the inclination angle of the external microwave field has a minimum at α ∼20°–30°.

The coefficient of microwave power absorption by a single-sided multipactor discharge on a dielectric surface is studied analytically and numerically as a function of the incident microwave power. It is shown that taking into account electron reflections from the dielectric surface leads to a substantial increase in the absorption coefficient. The analytical and numerical results are compared with experimental data.

The interaction of microplasma discharges with samples made of VT1 commercial titanium was studied experimentally. The amplitude of the current pulses of microplasma discharges was 200 A, the pulse duration was 20 ms, and the number of current pulses was from 1 to 10. After microplasma processing, a 10-μm-thick solid remelted surface layer with an increased hardness formed on the titanium samples. As compared to the initial state of titanium samples, the surface layer hardened by microplasma discharges possesses improved parameters: the microhardness increases fivefold, the maximum admissible pressure applied to the samples during friction increases more than 20-fold, the friction wear rate is reduced by three orders of magnitude, and the friction coefficient decreases sixfold.

Excitation of microplasma discharges on the surfaces of V95 aluminum alloy samples placed in a uniform pulsed plasma flow was studied experimentally. Strong localized interaction of microplasma discharges with aluminum leads to the melting and subsequent fast cooling of micrometer-size regions on the sample surface. Due to the multiple action of microplasma discharges, a continuous remelted layer with a thickness of up to 20 μm forms on the aluminum surface. The physical, structural, and tribotechnical properties of this layer differ substantially from those before microplasma processing.

Experiments with the limiting density of a plasma generated in the Globus-M spherical tokamak operating in the ohmic heating regime show that, when the plasma density reaches a certain value at the center of the plasma filament, snake-type strong magnetohydrodynamic instability exciting the m/n = 1/1 mode arises. A further rise in the density causes an internal disruption of the plasma. Numerical simulation of the instability dynamics is performed, and the results of simulation are compared with experimental and theoretical data. Experimental and theoretical data are in good agreement. It is shown that the instability is associated with an increased amount of impurities in the plasma and that the limit in density is of a “technological,” rather than of a fundamental, character. With the vacuum chamber cleaned more thoroughly, this limit disappears and the snake instability is not observed. Accordingly, a plasma filament density of 1 × 10 20 m−3 was attained with a Greenwald number as high as 95%.