S Yu Kazantsev

Generation of an Electrical Signal Upon the Interaction of Laser Radiation With Water Surface

A new physical effect lying in the generation of an electrical signal (ES) upon the interaction of IR laser radiation with the water surface at a laser fluences lower than the plasma formation threshold is studied. An ES amplitude exceeding 15 V is detected. A one-to-one relationship between the observed effect and the bulk explosive boiling of water is established, and a qualitative interpretation is proposed. In the case of irradiation of an open surface, the ES is generated due to bulk explosive boiling accompanied by the evacuation and splashing of the surface layer, the destruction of the double electric layer on the surface, and the spread of an electrified vapor-drop mixture (balloelectric effect). When the surface is covered with a transparent plate, ES generation can be caused by the charge separation upon the detachment of the water surface from the plate by a vapor bubble resulting from boiling and the displacement of the charged water surface upon the expansion and contraction of the bubble.

Explosive boiling of water induced by the pulsed HF-laser radiation

The surface evaporation and explosive boiling of water induced by the radiation of a nonchain pulsed HF laser are studied using piezoelectric acoustic pressure transducers. The evolution of pressure signals is studied and the relative contributions of thermal (photoacoustic) and evaporation mechanisms to these signals are determined for a wide range of the laser energy densities. A threshold of bulk explosive boiling with respect to laser pulse energy density (W0 = 0.23 J/cm2) is determined.

Laser on single-crystal ZnSe:Fe2+ with high pulse radiation energy at room temperature

A laser on single-crystal ZnSe:Fe2+ was investigated at room temperature. Pumping of the laser was performed by a pulsed electrodischarge HF laser. In experiments, the spot diameter of HF laser radiation incident to the surface of the crystal varied from 5.6 to 17 mm. Generation energy of ~1.2 J was obtained and the efficiency with respect to the energy arriving at the crystal was η in ≈ 25%. The slope efficiency with respect to the energy absorbed in the crystal at large spot dimensions was η slope = 45%.

Ignition of a combustible gas mixture by a laser spark excited in the reactor volume

Ignition of a stoichiometric CH4: O2 mixture by a laser spark excited in the reactor volume is studied experimentally. It is found that the spark initiates a feebly radiating incomplete-combustion wave, which is much faster than the combustion wave, but is substantially slower than the detonation wave. With a time delay of 500–700 μs, a bright optical flash occupying the entire chamber volume is observed, which indicates fast (involving branching chain reactions) ignition of the gas mixture. A conclusion is drawn regarding the common nature of the process of ignition of a combustible gas mixture by a laser spark excited in the reactor volume and the previously investigated initiation of combustion by laser sparks excited at solid targets, high-power microwave discharges, and high-current gliding discharges.

Ion — ion recombination in SF6 and in SF6 — C2H6 mixturesfor high values of E/N

Ion—ion recombination coefficients in a decaying SF6 plasma and in SF6 — C2H6 mixtures are measured in the pressure range 15 — 90 Torr for reduced field strengths 100 — 250 Td. The charge composition is analysed and dominating ion — ion recombination channels in such plasmas are determined. Relations for estimating the potential drop at the electrodes are obtained for decaying plasma in strongly electronegative gases. The results of measurements are extrapolated for estimating the ion — ion recombination coefficient in SF6 for nearly critical field strengths. It is concluded that the ion — ion recombination should be taken into account in calculations of the discharge characteristics in non-chain reaction HF lasers.

High-energy room-temperature Fe2+:ZnS laser

Characteristics of a room temperature laser on polycrystalline ZnS:Fe2+ subjected to diffuse doping from two sides were investigated. The sample was pumped by a non-chain electrodischarge HF laser with the FWHM duration of the radiation pulse of ~140 ns. The diameter of the pumping radiation spot on the surface of the crystal was 3.8 mm. Further increases in the size of the pumping spot were limited by parasitic generation arising due to a high concentration of Fe ions in the near-surface layer of the sample at a relatively small depth of doping (short length of active medium). The generation energy of 25.5 mJ was obtained at a slope efficiency of 12% with respect to the energy incident on the sample. Characteristics of lasers on polycrystalline ZnS:Fe2+ and ZnSe:Fe2+ have been compared in equal pumping conditions. The slope efficiencies of ZnSe:Fe2+ and ZnS:Fe2+ lasers with respect to the absorbed energy were 34% and 20%, respectively. At equal pumping energy absorbed in the samples, the duration of the ZnSe:Fe2+ laser radiation pulse was longer than that of the ZnS:Fe2+ laser. Possibilities of increasing the efficiency of ZnS:Fe2+ laser operation at room temperature by improving the technology of sample manufacturing and reducing the duration of the pumping pulse are discussed in this letter.

A nonequilibrium plasma accompanying the ignition of methane-oxygen mixtures

Measurements of electron density and electron‐neutral collision frequency in combustion-produced plasmas in methane‐oxygen stoichiometric mixtures are presented. The electron density in the developed flame was found to be at a level of ne ≈ (2‐6) × 10 12 cm −3 , which exceeds the values obtained or predicted in the previous experimental and theoretical works. The collision frequency between electron and neutral components is νen ≈ 10 12 s −1 , thus exceeding that in the preflame mixture by more than an order of magnitude. Based on the calculations performed within the framework of a multicomponent kinetic scheme the most important processes of chemi-ionization in methane‐oxygen flames are determined. It is shown that the maximum electron density in the developed flame is close to ∼3 × 10 12 cm −3 , which is in reasonable agreement with the measured values. (Some figures may appear in colour only in the online journal)

A self-sustained volume discharge in vibrationally excited strongly electronegative gases

This paper reports on the first experimental investigations into the physics of a self-sustained volume discharge (SSVD) in strongly electronegative gases on their vibrational excitation by irradiation of TEA CO2 laser. Examined are pure SF6, mixtures of SF6 with He, Ne and C2H6, and pure C2HCl3. In these gases, the discharge voltage and current oscillograms have been obtained for different delays between the laser and voltage applications, and on varying the laser fluences over a wide range. The observed effects of increasing the burning voltage and decreasing the discharge current in a CO2 laser-irradiated SSVD are treated in terms of enhanced electron attachment due to electron capture by vibrationally excited molecules of SF6 and C2HCl3. For the first time the effect of radically affecting a spatial electric current distribution over the discharge gap in strongly electronegative gases by irradiation of a specially profiled CO2 laser beam is found. Starting from this finding, the possibility of performing laser-controlled SSVD spatial structures in SF6 and SF6-based mixtures has been experimentally demonstrated. The problem of stability of a laser-irradiated SSVD in strongly electronegative gases is also touched upon.

Electron detachment instability and self-organization in strongly electronegative polyatomic gases

This paper reports on an investigation of the ionization instability and related phenomena in a self-sustained volume discharge (SSVD) in SF6-based mixtures pre-irradiated by a pulsed CO2 laser. The illumination geometry with a diaphragm slit arranged normal to the central electric field line of the discharge gap is considered. The delay times between the laser and voltage applications were varied from a few to several tens of microseconds. The formation and some special features of quasi-periodic filamentary structures in laser-heated SF6-based mixtures at different delays are studied. It is hypothesized that the self-organization observed is caused by the ionization instability developed in the SSVD plasma due to an imbalance between the rates of electron detachment by electron impact and electron–ion recombination. The mathematics of this instability in a zero-dimensional approximation and the characteristic time of its development are given. An imbalance-caused mechanism of conducting channel propagation in SF6-based mixtures at room temperature under the conditions approaching those of the HF laser performance is suggested. It starts from an appreciable decrease in electron–ion recombination rate in an enhanced electric field in the vicinity of the channel tip. The effect of a two-step ionization is assessed.

Room-temperature laser on a ZnSe : Fe2+ polycrystal with undoped faces, excited by an electrodischarge HF laser

Characteristics of a laser on a ZnSe : Fe2+ polycrystalline active element with undoped faces (the concentration of Fe ions was maximal inside the crystal and zero at the faces) were studied. The laser was pumped by a non-chain electrodischarge HF laser at room temperature of the crystal. The active element was fabricated by the method of diffuse doping, which prevented the iron film newly deposited to a ZnSe substrate from interacting with atmospheric air (moisture and oxygen) and hindered the subsequent penetration of oxygen into the ZnSe matrix in the course of the high-temperature annealing of the sample. This method was used instead of those employed earlier for doping polycrystalline samples; it noticeably increased the efficiency and generation energy of a ZnSe : Fe2+ laser at room temperature of the crystal. The generation energy was 298 mJ at the slope efficiency of η slope = 45% and the total efficiency with respect to the absorbed energy of η abs = 40%.