Supersonic Laser Research Articles

Cavity flow control for supersonic lasers

The heat deposited in the cathode sheath of a supersonic discharge laser creates a thickening of the electrode boundary layer, which in turn causes a compression wave of several-percent pressure jump at the electrode leading edge. Attention is presently given to the effects of controlled variations in cavity gas temperatures and pressures on well ordered wave strength in the main flow. The most efficient method of flow distortion reduction is the enlarging of the flow channel downstream of the cavity while balancing the pressure drop by a matched purge flow. 7 references.

Modeling a continuous-action chain chemical HF laser based on a standing detonation wave

The present investigations show that, in optimal conditions, the given system may ensure a specific energy supply to the mixture of 300 J/g, with a chemical efficiency of about 13%. Although these indices fall far short of the potential of supersonic chemical HF lasers of diffusional type, nevertheless the fundamental features of lasers of this class (the lack of a combustion chamber, spatial demarcation of the mixing and generation regions, resulting in higher optical homogeneity of the gas medium, etc.) mean that the continuous-action chain chemical HF laser based on a standing detonation wave may be regarded as a very interesting and extremely promising system. The high amplification properties of the active medium allow unstable resonators and resonators with field rotation to be successfully used in such lasers.

Calculation of the energy characteristics of a supersonic mixing chemical CO laser

The results are given of a calculation of the energy characteristics of a chemical CO laser in which the products of dissociation of carbon disulfide are mixed supersonically with oxygen. Calculations predict a weak influence of the vibrational--vibrational exchange between the CO molecules on the resonator conversion of the vibrational energy of CO into radiation. It is shown that, in principle, a laser of this type can operate at a near-atmospheric pressure in the resonator.

Energy characteristics of a self-contained cw CO chemical laser

Investigations were made of the energy characteristics of a self-contained supersonic cw CO chemical laser. A calculation method is described, the amplifying properties of the active medium are analyzed, and the spectral characteristics are examined. A comparison is made between the energy characteristics of planar and ring models of a cw CO chemical laser. The results show that the ultimate energy characteristics of this type of laser are fairly good: the specific laser energy (per 1 g of mixture) is ~300 J/g and the chemical efficiency is ~25%. An allowance for thermal losses in the gas generator reduces these characteristics by a factor of approximately one and half.

Design for improved resolution in a time-of-flight mass spectrometer using a supersonic beam and laser ionization source

This article describes the design of a time-of-flight mass spectrometer (TOFMS) constructed for optimum use with resonance-enhanced multiphoton ionization spectroscopy in supersonic beams. The use of the supersonic beam results in a great improvement in resolution, i.e., at least 800 at mass 93. Ion packets on the order of 10 ns FWHM or less are observed at our flat dual channel plate detector and the resolution ultimately appears to be laser-pulse-width limited. The design of this TOFMS utilizes fast-pulsed molecular beam techniques to allow the use of reservoir pressures >1 atm with a large orifice (0.5 mm) in order to provide high on-axis intensity for maximizing the photoionization signal produced by our pulsed laser system with the use of only modest pumping capacity. It also uses a skimmed beam, differential pumping, and a liquid-N2 baffle in order to maintain the flight tube at pressures on the order of 5×10−7 Torr and to eliminate organic contamination from the ionization region.

Gain saturation in gas flow and chemical lasers

A theoretical model for gain saturation in gas and chemical lasers is presented. The theory is applicable to all possible numerical values of τ/τ_c, where τ is the characteristie time for the flowing gas to move across the laser action region and τ_c is the characteristic collision relaxation time. The saturation effects of the convection and the source flow of the inverted population are revealed. A general relation of gain coefficient and some new gain saturation laws are obtained. For the special case of τ/τ_c1, the present theoretical results agree with the experimental results on the anomalous saturation phenomena in the supersonic diffusion HF chemical laser determined recently by Gross and Coffer~[8]. The theory also agrees with the measured results of saturation intensity varying with τ/τ_c in gas CO_2 lasers~[7]. For the special case of τ/τ_c1, the present theory is consistent with both the standard theory~[1] for gas lasers where the gas has no macroscopic motion and the known gain saturation theory~[2-5] for gas and chemical lasers.

An optically pumped supersonic iodine beam laser

We report the realisation of an optically pumped I 2-laser using a supersonic beam as active medium. Threshold pump power of the order of 10 mW and output power at λ d=1.34 μm of up to 2 mW is observed with λ p=514.5 nm as pump radiation. In contrast to the similar Na 2-beam laser, the output of a ring cavity is bidirectional with a forward to backward intensity ratio of about 10:1.

Effects of shock waves on the performance of a cw supersonic HF chemical laser

The effects of oblique shock waves on the gain and the output power of a cw supersonic HF chemical laser are theoretically investigated. A one-dimensional model is used to solve the flow of a premixed H2–SF6–He or H2–F2–He mixture through the resonator, allowing the presence of oblique shock waves in the flow. It is shown that a shock wave significantly affects the laser performance due to the gas compression at the shock front and due to acceleration of the chemical and energy transfer processes. It was found that immediately behind the shock front the gain coefficients and the photon energy sharply increase and further downstream they fall off rapidly, thus the profiles of the gain and radiation flux become narrower along the flow direction. Integration of the radiation flux along the cavity coordinate reveals that in some cases the total available power is significantly higher than the available power obtained in the undisturbed flow. For HF lasers pumped by the chain reactions it was found that choking occurs over a wide range of flow parameters including very weak shock waves. This is due to the large amount of heat released by the exothermic reactions.

The Use of Nitrogen as a Diluent in cw DF Lasers

The ineffectiveness of N2 as a secondary diluent in CW combustion-driven supersonic HF diffusion lasers is investigated experimentally, measuring the rotational temperatures of the DF molecules spectroscopically via the first-overtone emission and comparing the results of N2 and He systems. The results are presented graphically, plotting temperature versus the molar ratio of N2 or He to available F2. It is shown that increasing the ratio of He produces a significant drop in temperature, but increasing the N2 ratio has none. The drop in laser power and specific power (per total mass flow) with increasing N2 ratio reported by Spencer et al. (1972) and by Voignier and Regnier (1980) is confirmed. This effect can now be attributed to the inability of N2, because of its slow diffusion and low conductivity, to absorb the heat of the laser reaction.

Comparison of geometric and physical optics closed-cavity calculations with HF experiments

Geometrical and physical optics models for calculating the closed-cavity power of a cw supersonic diffusion chemical laser are presented. The mixing and kinetic gain medium formulation employed in these calculations is described along with its anchoring to HF small signal gain data. Mixing parameters thus established are used to compute the closed-cavity power and spectral distribution with the two models, which agree reasonably well with experimental data. The reasonably good agreement between the two models in their computed spectra, intensity, and loaded gain distributions indicates that in many applications the use of the more economical geometric model may be adequate for extensive closed-cavity power computations and performance analyses of chemical lasers.

Temperature requirements and corrosion rates in combustion driven hydrogen fluoride supersonic diffusion lasers

A maximum F-atom yield from F2 occurs in a combustion driven hydrogen fluoride supersonic diffusion laser (HFSDL) because the amount of fluorine reacted with hydrogen (or deuterium) continues to increase with temperature after most of the unreacted fluorine has been thermally dissociated. A small decease from the maximum combustor F-atom yield allows a significant decease in the required temperature and in the corrosion rates that uncooled laser nozzles would display. The temperatures that give F-atom yields equal to 95 percent of the maximum values were calculated for typical HFSDL combustor pressures and F-atom mole fractions and the corrosion rates of uncooled nozzles were evaluated at these temperatures. The corrosion rates of materials resistant to fluorine attack at the highest temperatures would allow HFSDL applications or test experiments up to several hours duration.

A supersonic multikilohertz pulsed HF chemical laser

Operation of a pulsed, multikilohertz-repetition-rate, supersonic HF chemical laser is described. Continuously flowing reactants are premixed and then expanded through a supersonic nozzle to the laser cavity where a pulsed electrical discharge initiates the chemical reaction producing the lasing species. An average power of 12 W has been obtained from a hydrogen/sulfur-hexafluoride/helium mixture. It is suggested that this apparatus can be used to overcome some of the inherent problems in achieving quasi-continuous laser operation based on a chain reaction in fluorine/hydrogen mixtures.

Numerical modeling of processes in a continuous chemical HF laser resonator on the basis of the Navier-Stokes equations

An effective finite-difference method for numerical integration of the complete system of Navier-Stokes equations is proposed for multicomponent chemically reacting gas mixtures in the presence of relaxation processes and coherent radiation. This method is used to investigate processes proceeding in a continuous supersonic chemical laser resonator using HF molecules. The problem is formulated using five assumptions regarding flow-mixture, diffusion, vibration level, and generation. Within this framework the system of Navier-Stokes equations is written out. Special questions of the mechanism, in particular the degree of influence on the local stream parameters and the output energetic characteristics of the laser effects associated with the off-design of the exhaust regime of the supersonic jets of reagents and with the origination of transverse pressure gradients in the stream due to heat liberation during the exothermal chemical reactions were investigated.

Gasdynamic chemical laser utilizing D–O3–CO2and H–O3–CO2mixtures. I. Experimental investigation

Results are presented of an experimental investigation of a cw supersonic chemical laser utilizing D–O3–CO2 and H–O3–CO2 mixtures. For a D–O3–CO2 mixture the highest small-signal gain was 2.65 ± 0.13 m−1 and the lasing power reached 960 ± 50 W. In this case, the specific lasing power and chemical efficiency of the laser were 60 ± 3 W·sec·g−1 and 18 ± 1%, respectively. For an H–O3–CO2 mixture under the same experimental conditions, the maximum gain of 2.45 ± 0.13 m−1 was achieved, which was only 8% lower than that for a D–O3–CO2 mixture. The downstream length of the inversion zone exceeded 20 cm. The mixing scale of the reagent streams was determined from the absorption of ultraviolet probe radiation by ozone.

High specific power long-pulse supersonic flow XeCl laser at 308 nm

Quasi cw laser action has been achieved at several wavelengths near 308 nm in an electron-beam excited supersonic flow at very low temperature and high density. The 0–1, 0–2, and 0–3 XeCl laser emissions have been obtained when Ne/Xe/HCl mixtures aerodynamically cooled to 120 K and at a density up to 1.7 amagat were excited by an electron beam of medium intensity (jeb≃10 A cm−2, 600 ns). Up to 15 mj of laser energy (≃1 J/l) was extracted in 500-ns pulses from the 16-cm long and 1-cm-diam useful volume of this laser. The highest intrinsic efficiency (laser energy out / e-beam energy deposited in the active medium) observed was over 2.5%.

Laser-induced I2 fluorescence measurements in a chemical laser flowfield

A laser-induced fluorescence experiment is described which was performed in the flow field of a 1-kW supersonic chemical laser. In the experiment molecular iodine vapor was injected into the chemical-laser cavity and made to fluoresce by Ar(+) laser excitation. The results obtained demonstrate that laser-induced fluorescence can be applied successfully to study mixing zones in chemical-laser cavities. The following potential seeding candidates for studying hot flows in chemical lasers are suggested, in addition to I2: popop dye vapor, SiH4, and H2S.

Chemical production of vibrationally excited HBr for a continuous wave laser

Bromine has been added to the hydrogen cavity fuel of a cw supersonic mixing HF chemical laser. The hydrogen atoms produced in the F+H2→HF(v⩽3)+H reaction are used to continuously produce vibrationally excited HBr by the reaction H+Br2→HBr(v⩽6)+Br. Infrared chemiluminescence measurements indicate unsaturated gain coefficients of about (2±1) ×10−3 cm−1 for J=4 through J=8 for v=3, 2, and 1 levels of HBr. The current 3-cm gain length is not sufficient for lasing without the use of high-reflectivity mirrors, but a scaling to 18-cm gain length would be appropriate for a cw HBr chemical laser operating in the 4.0- to 4.5-μm region.

Electrical and laser diagnostics of an 80-kW supersonic cw CO electric laser

Detailed spatial and time-dependent measurements of the electrical parameters and the gas properties have been made on a high-average-power quasisteady supersonic cw carbon monoxide electron-beam-stabilized electric-discharge laser. The electrical and optical measurements show that there are major changes in the current-density distribution and in the supersonic flow field due to the high electric-discharge power input to the gas. A multipin resistor-ballasted electrode has been used to make the current density much more uniform, resulting in a factor-of-2 increase in electrical energy input and in laser power output. Small-signal gain measurements were made on the J=7 to J=13 rotational lines of the V=7 to V=6 CO vibrational transition at various locations in the electrical discharge. The measured rotational temperature showed that only 12% of the discharge power went into gas heating in the supersonic free-stream region. The measured small-signal gain was as high as 0.08 cm−1, the maximum electrical energy density input was 430 J/l amagat, and the maximum laser power density extracted was 120 kW/l.

Pulsed blowdown—electron gun facility for low-temperature and high-pressure supersonic flow electronic transition lasers

The design of a small compact pulsed blowdown apparatus coupled to a cold cathode electron gun is described. This system allows the excitation of a gas mixture in supersonic flow at temperatures of 80 and 120 K and density of 1 amagat with a maximum electron beam current density of 20 A cm(-2) at 300 kV and pulse length of 0.5 mus. With this facility it has been possible, using aerodynamic cooling, to achieve high-power, long-pulse ultraviolet laser operation in a high-pressure active medium on XeF and Ne ii ion transitions. A strong increase in fluorescence intensity was also obtained for numerous atomic lines and molecular bands of interest for the developement of high-power, high-efficiency, visible and ultraviolet lasers.

Performance and diagnostic measurements of a supersonic HCl chemical laser

A supersonic purely chemical continuous-wave HCl laser is described in which nonequilibrium concentrations of atomic chlorine are generated by the branch-chain reaction of NO with ClO2. The ClO2 is injected into a stream of NO in helium immediately upstream of the throat of a slit nozzle. The reacting gases then expand as a free jet into a low-pressure region where HI is added to rapidly form vibrationally excited HCl. A maximum HCl multiline output power of 35 W is obtained, which represents a chemical efficiency of ∼4.5% based on the total exothermicity of the pumping reaction and the limiting flow rate of HI. An efficient technique for visualizing the flow in the expanded region is described. This technique is based on the visible chemiluminescence resulting from the addition of H2S to the NO/ClO2 reaction system.