A. V. Saifulin

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.

Scaling up of nonchain HF(DF) laser initiated by self-sustained volume discharge

Scaling problems are considered for non-chain HF laser operating on mixtures of SF6 and hydrocarbons in which a chemical reaction is initiated by self-sustained volume discharge. The possibility of obtaining a volume discharge in SF6 and in corresponding mixtures without preionization, i.e., self- initiated volume discharge (SIVD) is a new qualitative result in solving the scaling problem for non-chain lasers. The dynamics of SIVD evolution has been investigated. The possibility of obtaining SIVD is determined in SF6 by mechanisms that limit the density of current, are related to specific energy release, and prevent the total energy from flowing through a single channel. A simple mode is developed for calculating the discharge characteristics in non-chain laser. The model provides a good agreement with experiment data. The obtained energy of nonchain HF-laser is 407 J and that of DF-laser is 325 J with the electric efficiency 4.3 percent and 3.4 percent, respectively. A possibility is estimated of creating non-chain HF lasers with the output energy of the order of kilojoules and higher on the basis of experimental data obtained.

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.

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.

Advanced studies on powerful wide-aperture nonchain HF(DF) lasers with a self-sustained volume discharge to initiate chemical reaction

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÷6cm, 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 ~ 1kJ and above.

Self-initiated volume discharge for production of atomic iodine in pulsed oxygen-iodine lasers

This paper reports on investigating self-sustained volume discharge (SSVD) characteristics in CH3I, C3H7I, C4H9I, CF3I and their mixtures with SF6 and N2, employed as buffer gases, and with O2. The investigations performed in the plane-plane electrode system displaying high electric field edge enhancement have shown that in C3H7I, C4H9I and their mixtures with SF6, N2 and O2 SSVD is realized in the form of a self-initiated volume discharge (SIVD)- SSVD with no any preionization. Addition of SF6 or N2 in C3H7I, C4H9I leads to increasing the discharge stability, the latter being not adversely affected by addition of O2 in amounts of up to 300% of the iodide partial pressure. The fact that SSVD in C3H7I and C4H9I develops in the form of SIVD is indicative of these discharges to be promising for creation of high power pulsed and pulsed-periodic COIL. SIVD has been performed at total mixture pressures of up to 72 Torr and energy depositions of up to 130J/l in a volume of 1.5 l. The performed experimental modeling involving laser geometry of the discharge gap gives firm evidence that SIVD is promise for being used in creation of pulse and pulse-periodic COIL.

Continuous long laser spark produced with a conical mirror and CO2 laser

The feasibility of producing continuous laser sparks (CLSs) with a resistance per unit length of 100-400 Ω/cm by focusing radiation from CO2 laser with a conical mirror is demonstrated. The laser energy input per unit length required for this is experimentally found to be equal to ~200 J/m. The possibility to efficiently control the trajectory of an electric discharge by means of a CLS is demonstrated. A CLS is found to be an analogue of a high-conductivity metal rod during the electric breakdown and electric potential transfer. The effect of polarity in the electric breakdown of air gaps between the CLS plasma channel and a metal rod is discovered and interpreted. The transverse structure of the CLS conductivity is investigated. Most likely the CLS conductivity at the initial state is due to the photoionization of air by the radiation of primary nuclei of the optical breakdown.

UV-preionization in nonchain HF lasers with a self-sustained volume discharge to initiate chemical reaction: Is it actually essential?

A role of UV preionization in non-chain HF(DF) lasers has been investigated. The influence of preionization on the discharge uniformity and on the duration of stable burning of a self-sustained volume discharge (SSVD) in working mixtures of non-chain HF lasers as a function of the energy stored in capacitors are comprehensively studied. Also presented are discharge photographs taken under identical conditions with and without preionization. It is shown that SSVD characteristics and laser energy are scarcely affected by preionization if the discharge lasts more than 150ns and the cathode surface area is in excess of 300 cm2. In HF lasers with the cathode surface area and discharge duration of less than 300cm2 and 150ns, respectively, preionization is essential for nothing but stabilization of the discharge voltage and breakdown delay and for improving discharge uniformity. If the opposite takes place preionization has no influence on the laser characteristics.