CF4 Laser Research Articles

Characteristic features of the photolysis of UF6 molecules in a mixture with hydrogen by radiation from a repetitively pulsed CF4 laser

Uranium hexafluoride molecules were photolysed in the presence of hydrogen by focused radiation of a repetitively pulsed CF4 laser (λ=16 μm) with an energy of ~100 mJ per pulse at a pulse repetition rate 2 — 3.5 Hz. In the presence of hydrogen the yield of the photolysis of UF6 molecules was an order of magnitude higher than without hydrogen and the number of molecules photolysed per pulse was 3 — 5.5 times higher than their number in a zone where the radiation density exceeded the dissociation threshold. This effect was attributed to laser activation of UF6 molecules inside and outside this zone, and to a subsequent reaction of the activated molecules with hydrogen. In the presence of hydrogen the yield of the photolysis of UF6 molecules decreased from pulse to pulse practically proportionally to the change in the density of UF6 molecules, which was the result of reactions of the newly formed hydrogen radicals with activated and nonactivated UF6 molecules, and the rates of these reactions depended quadratically on the UF6 density before each pulse.

Characteristic features of the dissociation of UF6 molecules in the radiation field of a pulse-periodic CF4 laser

An investigation was made of changes in the dissociation yield of UF6 molecules from pulse to pulse in the radiation field of a pulse-periodic CF4 laser. The energy per pulse was 25 — 130 mJ and the UF6 pressure was 0.3 — 0.6 Torr. The dissociation yield was proportional to the volume of the focused-radiation zone inside a cell where the photon energy density Φ exceeded the threshold value Φ0 throughout the investigated ranges of the energy per pulse and of the UF6 pressure. The first pulse dissociated 28% ± 3% molecules in this zone for Φ0=1 J cm-2 and 53% ± 5% of molecules for Φ0=1.5 J cm-2. A steep fall of the UF6 photodissociation yield from pulse to pulse was observed for the first time. An analysis was made of the processes which accompanied dissociation and estimates were obtained of the influence of the relevant reactions on the observed fall of the UF6 photodissociation yield from pulse to pulse.

Optically pumped pulse-periodic CF4 laser

Some design features and the parameters of the latest model of a pulse-periodic CF4 laser are reported. The working substance is circulated along a two-loop system and the temperature of an optical cell is stabilised automatically. The pumping is provided by a pulse-periodic TEA CO2 laser with an energy of 5.5 J per pulse in the form of the 9R12 line narrowed down to ~100 MHz by external injection of radiation from a low-pressure cw CO2 laser. The maximum radiation energy carried by a single CF4 laser pulse is 250 mJ and the average power at a pulse repetition frequency of 50 Hz is up to 6 W.

Optimum operating conditions for an optically pumped CF4 laser amplifier

We report the results of a theoretical study of the performance characteristics of an optically pumped CF4 laser amplifier in terms of a rate equation model. In order to achieve the best output from such an amplifier it is necessary to optimize its operating parameters. Our calculations indicate that, for a given pump rate, there exists an optimum gain length for the amplifier at which the output energy is a maximum. This optimum gain length is a function of the amplifier pump rate as well as its operating pressure and temperature. Our results also show that proper synchronization of the CO2 pump pulse and the CF4 laser pulse leads to the highest amplified output energy.

Effect of superfluorescent emission on the operation of an optically pumped CF4 laser

A theoretical study investigating the effect of superfluorescent emission on the operation of an optically pumped CF4 laser is reported. The coupled space- and time-dependent rate equations for the molecular population densities and the photon fluxes have been numerically solved to study the dependence of the superfluorescent emission contents in the CF4-laser output on the various laser parameters such as the temperature, pressure, pumping rate, and the feedback factors. We have studied the effect of the temporal shape of the CO2 pump pulse on the superfluorescent emission and the laser-pulse energies. Our calculations indicate that a considerable reduction in the superfluorescent emission is possible by shaping the pump CO2-laser pulse and by choosing the laser operating parameters appropriately. We have also investigated the effect of the addition of buffer gas on the superfluorescent emission and laser emission.

Design and the operational characteristics of a 16 µ CF4 laser

The design and the operational characteristics of a CO2 laser pumped CF4 laser developed at BARC are reported. Output energies of up to 20 mJ have been obtained at 615 cm−1 with an absorbed energy conversion efficiency of 10%.

Influence of the laser radiation field on the absorption of pump radiation in a three-level system

A reduction in the absorption of the pump radiation under the influence of the laser radiation field was observed for a pulsed CF4 laser. The effect was due to two-photon processes and appeared for a small detuning of the radiation frequencies from a resonance. The results of a calculation carried out on the basis of a steady-state three-level model agreed well with the experimental data.

Experimental and theoretical investigation of a pulsed laser utilizing the CF4molecule

An experimental investigation was made of the characteristics of a pulsed CF4 laser at high pump power densities. A radiation intensity of 0.5 MW/cm2 was obtained at a frequency of 615 cm−1 when the pump power conversion efficiency was 12%. Calculations are made of the laser parameters using steady-state and complete models. It is found that two-photon processes may play a decisive role in the lasing mechanism. The feasibility of using amplifiers to improve the radiation power in the 16μ range is determined.

UF6 photodissociation through measurement of visible and infrared luminescence.

Infrared multiple-photon dissociation of UF6 was induced at room temperature by irradiation simultaneously with optically pumped CF4 laser at 615 cm−1 and TEA CO2 laser at 1,073 cm−1. The dissociation of UF6 was verified from evidence of fluorine generation, obtained by its reduction with H2 and observing infrared emission from the resulting excited HF molecules. The rate of UF6 dissociation was determined from the decrease of infrared absorption shown by UF6. Quantitative indication of the dissociation yield of UF6 is known to be provided by the more readily measurable visible luminescence from the irradiated zone. The intensity of the visible luminescence was therefore measured, to seek the dependence of dissociation yield on such factors as the fluences of the CF4 and CO2 lasers, and the pressures of UF6 and of Ar buffer gas. A clear threshold proved to exist around 70 J/cm2 for the fluence of CO2 laser, but not for that of the CF4 laser, indicating that a very slight excitation by the latter laser serves to induce UF6 dissociation. This suggests that the dissociation energy is supplied in large part by the COZ laser. In the range of UF6 dissociation, the visible luminescence intensity was found to rise roughly proportionally with CF4 and CO2 laser fluences, as well as with the pressures of UF6 and of added Ar buffer gas.

Isotopically Selective Multiphoton Dissociation of UF6 in Static Gas Cell at -35°C

In molecular laser isotope separation of uranium (MLIS), the compound UF6 is a natural choice since it is the most familiar material in conventional uranium enrichment processes. The UF6 has the sufficient chemical stability and the highest vapor pressure of all known uranium compounds. However, the studies on MLIS have not been as successful as those on AVLIS (atomic vapor laser isotope separation) until recently owing to the lack of the suitable infrared lasers that can be tuned precisely to the absorption center of the 235UF6ν3 band near 16 pm.Many attempts have been made to develop the 16 μm lasers. Continuously tunable 16 μm radiation was obtained using an optical parametric oscillator(1)(2). Since the available energy was very small, however, the second laser for dissociation of the vibrationally excited UF6 was needed. The groups at Los Alamos National Laboratory and Centre d'Etudes Nucleaires at Saclay both utilized the UV laser for this purpose. But it was found(3)(4) that the use of the UV laser deteriorates the isotopic selectivity because even unexcited UF6 absorbs the UV laser very strongly. Although infrared multiphoton excitation without using UV lasers was expected to yield better selectivity, no high-power gas laser had the capability of tuning the laser frequency to the center of the 235UF6 16 μm band until the advent of the p-H2 Raman laser. For example, the CO2-pumped CF4 laser was found to be successful in dissociating UF6 only in the region between 610 and 620 cm-1(5), which corresponds to the lower frequency tail of the absorption of 238UF6.Immediately after the proposal(6) and the following successful demonstration(7), it was recognized that p-H2 Raman laser is the most suitable 16 μm laser which is capable to tune over the region of 235UF6 absorption. It is a scalable gas laser based on well established CO2 laser technology and on the very efficient stimulated rotational Raman scattering conversion in a gaseous medium.Using this p-H2 Raman laser, Rabinowitz et al.(8) demonstrated infrared two-color multiphoton excitation to dissociate UF6 selectively at ambient temperature. Although their choice of all-infrared excitation scheme itself is more advantageous than IR-UV excitation scheme, the selectivity was not very high probably owing to the fact that UF6 was not cooled with an expansion nozzle. They reported a selectivity of 1.20. Since the amount of UF6 treated in their experiment was small, they developed a complicated on-line detection system for the isotopic selectivity measurement.It is always of great interest from a practical viewpoint whether the selectivity observed at the time immediately after dissociation is deteriorated by the secondary reactions or not. In this note, an attempt was made to irradiate UF6 contained in a static gas cell with one-frequency p-H2 Raman laser and to obtain the selectivity based on the stable compound remaining in the cell after irradiation.

Pressure dependence of laser oscillation and superfluorescent emission from an optically pumped CF4 laser

Experiments on pressure dependence of a CF4 laser have been carried out to elucidate dynamic characteristics of CF4 molecules pumped by CO2 lasers. The results are followed by a model calculation which characterizes collisional quenching of the energy levels related to the stimulated emission. The enhancement of the laser output induced by buffer-gas addition, which was found under specific conditions, is discussed in relation to the competition between laser and superfluorescent emissions.

Two-frequency multiphoton absorption in UF6

The multiphoton absorption of 9.3-μm CO2 laser radiation induced by multiphoton absorption of 16-μm CF4 laser radiation in UF6 is reported. An average vibrational energy deposition in the range of 1 to 2.8 eV per molecule is found, which is sufficient to cause the observed dissociation either by collision-assisted or unimolecular decomposition processes.

Laser-pumped molecular laser

The parameters of laser-pumped molecular lasers are investigated. It is established that the energy and tuning characteristics of an NH3 laser (Eg = 1.5 J, efficiency 20%, Pav = 20 W, radiation frequency tuning band 753–890 cm-1) are decisively influenced by the addition of N2. A focusing raster optical-pumping system has made it possible to obtain a specific lasing energy 12 J/liter. A CF4 laser with lasing energy 40 mJ operates in the 612–655 cm-1 band. Experiments on dissociation of the molecules CCl4 and UF6 were carried out with the aid of NH3 and CF4 lasers. The systems and methods of producing Raman lasers (RL) are presented. An effective RL amplifier on rotational transitions in compressed H2, which transforms tens of beams of Nd lasers into one coherent beam of the first Stokes component with λ ≅ 1.13 μm at an efficiencyup to 70%, is described.

Isotopes separation experiments in natural UF6 by CF4 and CO2 lasers, analyzed by gamma-ray spectrometry

Multiphoton dissociation of natural UF6 was obtained by simultaneous irradiation with a CF4 laser at 615.2 cm−1 and a transverse, electric, atmospheric (TEA)-CO2 laser at 1073.3 cm−1 using various gas pressures and laser fluences. The dissociation percentages D235 and D238 of 235UF6 and 238UF6, respectively, were determined by comparing results of an irradiated sample to those of a nonirradiated reference sample prepared under identical conditions. For the analysis we employed a simple, high-precision nuclear technique based on gamma-ray spectrometry. The values of D235,D238 and the resulting enrichment factor F = D235/D238 were found from an analysis of the 186-keV peak, originating in 235U, and the 63-keV peak, originating in 234Th, the daughter nucleus of 238U. A representative result shows a dissociation of 41±1% and 235U enrichment factor of 1.049±0.016 yielded by the excitation of 0.47 Torr gas by 25 laser pulses at 20-mJ cm−2 CF4 fluence and 1-J cm−2 CO2 fluence.

High repetition rate CH4 laser

A 16-μm CF4 laser oscillator has operated at 1 kHz in a cooled static cell. Pump energies required from the low pressure, Q-switched, cw discharge CO2 laser were as low as 60 μJ. The laser cavity employed a multiple-pass off-axis path resonator in a ring configuration. CF4 laser power at 615 cm−1 and a 1-kHz repetition rate exceeded 300 μW.

Parametric characterization of an optically pumped CF4 laser operated in a nonsaturating pump regime

The operation of a 16 μm pulsed CF4 laser, optically pumped with a nonsaturating tunable hybrid CO2 laser, is described. The influence of CO2 pump pulse energy and shape, CF4 gas pressure and temperature, cavity configuration and addition of buffer gases on the CF4 laser radiation characteristics is discussed. From our experimental findings a set of criteria for optimum performance of CF4 lasers are given.

Investigation of the spectral dependences of the excitation of high vibrational levels of the UF6molecule

An experimental investigation was made of the excitation of high vibrational states of the UF6 molecule by infrared laser radiation. The spectral and energy dependences were determined of the absorption of 16 μ radiation by UF6 gas undergoing single-frequency interaction with CF4 laser radiation, and of the visible luminescence of the UF6 molecules in the case of two-frequency interaction with CF4 and CO2 laser radiation in a static gas cell at 300 °K. It was found that the visible luminescence can serve as a quantitative measure of the number of UF6 molecules excited above the first dissociation limit. A possible isotopic dissociation selectivity was estimated and the deexcitation times of electronic and vibrational excitation of the UF6 molecule were measured.

Pulse-periodic CF4laser with circulation of the working gas

A pulse-periodic CF4 laser with circulation of the working gas was constructed and investigated. The CF4 molecules were pumped optically by a pulse-periodic CO2 laser of moderate power (up to 1 kW). The CF4 gas was circulated through the laser cell by a specially developed centrifugal compressor. When the gas velocity was 22 m/sec, the maximum output power (0.6 W) of the CF4 laser was obtained when the pulse repetition frequency was 110 Hz. Stabilization of the frequency of the CO2 laser increased the average output power of the CF4 laser radiatiion to 2.5 W.

Absorption of CF4laser radiation by UF6molecules

A study was made of the absorption by UF6 molecules of CF4 laser radiation falling within the P branch of the ν3 band. Measurements were made of the absorption selectivity for the isotopic variants 238UF6 and 235UF6. The results were compared with the published data. A discrepancy is attributed to accumulation of selectivity on successive transitions in multiphoton absorption.

Investigation of the characteristics of a pulsed CF4laser

A study was made of the characteristics of a pulsed CF4 laser, optically pumped by a pulsed CO2 laser whose frequency was stabilized by injecting a signal from a low-pressure cw CO2 laser. The laser output energy was observed to saturate with increasing pump energy, the maximum value obtained being 0.1 J at an efficiency of up to 1%.