In this study, correlation reception of thermal acoustic radiation by a pair of sensors was carried out. The experiment used receivers with different bandwidths, varied the size of the heated sources and the distance from the sources to the receivers, and also shifted the sources in the transverse direction perpendicular to the acoustic axis of the system. For each case, applying the relations used in radio astronomy, the correlation functions of thermal acoustic radiation were calculated. It is shown that the experimentally obtained and calculated cross-correlation functions are close, taking into account the measurement error.

Контроль внутренней температуры активных элементов (АЭ) мощных лазеров необходим для их безопасной работы. В статье описана методика и устройство для мониторинга внутренней температуры АЭ лазеров. Измерения основаны на импульсном ультразвуковом (УЗ) зондировании и зависимости от температуры скорости звука в материале АЭ. Изменение скорости звука приводит к изменению фазы УЗ сигнала, прошедшего через объект, которое регистрируется описываемым устройством. Представлены результаты мониторинга температуры АЭ с помощью ультразвукового зондирования в процессе работы действующей лазерной установки.

In the developed waveguide plasmatron with combined gas influx, a nonequilibrium discharge maintained at atmospheric pressure by continuous microwave gyrotron radiation at 24 GHz was obtained and studied for the first time. It is shown that in the regime of argon discharge stabilization by carbon dioxide flow the temperature of electrons is 0.2-0.3 eV at gas temperature 1200-1500 K and the electron density does not exceed the cutoff value for the heating frequency in a wide power range.

For the first time, experiments were carried out on the decomposition of carbon dioxide in a plasma torch maintained by continuous focused gyrotron radiation with a frequency of 263 GHz in an argon flow at atmospheric pressure. It is shown that, despite the decrease in the electron density by a factor of 5, when 3% carbon dioxide is added to the plasma-forming gas, it is possible to achieve its degree of conversion of 22% due to the nonequilibrium nature of maintaining the discharge. Thus, the prospects of using high-power electromagnetic radiation in the sub-terahertz range for solving plasma-chemical problems of the decomposition of highly stable molecules have been demonstrated. Keywords: microwave discharge, nonequilibrium plasma, gyrotron, subterahertz radiation, CO2 decomposition.

This paper presents the results of an experimental study of the spatial structure of a microwave discharge maintained in an argon flow by gyrotron radiation in a continuous mode with a frequency of 24 GHz at atmospheric pressure. In the structure of the plasma plume, stationary filamentary channels are observed, elongated along the direction of the argon flow, regardless of the orientation of the external electric field of the wave, surrounded by a diffusion halo. Measurements of the electron density, vibrational and rotational temperatures of gas molecules in plasma filaments have been carried out. The role of gas-dynamic mechanisms responsible for the formation of the inhomogeneous static structure of the plasma torch and the maintenance of a substantially nonequilibrium distribution of temperature characteristics in the discharge is discussed.

The possibility of converting methane into hydrogen using a high-frequency induction plasma torch at atmospheric pressure has been experimentally studied. The dependencies of the degree of methane conversion and the rate of hydrogen production were studied depending on the process conditions. It has been demonstrated that the degree of conversion of methane to hydrogen can reach values close to 100%.

A method of obtaining highly dispersed tin (II) oxide (SnO) powder by the evaporation-condensation method was investigated when the initial material was heated by focused radiation of a subterahertz gyrotron with a frequency of 0.26 THz and a power of 1 kW. The process was carried out with a purge with nitrogen inert with respect to SnO. A powder with a SnO weight content of 92% was obtained. The specific surface was 7.20 m2/g, which corresponds to an average particle size of 130 nm. The characteristics of the obtained powder are analyzed. The applicability of the experimental setup for the production of nanopowders from compounds requiring special gas conditions during the evaporation-condensation process has been demonstrated.

The temperature dependences of the absorption coefficient of Al2O3, ZnO, TiO2 powders at a frequency of 527 GHz are investigated. A gyrotron with an output power of up to 240 W in CW mode was used as a radiation source for heating the samples. The power absorbed in the samples was measured in the temperature range from 300 to 1000 ° C (for some samples). Compared with experiments at a frequency of 263 GHz, a significant increase (by a factor of 4 - 6) in the absorption coefficient is observed. The results may be of interest to researchers involved in microwave heating of materials.

Controlling deep temperature in parts of the human body is necessary for hyperthermia and thermal ablation used in oncology. UHF heating of single hands of patients was chosen as the model for this procedure. For the control, a plastisol cylinder was heated in the same manner: a substance with acoustic and thermophysical properties close to those of soft tissues of the human body. Passive acoustic thermometry was used to measure the deep temperature of the hand, and infrared thermometry was used to measure the surface temperature. After 5 min of UHF heating, the deep temperature of the hand increased by an average of 0.7 ± 0.6°C, and the surface temperature, by 0.8 ± 0.6°C. The same methods, as well as independent measurements, were used to determine the plastisol temperature. After the same procedure, the deep temperature of the plastisol increased by 4.3 ± 0.4°C; the surface temperature, by 3.2 ± 0.2°C; the temperature measured with a thermometer at the center of the object, by 3.3 ± 0.5°C. The smaller heating of the hand compared to the model object is related to effect of blood flow, which should be adequately taken into account in further studies. The noninvasive methods indicated in the study can be used to control temperature in oncology during hyperthermia and thermal ablation under the effect of a high-frequency electromagnetic field.

A method for the preparation of a highly dispersed powder of tin monoxide (SnO) by the evaporation/condensation method under heating the initial material with focused radiation of a subterahertz gyrotron with a frequency of 0.26 THz and a power of 1 kW is investigated. The process is carried out with a purge with nitrogen that is inert with respect to SnO. A powder with a SnO content of 92 wt % is obtained. The specific surface area of the powder is 7.20 m 2 /g, which corresponds to an average particle size of 130 nm. The characteristics of the obtained powder are analyzed. The applicability of the experimental setup for the production of highly dispersed powders from compounds that require special gas conditions in the evaporation/condensation process is demonstrated.

The paper describes the features of the metal oxide nanopowder obtaining by an evaporation-condensation method with pulsed heating of the material by focused subterahertz radiation. It has been experimentally shown that optimal conditions for the rapid evaporation of a substance are achieved at the highest possible pulse power and the lowest possible duty cycle at a fixed average power. Under these conditions, it is possible to realize the heating and evaporation of a substance in a focused radiation beam in a relatively short time, while the adjacent layers of the substance maintain a relatively low thermal conductivity. A linear increase in the evaporation rate with a decrease in the duty cycle of heating pulses was demonstrated, the increase reached 5 times relative to the continuous heating mode.

In this study, we used passive acoustic thermometry to measure the core (acoustic brightness) temperature in a subject’s forearm while applying warming ointments to the skin. A decrease of 3–6°C in the acoustic brightness temperature was recorded in the first 5–10 min. After this, the acoustic brightness temperature returned to the level prior to application of the ointment. We attribute the decrease in core temperature to the use of a gel at room temperature in the measurements. On the one hand, the application of a cold gel to the skin should reduce the superficial blood flow. However, the use of warming ointments temporarily blocked this process. As a result, cold blood from near-surface capillaries entered the internal tissues of the forearm, cooling the tissue. The effect was taken into account in the heat conduction equation with blood flow by changing a parameter used in the equation: the temperature of flowing blood. The time dependence of the acoustic brightness temperature calculated with the heat conduction equation is consistent with the experimental data.

The 1D internal (core) temperature profiles for the model object (plasticine) and the human hand are reconstructed using the passive acoustothermometric broadband probing data. Thermal acoustic radiation is detected by a broadband (0.8–3.5 MHz) acoustic radiometer. The temperature distribution is reconstructed using a priori information corresponding to the experimental conditions. The temperature distribution for the heated model object is assumed to be monotonic. For the hand, we assume that the temperature distribution satisfies the heat-conduction equation taking into account the blood flow. The average error of reconstruction determined for plasticine from the results of independent temperature measurements is 0.6 K for a measuring time of 25 s. The reconstructed value of the core temperature of the hand (36°C) generally corresponds to physiological data. The obtained results make it possible to use passive broadband acoustic probing for measuring the core temperatures in medical procedures associated with heating of human organism tissues.

The time-varying temperature profiles were reconstructed in an experiment using a thermal acoustic radiation receiving array containing 14 sensors. The temperature was recovered by performing similar experiments using plasticine, as well as in vivo with a human hand. Plasticine preliminarily heated up to 36.5°C and a human hand were placed into water for 50 s at a temperature of 20°C. The core temperature of the plasticine was independently measured using thermocouples. The spatial resolution of the reconstruction in the lateral direction was determined by the distance between neighboring sensors and was equal to10 mm; the averaging time was 10 s. The error in reconstructing the core temperature determined in the experiment with plasticine was 0.5 K. The core temperature of the hand changed with time (in 50 s it decreased from 35 to 34°C) and space (the mean square deviation was 1.5 K). The experiment with the hand revealed that multichannel detection of thermal acoustic radiation using a compact 45 × 36 mm array to reconstruct the temperature profile could be performed during medical procedures.