Effect of an Electric Field on an Optical Breakdown in the Air Stream

Experimental data on the effect of an electric field on the plasma of an optical discharge in an air flow have been obtained. Two configurations of an external electric field under the action of an optical discharge on the plasma are considered. To create an electric field, flat (the field along the beam and across the flow) and ring electrodes (the field across the laser beam and along the flow) were used. It was found that there are two modes of combined discharge (optical and electrical). When the field was created symmetrically with respect to the flow axis, an electrical breakdown was observed from the nozzle exit (positively charged electrode) to the focusing point of the laser beam, while no streamers were observed in the optical discharge wake. In another case, an electric discharge is realized between flat electrodes simultaneously with optical breakdown. In a field of constant strength above 3 kV/cm, the presence of an optical discharge plasma promoted electrical breakdown of the medium. In this case, the parameters of the electrical breakdown depended on the shape of the electrodes, the polarity of the applied voltage, and the air flow rate.

Влияние внешнего электрического поля на оптический разряд в скоростном потоке

The influence of an electric field on the plasma of an optical discharge in subsonic and supersonic air flows has been studied experimentally. The presence of a weak electric field practically does not affect the size of the plasma formation, but, regardless of the configuration of the field lines and the polarity of the applied voltage, it leads to a decrease in the probability of optical breakdown. The experiment has shown that the plasma created by focused laser radiation is very sensitive to the presence of an electric field. When a voltage exceeding 22 kV was applied to the ring electrodes, powerful quasi-stationary streamers were formed in the flow. The presence of an optical discharge plasma made it possible to create an electric discharge in fields with an intensity below the breakdown threshold of the medium. The effect of quenching and the processes of development of an optical discharge were studied depending on the speed and characteristics of the electric field. Quenching of the optical discharge was observed when a voltage of 22 kV and higher was applied. Despite the preservation of the geometric dimensions of the optical discharge, the high-temperature region in the flow can be increased by using electric streamers. This leads to an increase in the energy supplied to the flow, and thus allows combustion to be initiated and flame stabilized at higher flow rates. Key words: experimental modeling, laser radiation, optical breakdown, electric field, electric discharge, sub- and supersonic air flow.

Comparative study on pharmacokinetics and toxicity of intravitreal and sub-Tenon injection of triamcinolone acetonide in ocular tissues

AIM: To compare the differences in kinetics, distribution, and toxicity of triamcinolone acetonide (TA) between the injection methods, sub-Tenon and intravitreal injections in rabbit ocular tissues. METHODS: TA was injected into the vitreous or the sub-Tenon in rabbits. For pharmacokinetic study, rabbits were sacrificed periodically and then TA in blood and ocular tissues (retina/choroids, vitreous, and aqueous humor) were measured over 91d. For toxicological study, clinical signs, slit-lamp microscopic examination, ophthalmological test were performed. The eyeballs and surrounding tissues were collected and fixed with glutaraldehyde-formalin solution, and then paraffin embedded for histological investigation. RESULTS: Higher levels of TA were distributed in the intraocular tissues when injected into the vitreous compared to the sub-Tenon. Conversely, TA level was remarkably lower in the rabbits which received intravitreal TA injections than those treated with sub-Tenon injection throughout the study period in plasma. Optical discharge probably caused by systemic circulation of TA was observed by receiving sub-Tenon TA injection. Meanwhile, technic-associated toxicological ocular symptoms and findings were more frequently observed in intravitreal injection than in sub-Tenon injection. CONCLUSION: There are significant differences in kinetics and distribution of TA in vitreous body, aqueous humor and plasma, between the two injection methods. Although further study is needed to explain the species difference between human and rabbit, it is assumed that the difference in the frequency of intraocular pressure elevation and cataract formation by TA between the two injection methods are directly related to the TA concentrations in aqueous humor and vitreous body in each injection methods. Systemic toxicity and technic-associated toxicity are also closely related to kinetics of TA in plasma and each injection method itself, respectively.

Internal Surface Plasmon Excitation as the Root Cause of Laser-Induced Periodic Plasma Structure and Self-Organized Nanograting Formation in the Volume of Transparent Dielectric

A computer simulation of the dynamics of an optical discharge produced in the volume of a transparent dielectric (fused silica) by a focused femtosecond laser pulse was carried out taking into account the possibility of developing small-scale ionization-field instability. The presence of small foreign inclusions in the fused silica was taken into account with the model of a nanodispersed heterogeneous medium by using Maxwell Garnett formulas. The results of the calculations made it possible to reveal the previously unknown physical mechanism that determines the periodicity of the ordered plasma-field structure that is formed in each single breakdown pulse and is the root cause of the ordered volume nanograting formation in dielectric material exposed to a series of repeated pulses. Two main points are decisive in this mechanism: (i) the formation of a thin overcritical plasma layer at the breakdown wave front counter-propagated to the incident laser pulse and (ii) the excitation of the “internal surface plasmon” at this front, resulting in a rapid amplification of the corresponding spatial harmonic of random seed perturbations in the plasma and formation of a contrast structure with a period equal to the wavelength of the surface plasmon (0.7 of the wavelength in dielectric).

IGNITION AND STABILIZATION OF HOMOGENEOUS COMBUSTION IN HIGH-SPEED FLOW BY PULSE-PERIODIC LASER RADIATION

The paper present the results of an experimental study of the effect of focused pulsed-periodic radiation from a CO2 laser on the initiation and development of a combustion process in subsonic and supersonic flows of homogeneous fuel-air mixtures (H2 + air and CH4 + air). The radiation from the CO2 laser propagated across the stream and was focused on the jet axis. To register the flow structure, a schlieren imaging setup with a slit and a flat knife was used. The image was recorded by a high-speed camera with exposure time of 1.5 s and frame rate of 1000 fps. At the same time, spectrozonal recording (at the wavelength of OH* and CH* radiation) and emission spectroscopy (in the wavelength range of 210-780 nm) was carried out. Stable ignition of methane and hydrogen-air mixture has been obtained at supersonic outflow into the flooded space. The results of the spectrozonal registration indicate the occurrence of combustion reactions in the wake behind the optical discharge region. The analysis of the radiation spectrum of optical discharge in a supersonic flow revealed the main types of radicals present in the plasma. Strong intensity was found in the H lines, which cannot help but affect the development of the initiation and behavior of the combustion process in the wake of the optical discharge.