K N Firsov

Increasing the radiation energy of ZnSe?:?Fe2+ laser at room temperature

Characteristics of the laser on a polycrystalline ZnSe?:?Fe2+ sample pumped by the radiation of a non-chain electro-discharge HF laser at room temperature are investigated. The radiation spot of the HF laser on a surface of an active element had an elliptical shape with the axes a ? b = 2.8???3.2 ? 8.25???9.5?mm. A maximal generation energy E = 192?mJ was obtained at the efficiency with respect to the energy absorbed in the sample ?abs = 23% at the dimensions of the pumping spot a ? b = 6.8???7.5?mm. In this case, a maximal slope efficiency with respect to the absorbed energy was ?sl = 29%. At the spot dimensions a ? b = 2.8???3.2?mm the slope efficiency was ?sl = 34%. Possibilities of a further increase of the generation energy of a ZnSe?:?Fe2+ laser pumped by radiation of the HF laser at room temperature are discussed.

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%.

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.

High-power nonchain HF (DF) lasers initiated by self-sustained volume discharge

Initiation mechanisms of self-sustained volume discharge (SSVD) are analyzed in mixtures of SF6 with hydro-carbons (deuterocarbons). The possibility is shown to obtain SSVD in a large volume of SF6 and hydrocarbon (deuterocarbon) mixtures without preionization in the discharge gaps possessing high edge nonuniformity of electric field. The scaling characteristics of nonchain HF (DF) laser are investigated. The highest generation energy has been achieved, namely, 397 J for HF laser and 312 J for DF laser at the electric efficiency 3.8% and 3%, respectively.

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%.

Self-initiated volume discharge in mixtures of SF6 with hydrocarbons to excite nonchain hf lasers

The paper studies a self-sustained volume discharge without preionization-self-initiated volume discharge (SIVD)--to excite non-chain HF lasers on SF6-C2H6 mixtures. Once initiated by a local discharge gap breakdown, SIVD is found to propagate then over the whole gap normally to the applied electric field through successively starting diffuse overlapping channels at a voltage close to the quasi- stationary value. With forming new channels, the current through those previously originated decreases. The volume occupied by SIVD tends to expand with increasing the energy released within the discharge plasma, whereas a discharge bounded by a dielectric surface shows a simultaneous increase both in burning voltage and current. All these features combined allow a concept to be put forward of the existence of certain restriction mechanisms depending on the specific energy released and not permitting the total deposited energy to pass through a single channel. It is suggested that SF6 dissociation by electron impact and the electron attachment to vibrationally excited SF6 molecules are just those mechanisms. Simple analytical models have been developed allowing these mechanisms to be qualitatively described.

Room-temperature laser on a ZnS:Fe2+ polycrystal with a pulse radiation energy of 0.6 J

A laser on polycrystalline ZnS:Fe2+ samples pumped at room temperature by a pulsed electrodischarge HF laser was investigated. Samples of zinc sulphide grown by the method of chemical vapour deposition (CVD) were doped with iron ions from two sides by the diffusion method. The doping of the crystals was performed using the process of hot isostatic pressing (HIP treatment) at an argon pressure of 100 MPa and a temperature of 1265 °С. A longer duration of HIP treatment resulted in a greater depth of sample doping and higher slope efficiency of the laser. For a crystal subjected to HIP treatment for 151 h, a generation energy 660 mJ was obtained. The values of the slope and total efficiency of the energy absorbed in an active element were, respectively, η slope = 36% and η abs ≈ 26%. Based on the results obtained, and data from earlier published works, a conclusion has been made that to enhance the generation characteristics of a ZnS:Fe2+ laser it is necessary to increase the temperature of the HIP treatment of CVD ZnS.

On the current density limiting effect in SF6-based mixtures

This paper reports on the investigation of the current density limiting effect in a self-sustained volume discharge in SF6-based mixtures due to a self-action of the discharge current on the media it passes through. The mechanism of this phenomenon in terms of electron-impact dissociation is examined. The argument is advanced that in the planar geometry the limiting effect treated may provoke triggering a spatially uniform volume discharge in the media stated even in the absence of pre-ionization. It is shown that the dissociation mathematics developed by the authors allows the quantitative estimation of the averaged expenditures of energy per formation of a dissociation fragment. Specifically, when applied to pure SF6 this finding enables the assessment of the energy spent for F-atom formation. For the first time the influence of current pulse duration on the discharge voltage has been experimentally revealed. Both thorough qualitative and quantitative considerations including relevant numerical simulation have been performed.

HIGH-POWER PULSE AND PULSE-PERIODIC NONCHAIN HF(DF) LASERS

This paper reports on the physics of a self-sustained volume discharge without preionization, self-initiated volume discharge (SIVD), in working mixtures of nonchain HF(DF) lasers. Dynamics of SIVD in discharge gaps of different geometry is thoroughly described. The mechanisms of restricting current density in a diffuse channel in electric discharges in SF6 and SF6 based mixtures determining the possibility of the existence of SIVD were suggested and analyzed using simple models. It is shown that the most probable mechanisms are the electron impact dissociation of SF6 and other mixture components, electron-ion recombination and electron attachment to vibrationally excited SF6 molecules. Starting from a comparison analysis of the rate coefficients of these processes, it was found that the electron-ion recombination is capable of compensating for electron detachment from negative ions by electron impact. It is established that SIVD can be observed not only in SF6, but in other strongly electronegative gases, e.g., in C3F8 and C3HCl3. Analysis is given of the factors determining uniformity of active medium in nonchain HF(DF) lasers. Some special features of operating nonchain HF(DF) lasers with small, 2 divided by 6 cm, apertures are carefully examined and the results of measuring the nonchain HF(DF) laser divergence are presented. Consideration is given to the problem increasing the aperture and discharge volume of nonchain HF(DF) lasers and, based from the experimental results, the possibility is shown of increasing their energy to a level of approximately 1 kJ and above.