The results of studying the self/non-self-sustained gas discharge (SNSS discharge) excited by microwave beams at high gas pressure (atmospheric or higher than atmospheric) under conditions of a ...substantially subthreshold radiation power are presented. The discharge has a number of properties that distinguish it from all microwave discharges described in literature. The physical model of the discharge built at the Prokhorov General Physics Institute, Russian Academy of Sciences, describes its excitation at pressures high enough that the frequency of electron-neutral collisions is appreciably higher than the cyclic frequencies of microwaves, and the microwave beam power is much lower than the threshold for the excitation of a self-sustained plasma formation. For the first time, the article presents the results of “shadow” photography demonstrating one of the main properties of discharge—its structure, which is a set of “microexplosion” formations moving against the microwave radiation.
The formalism of compound Cox processes was used for statistical analysis of turbulent density fluctuations of plasma during a transient process at electron cyclotron resonance heating of L-2M ...stellarator plasmas. Short-wavelength (k = 20-30 cm−1) density fluctuations in the central region of the plasma column were measured by the collective backscattering technique. The transient process was caused by sputtering of wall coating and subsequent pulsed impurity injection into the plasma after a step-like increase of the heating power. The evolution of the following parameters was studied: probability density function (PDF) and dynamic and diffusive components of variance of density fluctuations increments. It was found that density fluctuations increments increase simultaneously with average electron density increase and electron temperature decrease during the transient process. It was shown that the PDF of fluctuation increments is characterized by the existence of heavy tails and it can be represented as a composition of four normal laws. Deviation from normal law that happens in the form of random bursts was observed. Duration and frequency of the bursts increases with the level of fluctuation increments and this increase is accompanied by the increase of both components of the variance.
The results are presented from studies of the extraordinary wave (X-wave) absorption in plasma of the L-2M stellarator, as well as the power loss of microwave beam introduced into the plasma. The ...X-wave occurs as a result of splitting the linearly polarized gyrotron radiation, as it intersects the plasma boundary. Different electron cyclotron resonance heating (ECRH) regimes were studied: heating by a sequence of microwave pulses and heating that involves an auxiliary ECRH pulse. It was ascertained that for the X-wave, the single-pass absorption coefficient along the central chord of the plasma cross section is higher than 95%, and in the axial ECRH regime, the power losses correspond to the O-wave power. It was ascertained that the power loss of the beam inputted into the plasma increases up to 20–23% when the gyroresonance region was displaced towards the inner wall of the vacuum chamber and the ECRH pulse duration was increased. At the same time, the radiation loss drastically increased. It was hypothesized that an increase in the power loss is caused by a decrease in the X-wave absorption coefficient integrated over the beam cross section, which occurs due to a decrease in the electron temperature across the microwave beam cross section. In the regime with the auxiliary ECRH pulse, the pulsed sputtering of the vacuum chamber coating occurs, and the electron density and temperature change; nevertheless, the absorption coefficient along the central chord exceeds 90%.
At the specific power of electron cyclotron resonance (ECR) heating of 3.2 MW m
–3
(plasma density of 2 × 10
19
m
–3
, electron temperature of 0.6 keV), an increase in the plasma energy lifetime by ...not less than 30% is accompanied by a two-time-decrease in the level of short-wave turbulent density fluctuations. In such a shot, before the beginning of the quasi-stationary confinement stage, the turbulent state of density fluctuations is characterized by the stronger deviation from zero of the coefficient of excess of fluctuation increments than it is in shots without transport transitions. This indicates the stronger deviation of the probability distribution function of density fluctuation increments from the normal law in shots with transport transitions. Based on the analysis of increments of short-wave fluctuations using the special method for separating the continuous components in stochastic processes, a qualitative difference was established between the behaviors of the structural components forming the plasma turbulence in shots with and without transport transitions. In addition, for shots with transport transitions, a change in the shape of the approximating finite mixture of normal distributions and parameters of its component densities is demonstrated.
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A microwave beam (wavelength 4 mm, power 150–300 kW) generates an ionization wave in air in the subthreshold field in the shape of a conglomerate of thread-like channels that moves toward the beam. ...The periphery of the beam refracts in the plasma of the non-self-sustained discharge in the UV halo of the thread-like discharges and self-focuses in the recombining plasma behind the front of the plasmoid that consists of thread-like channels. The self-focusing of the peripheral regions of the wave beam causes local bursts of ionization behind the front of the leading plasmoid.
Frames of a subthreshold microwave SNSS discharge were obtained in experiments. They showed that, behind the front of the head plasmoid with a fine-cell glow structure, there is an extended region of ...diffuse glow. In this region, separate localized luminous formations are also found. Photodetectors in the ultraviolet range (240–360 nm) recorded the glow of pulses arising on the path of the microwave beam behind the head plasmoid of the discharge. The observed effect can be qualitatively explained by the local excitation of glow in a microwave field under self-focusing of an annular wave beam behind the head plasmoid. A decrease in the transmitted radiation intensity behind the discharge at the axis of the microwave beam accompanied by an increase in the length of the region behind the front of the head plasmoid is established.
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A layer of regolith with a 1.35% Al content was irradiated from the side of the reactor supporting plate with a microwave pulse with an intensity of 10–15 kW cm
–2
(the wavelength is 4 mm) and a ...duration of 1.5 ms and 3.5 ms. An air rarefaction of ~76 Torr was created in the reactor. A gas discharge occurred when the ratio
E
0
/
N
0
= 1.03 × 10
–15
V cm
2
was exceeded (
E
0
is the electric field at the reactor axis,
N
0
is the concentration of molecules in air). The discharge above the regolith surface occurred only after microwave breakdown of the air with a delay of more than 200 μs. In this case, evaporation of regolith particles and their ejection into the reactor volume took place. It was found that an increase in the microwave pulse duration to 3.5 ms leads to long-term sparking of regolith particles. This can be explained by the energy release during the reaction 2Al + 3FeO → Al
2
O
3
+ 3Fe initiated by the evaporation of iron oxide under the combined action of microwave radiation and UV radiation from the gas discharge.
An analysis of a set of experiments on studying subthreshold microwave self/non-self-sustained (SNSS) discharges made it possible to estimate the volume of the active zone of self-sustained ...thread-like discharges resulting from the development of the ionization–overheating instability. It is shown that this volume is 0.04–0.1 of the wave beam volume. It is shown that in the processes of the plasma-chemical action of the SNSS discharge, mixing of the substance reacted in the active regions with the gas outside the active regions increases the cost of the plasma-chemical process. A decrease in the energy cost of plasma-chemical processes requires an increase in the ratio of the microwave beam volume to the plasma-chemical reactor volume. A scheme of a flow-through reactor with a multimode waveguide is proposed and the possible performance of such a design in the destruction of harmful impurities in gas mixtures is estimated.
At microwave beam intensities in the range of 3.3–4.8 kW/cm
2
, changes in the characteristics were recorded of the subthreshold self-non-self-sustained microwave discharge (SNSS discharge) in air at ...atmospheric pressure. Qualitative changes were observed in the discharge glow structure when going over from higher to lower intensities: in the discharge head, the structure in the form of a set of contracted channels disappears and there appears the inhomogeneous diffuse glow along the entire discharge length. The changes in the discharge structure occurring as the beam intensity decreases from 4.8 to 3.3 kW/cm
2
are accompanied by a decrease in the propagation velocity of the discharge front from 1.7 × 10
3
to 2.8 × 10
2
cm/s, that is, the quadratic dependence of the discharge front velocity on the microwave intensity becomes violated. In this case, behind the discharge front, a decrease in the fluctuation level of the UV radiation intensity was also recorded.
The subthreshold self-non-self-sustained discharge in air was studied as the basis of the system for cleaning urban air from ecologically harmful admixtures. A cycle of experiments was carried out ...and the efficiency of the destruction of small amounts of hydrogen sulfide mixed with air (about 8 ppm) introduced into the processed air volume was determined. Experiments demonstrated a relatively high degree of H
2
S decomposition (its concentration decreased 41.5 times at unit energy deposition of 8 J/cm
3
in the SNSS discharge).
