A A Belevtsev

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.

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.

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.

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.

On the temperature dependence of the critical reduced electric field in SF6 and mixtures of SF6 with C2H6

This paper reports on the determination of the temperature dependence of the critical reduced electric fields (E/N)cr in SF6 and mixtures of SF6 with C2H6 at gas temperatures ranging from 293 to 1500 K. The (E/N)cr values are derived from the quasi-stationary burning voltages of a self-sustained volume discharge in these gases after their being irradiated by a pulse CO2 laser. The mechanism of a laser-induced heating of the mixtures treated, and evaluation of the gas temperatures established due to absorption of the laser energy, are described in detail. The temperature dependence of (E/N)cr established for SF6 is compared with those previously obtained in other studies. A topological approach based on Thoms catastrophe theory is involved to predict and qualitatively describe all possible structurally stable features, which can only be expected in the behaviour of (E/N)cr on changing the time of the thermal action on SF6 from vanishingly small values to infinitely large ones. On this basis, some inferences about the peculiarities of atomic fluorine generation on heating SF6 are drawn.

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.

Investigation of a self-sustained volume discharge in c-C4F8

This paper reports the first experimental study of a self-sustained volume discharge (SSVD) in c-C4F8. The discharge voltage and current oscillograms are taken over a wide range of gas pressures. For the first time an SSVD in c-C4F8 preheated by CO2-laser radiation has been investigated. Some special features and temperature-dependent characteristics of this discharge are revealed. There is discussion on the peculiarities of an SSVD in a preirradiated c-C4F8. To refine the static limiting field in c-C4F8 the static dielectric strength of c-C4F8 is measured on changing the gas pressure by nearly two orders of magnitude.

Dynamics of a self-sustained volume discharge in laser-shock-disturbed SF6-based mixtures

This paper reports on first systematic investigations into the dynamics of a self-sustained volume discharge (SSVD) in SF6-based mixtures pre-irradiated by a pulse CO2 laser. Two different illumination geometries are considered, one with a diaphragm slit arranged normal to the central electric field line of the discharge gap and the other with the discharge gap being wholly irradiated. The pattern of a laser-induced gas dynamic disturbance has been theoretically drawn starting from a shock wave theory and visualized by taking SSVD photographs. The delay times between the laser and voltage applications were varied from a few to several tens of microseconds. It has allowed us for the first time to obtain information, based on measuring the discharge voltage, about how long the mixtures considered are capable of being exposed to high temperature with no dissociation. The dynamic barrier effect found in our previous works is thoroughly investigated in both illumination geometries and the observed discharge patterns are qualitatively treated in terms of the electric field distortion by the surface charges at the shock wave fronts. The corresponding surface charge density is numerically estimated. The concept of controlling the electric field distribution over the discharge gap using the dynamic barrier effect is put forward. (Some figures in this article are in colour only in the electronic version)

Burning voltage of self-sustained volume discharge in CO 2 laser irradiated SF 6 and mixtures of SF 6 with C 2 H 6

The burning voltages of a self-sustained volume discharge (SSVD) in SF6 and SF6-C2H6 mixtures subjected to a 10.6 μm pulse CO2 laser irradiation are measured on varying the laser fluences and delays between the laser and voltage applications over a wide range. These voltages are found to considerably increase as the specific absorbed laser radiation energy increases. The observed effect is treated in terms of enhanced electron attachment due to the capturing of electrons by vibrationally excited molecules of SF6. Also given are the results of measuring the burning voltages in a non-irradiated confined SSVD in the same mixtures versus the specific electric energy deposited. Most plausible scenarios of vibrational relaxation in SF6 are qualitatively discussed and, on this basis, some inferences about the peculiarities of electron attachment to vibrationally excited molecules of SF6 are drawn. At lowest values of the specific absorbed laser radiation energy, realized in the present experiments, no significant impact on the discharge voltage was observed. Together with data on the burning voltages in a non-irradiated SSVD this fact has allowed some conclusions about the role of electron attachment to the discharge-produced vibrationally excited molecules of SF6. In particular the results obtained appear to give no evidence for the electron impact vibrational excitation of SF6 is capable of considerably affecting the operating characteristics of current electric discharge non-chain chemical HF lasers using SF6 and SF6-based mixtures.