Pulsed HF Chemical Laser Research Articles

A new method for enhancing the output energy of pulsed hf chemical lasers

AbstractThe HCl chemical laser is extremely useful because of its output wavelength 3.5‐4.0 μm (transparent window of the atmosphere) and its easiness in handling a gas. However, its output is smaller by one digit than the HF laser output. This paper proposes a use of a vibrationally excited H2 as a H donor of HCl and presents the computer simulation result using the equal‐gain model. It is found that the output can be increased considerably if H2 of 10‐2 (1%) is vibrationally excited to v = 1. The laser characteristics (output energy, power, pulse duration, etc.) are calculated under various operating conditions such as initial ignition (the initial H and Cl concentrations), the gas (H2/Cl2) ratio, etc.

Calculations of the energy characteristics of a pulsed HF chemical laser with a spherical telescopic resonator

A method of calculating the energy characteristics of a pulsed HF chemical laser with a telescopic resonator formed by spherical mirrors is described and results are presented. A comparison is made with the case of a planar resonator. An investigation is made of photoinitiated short-pulse emission conditions. It is shown that, in order to improve the directionality of the radiation from these lasers, it is advisable to use telescopic resonators with high magnifications M~10–100.

Nanosecond switching and amplification of multiline HF laser pulses

Electrooptic gating and amplification of Pulses with durations from 1 to 5 ns were accomplished with a pulsed HF chemical laser system using SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> -hydrocarbon media and transverse-discharge energization. The system comprised an apertured, pin-discharge multiline oscillator, a MgF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Rochon prism polarizer, a Pockels-effect, fast electrooptic switch, a 5:1 magnifying telescope, and a 22-mm diam, ∼ 5-MW TEA amplifier, together with several arrangements of polarization analyzer elements. A 5-mm, square-sectioned electrooptic crystal of CdTe or LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> yielded polarization contrast ratios of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim 80:1</tex> under zero and half-wave electric fields. Without other measures, this contrast in gated amplitude was diminished in passage through the partially gain-saturated amplifier. However, 80:1 amplitude contrast in the gated, amplified pulse was preserved when file full oscillator beam intensity was used continuously to control the amplifier gain. This was done either with a polarization discriminator placed downstream of the amplifier, or with the two polarized components from the gate taking separate directions, intersecting within the amplifier.

Vibrational relaxation of HF(v=1, 2, and 3) in H2, N2, and CO2

The vibrational relaxation times of HF(v=1, 2, and 3) were measured in H2, N2, and CO2 by a laser-induced fluorescence technique. The upper vibrational levels were produced by sequential absorption in which HF(v=0) was pumped first to HF(v=1) and subsequently to HF(v=2) and HF(v=3) by photons from a pulsed multiline HF chemical laser. At T=295 K, the relaxation rates of HF(v=1), HF(v=2), and HF(v=3) in H2 were found to be, respectively, (1.43±0.15) ×10−2, (1.23±0.1) ×10−2, and (1.13±0.1) ×10−2 (μsec Torr)−1; in N2, (1.45±0.15) ×10−4, (8.1±1.0) ×10−4, and (2.92±0.3) ×10−3 (μsec Torr)−1; and in CO2, 0.039±0.004, 0.19±0.02, and 0.38±0.04 (μsec Torr)−1. Values of (7.5±1) ×10−4 and 0.4±0.04 (μsec Torr)−1 were obtained for the relaxation rates of HF(v=3) in O2 and HCl, respectively.

Vib–rotational energy distributions and relaxation processes in pulsed HF chemical lasers

The rate equations governing the temporal evolution of photon densities and level populations in pulsed F+H2→HF+H chemical lasers are solved for different initial conditions. The rate equations are solved simultaneously for all relevant vibrational–rotational levels and vibrational–rotational P-branch transitions. Rotational equilibrium is not assumed. Approximate expressions for the detailed state-to-state rate constants corresponding to the various energy transfer processes (V–V, V–R,T, R–R,T) coupling the vib–rotational levels are formulated on the basis of experimental data, approximate theories, and qualitative considerations. The main findings are as follows: At low pressures, R–T transfer cannot compete with the stimulated emission, and the laser output largely reflects the nonequilibrium energy distribution in the pumping reaction. The various transitions reach threshold and decay almost independently and simultaneous lasing on several lines takes place. When a buffer gas is added in excess to the reacting mixture, the enhanced rotational relaxation leads to nearly single-line operation and to the J shift in lasing. Laser efficiency is higher at high inert gas pressures owing to a better extraction of the internal energy from partially inverted populations. V–V exchange enhances lasing from upper vibrational levels but reduces the total pulse intensity. V–R,T processes reduce the efficiency but do not substantially modify the spectral output distribution. The photon yield ranges between 0.4 and 1.4 photons/HF molecule depending on the initial conditions. Comparison with experimental data, when available, is fair.

Spectroscopic diagnostics of laser-supported absorption (LSA) waves produced by a HF laser

LSA waves have been generated by a pulsed chemical HF laser beam (2.8 μ, 45 J, 7 μsec) when focused on solid surfaces in air. The emission spectrum is found to contain mainly N+ and O+ lines broadened by the Stark effect and superimposed on a strong continuum. The LSA wavefront travels at an average speed of 0.72 cm/μsec as measured by the time-of-flight method. Target species such as atomic Al emit light for 600 μsec after the laser pulse. In contrast with the results from a CO2 laser, however, no forbidden transition of AlI is observed.

Effect of cavity transients and rotational relaxation on the performance of pulsed HF chemical lasers: a theoretical investigation

Two rate-equation models of a pulsed HF laser are presented. Calculations with these models reveal the time evolution of the gain and intensity of lasing on all lines in vibrational-rotational bands studied and the time histories of concentrations of the chemical species. The present formulation permits an accurate assessment of the effect of vibrational relaxation and rotational relaxation. The model predictions are compared with earlier models to illustrate their unique features. Multiline lasing on vibrational-rotational bands is predicted; however, the general trends of J shifting are similar to those of earlier models. It is found that lasing does not begin when the medium gain reaches threshold, but intensity increases sharply after the gain is far above threshold. The gain subsequently drops rapidly and oscillates near the threshold value. The effect of rotational relaxation on laser performance is shown for multiline laser operation, and the enhancing effect of cascading is clearly revealed. The present models are found to predict output and performance consistent with experiment.

Pulsed HF Chemical Lasers from Reactions of Fluorine Atoms with Benzene, Toluene, Xylene, Methanol, and Acetone

The stimulated emission of vibrationally excited HF has been investigated in the pulse-discharged mixtures of SF6 and hydrogen donors: C6H6, C6H5CH3, C6H4 (CH3)2, CH3OH, and (CH3)2CO. The P2-1(J=3–8) and P1-0(J=4–8) lines, and P2-1(J=5–8) and P1-0(J=4–8) lines were found in the mixtures with C6H4(CH3)2 and C6H5CH3, respectively, while with C6H6 only the P1-0(J=4–8) lines were observed. The addition of C6H4(CH3)2 to the SF6/H2 mixture resulted in the enhancement of the HF laser power, when the concentration of H2 Was small. This was due to the fact that C6H4(CH3)2 acted as a hydrogen donor.

Theoretical investigation of the effect of rotational nonequilibrium on predicted performance of pulsed HF chemical lasers

Theoretical investigation of the effect of rotational nonequilibrium on predicted performance of pulsed HF chemical lasers

Temperature Dependence and Enhanced Output of Pulsed Discharge-Initiated HF Chemical Lasers Using SF6/H2, SF6/CH4 and SF6/C4H10 Mixtures

Energetic and spectral studies of pulsed discharge-initiated HF chemical lasers using SF6/H2, SF6/CH4 and SF6/C4H10 mixtures at temperatures from 228 to 573 K have been performed experimentally. The experiments have shown that at the optimum pressure for each mixture the HF laser output energy and spectral distribution are dependent on temperature ; optimum output energy is enhanced as the initial gas temperature is decreased. The output energy increases at 228 K (dry ice cooled) by 15 to 20% over that at 296 K (room temperature) for each mixture. The output energy dependence on temperature has been found to fit approximately an Arrheniustype form. The relative peak power distribution of the HF lasing lines at various temperatures is presented. A brief explanation for this behavior is offered.

Temperature dependence of pulsed discharge-initiated HF chemical lasers using SF6/H2, SF6/CH4, and SF6/C4H10 mixtures

Energy and spectral measurements of pulsed discharge-initiated HF chemical lasers are presented. SF6/H2, SF6/CH4, and SF6/C4H10 mixtures at 228, 296, and 373 °K are considered. At the optimum pressure for each mixture, HF chemical laser output has been found to depend weakly on temperature; optimum output energy was enhanced as the initial mixture temperature was decreased. A qualitative explanation for this behavior is offered. The peak power distribution of the HF laser lines at 228 and 296 °K is also discussed.

Improved performance of a double discharge initiated pulsed HF chemical laser

Operation of a pulsed HF chemical laser initiated by electric discharge is reported. Uniform transverse capacitive discharge through high-pressure mixtures of SF6 and H2 is effected by the use of a double discharge. For the mixture SF6, H2, He (250, 20, and 280 Torr, respectively) laser output of more than 1 J per pulse with 4% electrical efficiency from an active volume of 0.06 1 has been achieved. Pulse shapes of the different transitions, spectrum measurements, and SF6 decomposition rate are reported.

Atmospheric pressure pulsed HF chemical laser

Experiments are described on a pulsed chemical laser operating on the reaction between hydrogen and fluorine at pressures above the second explosion limit. A laser energy density of 80 J/liter-atm is obtained for a mixture of 0.1 F2/0.1 H2/0.8 He mole fraction at 1.1 atm total pressure. This performance corresponds to a chemical efficiency of 4% and an over-all electrical efficiency of 1.3% for the particular photolysis geometry employed. Various diagnostic experiments are developed to measure the initial fraction of F2 dissociated by the photolysis, the temporal extent of the over-all reaction, and measurements of medium homogeneity. These data together with measurements of over-all laser performance are compared with calculations from a comprehensive theoretical model which includes the photolysis process, chemical kinetic reactions, vibrational energy transfer processes, and stimulated emission. The major features of this laser appear to be understood and described by the existing kinetic rate data. However, certain important effects, such as the influence of rotational nonequilibrium and optical output coupling, remain to be fully understood.

Pulsed HF chemical laser linewidth measurements using time-resolved bleachable absorption of HF gas

The spectral linewidth at 2.7 μ of a transverse discharge pulsed HF chemical laser has been measured using the saturable absorption of low-pressure HF gas in an external cell. Using the ν = 0 to ν = 1 P(J) transitions, the laser linewidth was found to be 10−4 cm−1—120 times narrower than the Doppler spectral width. Heterodyne beating between two HF lasers was observed and the pulse-to-pulse frequency shift noted.

Temperature dependence of V-V and V-R, T energy transfer measurements in mixtures containing HF

A pulsed HF chemical laser has been used to excite HF molecules to their first vibrational level. The decay times of the infrared fluorescence from HF(ν = 1) have been used to determine the rate coefficients for energy transfer from HF to various collision partners including H2, D2, N2, O2, DF, HCl, CO2, NO, CO, and HBr. The experiments were performed at temperatures from 450 to 1000°K for most of the molecules. as well as at 295°K. For the higher temperature measurements, the experiments were performed behind a reflected shock wave in a shock tube. The compression heating capability of a shock tube has been used successfully as an alternative to a heated cell for laser-induced fluorescence measurements. The room-temperature probabilities of energy transfer from HF to the various molecules plotted versus energy mismatch fell along two distinct lines: one for homonuclear and one for heteronuclear molecules. The rate coefficients (cubic centimeters per mole second) for the molecules with the smallest energy mismatch had temperature dependences between T−1 and T0 near T = 295°K, while the molecules with larger energy mismatches and correspondingly smaller rate coefficients showed positive temperature dependences. The room-temperature relaxation rates of HF in the presence of the various molecules were measured to be H2, (1.7 ± 0.1) × 10−2; HCl, (1.7 ± 0.1) × 10−2; DF, (7.7 ± 0.4) × 10−2; CO2, (3.6 ± 0.2) × 10−2; D2, (3.1 ± 0.6) × 10−3; N2, (1.52 ± 0.15) × 10−4; O2, (4.5 ± 0.6) × 10−5; CO, (1.8 ± 0.2) × 10−3; HBr, (7.5 ± 1.0) × 10−3: and NO, (6.0 ± 1.0) × 10−3, in units of (μsec torr)−1. These rates represent the total relaxation rate of HF due to both V - V and V - R,T energy transfer. Relaxation rates of some of the chaperone molecules by HF as well as their self-relaxation rates were also measured.

Pulsed HF chemical laser with high electrical efficiency

A pulsed HF chemical laser utilizing the chain reaction between H2 and F2 has been operated with electric discharge initiation. A uniform transverse discharge is made possible through the use of spark preionization. For the mixture 1:1:10:0.06 (F2:H2:He:O2) at 120 Torr total pressure an output energy 1.01 times the electrical input has been achieved. The corresponding chemical efficiency was ∼1.0%. A tradeoff is observed between electrical efficiency and chemical efficiency with the maximum chemical efficiency of 2% being achieved at an electrical efficiency of 25%. Even higher electrical efficiencies are expected with further device optimization.

MA1 - Laser-excited vibrational energy transfer studies of HF, CO, and NO

An electrically initiated pulsed HF chemical laser has been used to measure vibrational relaxation of HF ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ), CO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ), and NO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) at 295 ± 2 K. The self-relaxation rate of HF highly dilute in argon is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(8.7 \pm 0.1) \times 10^{4} s^{-1}</tex> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and is independent of rotational level excitation over laser transitions <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P_{1 \rightarrow 0}(2) - P_{1 \rightarrow 0}(9)</tex> . The rates at which O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , CO, and NO quench HF( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) have been found to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">350 \pm 25, (2.5 \pm 0.2) \times 10^{3}</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(6.2 \pm 0.3) \times 10^{3}</tex> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively. There is indirect evidence of transfer from HF( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) to the ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 2</tex> ) levels of NO and O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Through the use of HF ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) as a collisional pumping source we have measured the deactivation rates of CO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) by HF (V → R, T) and N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (V → V) to be 480 ± 25 s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 130 ± 15 s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Similarly, HF ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) has been used to pump NO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) and the NO self-relaxation rate has been measured to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2.7 \pm 0.2) \times 10^{3}</tex> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ; this rate is about a factor of 5 faster than would be predicted by the kinetic spectroscopy results of Billingsley and Callear [43].

Effects of intense CO2radiation on pulsed HF chemical laser systems

Effects of intense CO₂radiation on pulsed HF chemical laser systems

Fluorine Pressure Change Monitor for a Reacting System

Recent experimental results on the performance of a pulsed HF chemical laser have indicated that the presence of HF in the chemical reactants prior to laser initiation degrades the laser performance. In order to monitor the amount of HF produced when the chemical reactants are mixed, a device has been developed that measures the change in concentration of one of the reactants. This device, which relies on the absorption of light by molecular F2, has been shown to have an accuracy of better than 5% in the measurement of the F2 pressure.

Parameter studies of pulsed HF lasers

The average output power of pulsed HF chemical lasers is found to depend strongly on the fluorine source used. The best performance was obtained from H 2 -C 2 F 6 -He mixes, which gave average powers of 850 mW and energies of 20 mJ/pulse. Peak gains of at least 0.036 cm-1are observed. Average powers of 450 mW from DF and 70 mW from HCl are reported.