Cw HF Chemical Laser Research Articles

Possibility of using tetrafluorohydrazine as an oxidiser in a supersonic cw chemical HF laser

We report a computational and experimental study of the possibility of replacing NF3 nitrogen trifluoride, which is used in the NF3 – D2 – He fuel composition in the atomic fluorine generator of a supersonic cw chemical HF laser, with N2F4 tetrafluorohydrazine. As a result of comparing the specific energy characteristics of an HF laser operating with each of the oxidisers, it is shown that the replacement of nitrogen trifluoride with tetrafluorohydrazine in the same laser model leads to an increase in the specific energy output by 13% – 15%. In this case, it is possible to reduce the specific mass of the oxidiser storage system (the ratio of the mass of the oxidiser storage system to the laser facility power), reduce the oxidiser tank dimensions, and simplify the active medium generator design by eliminating the ignition system of the NF3 oxidiser with a D2 primary fuel.

Comparative analysis of characteristics of a supersonic cw chemical HF laser on molecular fluorine and nitrogen trifluoride

We have experimentally studied the characteristics of a supersonic cw chemical HF laser with a flat nozzle array based on a nozzle – nozzle reagent mixing scheme operating with the use of F2 – D2 – He and NF3 – D2 – He fuel compositions in an atomic fluorine generator. The active medium flow field pattern, its gas-dynamic characteristics, and the laser radiation spectrum are considered. The comparison of these characteristics has made it possible to establish a strong impact on the flow field (lasing zone length) of the chemical composition of the combustion products from the atomic fluorine generator. The gas-dynamic parameters of the active medium formed by using a NF3 – D2 – He fuel have turned out to be less acceptable in terms of the gas flow pressure recovery than in the case of F2 – D2 – He fuel, while the radiation spectrum was more preferable from the viewpoint of passing through the atmosphere.

Energy characteristics of a supersonic continuous-wave chemical HF laser using hydrazine decomposition products

We report the results of calculations and experimental studies of specific energy characteristics of a supersonic cw chemical HF laser with a flat nozzle block corresponding to the nozzle – nozzle reagent mixing scheme, in which catalytic decomposition products of hydrazine N2H4 (H2 + N2 + NH3) are used as a secondary fuel. As a result of experiments with cold (300 K) model mixtures, the specific energy extraction of the laser is found to decrease in this case by 43 % compared with the laser utilising pure hydrogen. The calculations show that the main reason for a decrease in the specific energy of the laser is an increase in the temperature in the mixing zone of the oxidising gas and secondary fuel jets. A strong effect of ammonia is revealed. Taking into account the actual heating of the products (up to 1000 K) during the catalytic decomposition of N2H4 allows the level of specific energy extraction from the laser to approach that of energy extraction from the laser operated on pure hydrogen by 13 %, which will lead to a decrease in the specific energy extraction by only 30 % in the absence of NH3 decomposition products.

Tetrafluoroethylene as a fuel component for an atomic fluorine generator of a supersonic continuous-wave chemical HF laser

A cheap and widely accessible combination (one of non-saturated perfluorocarbons), i.e. C2F4 tetrafluoroethylene, is proposed as an alternative fuel component for an atomic fluorine generator of a supersonic cw chemical HF laser of self-contained type. Combustion products of the hypergolic F2 – C2F4 – He mixture do not contain active relaxants of vibrationally excited HF(v) molecules. According to estimates, the atomic fluorine yield under comparable conditions is about 70 % in relation to that of the atomic fluorine generator operating on the F2 – D2 – He mixture which is the ‘basic’ one for a HF laser. The specific power is reduced by 23 % with an improvement in a set of the fuel performance and a significant reduction in the cost of triggering the laser in the course of its bench tests.

Numerical simulation of a high efficiency combustion-driven HF laser with preheated fuels

We report a simulation calculation of the processes proceeding in a nozzle array and an optical cavity of a combustion-driven cw HF chemical laser with preheated fuels. Investigation results demonstrate that fuel preheating can dramatically improve the output characteristics of combustion-driven HF chemical lasers. The available output power and specific output power are improved by ∼13% and ∼46%, respectively, when fuels are preheated to 1200 K. It is shown that the fuel consumption in the combustor is reduced by ∼36%, which allows one to improve the performance of combustion-driven HF lasers and make them more compact due to fuel preheating.

About possibilities of clearing near-Earth space from dangerous debris by a spaceborne laser system with an autonomous cw chemical HF laser

The possibility of clearing hazardous near-Earth space debris using a spaceborne laser station with a large autonomous cw chemical HF laser is substantiated and the requirements to its characteristics (i.e., power and divergence of laser radiation, pulse duration in the repetitively pulsed regime, repetition rate and total time of laser action on space debris, necessary to remove them from the orbits of the protected spacecrafts) are determined. The possibility of launching the proposed spaceborne laser station to the orbit with the help of a 'Proton-M' carrier rocket is considered.

Numerical simulation of a Q-switched cw chemical HF laser

A mathematical model of a Q-switched cw chemical laser is developed. The model is based on a simplified description of the active medium and the exact wave description of the mode structure of radiation. The model is used to study numerically the spatial–angular parameters of the output beam of a HF laser. We considered an unstable cavity that provided a high stability of the axis of the laser-pulse radiation pattern upon Q-switching with the help of a rotating mirror. The results of numerical simulation showed that for typical dimensions of a plane or an unstable cavity of the positive branch in a cw chemical laser, the angular radiation divergence rapidly changes in time and on average exceeds the diffraction limit by an order of magnitude. The problem can be solved by replacing a rotating mirror Q switch by a fixed one with time-dependent transmission. We also considered a negative-branch unstable cavity which can provide a stable radiation divergence upon Q-switching produced by a rotating mirror.

Q-switched chemical cw HF laser

The characteristics of a cw chemical HF laser mechanically Q-switched at a frequency of up to 1000 Hz are studied. It is shown that the peak power of laser pulses increases by a factor of at least five for modulation frequencies from 17 to 250 Hz. The mean output power in the repetitively pulsed mode is appreciably lower than that in the cw lasing mode, but, as the modulation frequency increases (within the same frequency range), the mean power rises more rapidly than the peak power does. The possible factors causing the revealed discrepancies between the experimental and calculated laser characteristics are considered.

Characteristics of a cw chemical HF laser operating by using a new oxidising-gas production technique

The energy and spectral characteristics of an HF laser operating by using a new method of obtaining the oxidising gas based on a two-region mixing are studied experimentally. The investigation involved redistribution of the overall amount of the rare-gas diluent (helium) delivered to the laser between the gas generator (the first mixing region) and the subsonic parts of the nozzles of the nozzle unit (the second mixing region). In this case, the temperature in the gas generator was maintained constant and the experiments were conducted under conditions when either the mass flow of atomic fluorine or the pressure in the gas generator were maintained constant. It was shown that upon a nearly full transfer of helium from the first mixing region to the second one in the constant-pressure regime in the gas generator, the laser output power increased by 70%, while in the regime of a constant mass flow of atomic fluorine, the output power increased by 14% along with a simultaneous increase in the specific energy extraction by 40%. The lasing region was found to lengthen by no less than 35%.

Selection of spectral lines of a CW chemical HF laser

The spectral distribution of laser radiation power generated in two (initial and final) regions of the flow of an active medium formed by a three-jet nozzle array of a supersonic cw chemical HF laser embodying the nozzle-nozzle-nozzle mixing scheme is investigated experimentally. It is demonstrated that the spectral lines can be selected by merely varying laser operating parameters, such as the feedback factor of the cavity, the orientation of the optical axis of the cavity with respect to the exit plane of the nozzle array, and the degree of secondary dilution of the active medium with helium. This method results in the selection of one line P1−0 (6) with a power as high as 80% of the total radiation power and two lines P1−0 (5) and P1−0 (6) with a power of 100%.

Overtone cw chemical HF laser with an extended active medium

The operation on the first overtone transition of the HF(ν) molecule is demonstrated for a cw chemical laser with a three-jet nozzle unit using a nozzle — nozzle — nozzle reagent mixing scheme. The energy, spatial, and spectral characteristics of the laser, as well as the efficiency of conversion of the chemical energy of the pump reaction to the overtone radiation are experimentally estimated. Lasing with an output power of ~1.1 kW and a conversion efficiency of 18 % is obtained in the 1.321 — 1.34 μm wavelength range. The extent of the lasing region is 5.3 cm, which is several times longer than in a laser employing a two-jet nozzle unit with a radial expansion in a nozzle — injector scheme.

Study of the dynamics of spatial — energy and spectral characteristics of a cw chemical HF laser with a three-jet nozzle array

The development of spatial — energy and spectral characteristics of a cw chemical HF laser with a three-jet nozzle array that was built using the nozzle — nozzle — nozzle mixing scheme is experimentally studied as a function of the degree of secondary dilution of the active medium with helium. The energy and spectral characteristics of laser emission were found to be unstable in time and dependent on the cavity configuration. The length of the lasing region upon the secondary dilution reached 10 — 12 cm, whereas the spacing between the onset. of this region and the edge of the nozzle array was independent of the degree of dilution. The possibility of selecting spectral lines by varying the position of the optical cavity axis and the degree of secondary dilution was demonstrated.

Effect of the method of secondary-fuel supply on the characteristics of a cw chemical HF laser with a three-jet nozzle array

The effect of the angle of secondary-fuel injection into a flow of oxidising gas and pressure of an active medium on the energy and spatial characteristics of a cw chemical HF laser with a three-jet nozzle array are experimentally studied. It is shown that the intensification of reagent mixing at the output of the nozzle array with the three-jet nozzle—nozzle—injector scheme due to the secondary-fuel injection at an angle of 20° to the direction of oxidising-gas flow enables one to reach a maximum of specific output energy, with the mass hydrogen flow lowered by 20% compared to the case of parallel injection. This is accompanied by a 12% increase in the specific output energy and a 15% shortening of the lasing region. The output energy as large as 50% of the maximum value can be reached at the static pressure of the active medium of about 13 Torr, which enables one to increase the pressure at the output of a diffuser by a factor of two when evacuating exhausted reaction products into the environment.

Gas dynamics of the active medium of a supersonic cw HF chemical laser

Gas-dynamic characteristics of a 5-kW supersonic cw HF chemical laser with a nozzle array of size 25 cm × 2.8 cm and the nozzle — nozzle mixing scheme were experimentally studied. The distributions of Mach numbers, static pressure, total pressure behind the normal shock, and the loss of total pressure were measured in the flow of an active medium in wide ranges of variation of the flow rate of secondary fuel (hydrogen) and pressure in the atomic-fluorine generator. The energy parameters of the laser were found to be interrelated with the gas dynamics and the optical quality of the active laser medium.

Supersonic CW HF chemical lasers with an extended active medium

Spatial-energy, spectral, and optical characteristics of kilowatt supersonic cw HF chemical lasers of self-contained type with three-jet nozzle arrays using nozzle-nozzle-injector and nozzle-nozzle-nozzle configurations were experimentally studied. It was shown that such nozzle arrays enable one to form an active medium as long as 12.5 cm with a high optical quality. In a laser with a wide-aperture stable cavity, a laser beam with a nearly square cross section 11 × 10 cm in size was formed.

Cross-relaxation dynamics on anomalous saturation processes in low pressure supersonic cw HF chemical laser amplifier

It is assumed that both translational and rotational nonequilibrium cross-relaxations play a role simultaneoulsy in low pressure supersonic cw HF chemical laser amplifier. For two-type models of gas flow medium with laminar and turbulent flow diffusion mixing, the expressions of saturated gain spectrum are derived respectively, and the numerical calculations are performed as well. The numerical results show that turbulent flow diffusion mixing model is in the best agreement with the experimental result.

Influence of the oxidiser gas composition on the overtone generation efficiency of a supersonic cw chemical HF laser

An experimental investigation was made of the influence of the chemical composition of the oxidiser gas in an atomic-fluorine generator on the efficiency of generation of radiation representing the first overtone of the HF molecule in a self-contained supersonic cw chemical HF laser with the active medium 70 cm long. The optimal chemical composition was different for the fundamental and overtone transitions. A specific output energy of 84 J g-1 at a specific mass flow rate of 0.13 g s-1 cm-2 through the nozzle array was achieved by optimisation of a linear three-mirror optical cavity at the 1.33 — 1.35 μm wavelengths. The overtone radiation power generated in the whole of the active medium was 7.5 kW, corresponding to a 41% efficiency of energy conversion to an overtone.

Experimental investigation of a supersonic cw chemical HF laser with a three-jet nozzle array

An experimental investigation was made of a self-contained supersonic cw chemical HF laser with a 40 cm ×11 cm three-jet nozzle array and the nozzle—nozzle—nozzle configuration. Such a nozzle array made it possible to form an active medium ~12 cm long and of high optical quality. The use of a wide-aperture stable optical cavity resulted in generation of a laser beam of square (11 cm×11 cm) cross section. A specific output energy of ~80 J g-1 was reached.

Numerical modelling of a supersonic cw chemical HF laser with a three-jet nozzle array

A simultaneous calculation was made of the processes in the nozzle array and in the resonator cavity of a supersonic cw chemical HF laser. A numerical investigation and of the characteristics of a laser with a three-jet nozzle array and the nozzle—nozzle—nozzle configuration was carried out within the framework of the full system of the Navier—Stokes equations. This investigation yielded dependences of the specific output energy and of the lasing zone length on the coefficient of secondary dilution of the active medium with helium for different coefficients representing the excess of the secondary fuel.

Influence of the secondary-fuel feed method on the energy and gasdynamic characteristics of a supersonic cw chemical HF laser with a radial-expansion nozzle array

An experimental investigation was made of the energy and gasdynamic characteristics of a supersonic cw chemical HF laser with a radial-expansion (nozzle—sonic injector) nozzle array of 35 cm ×10 cm dimensions. The two types of secondary-fuel feed collector differing in respect of the angular orientation of the hydrogen injector axes relative to the direction of flow of the oxidant gas. The Mach number, the static pressure, and the total pressure distributions behind a plane shock wave were determined in the field of flow of the active medium. Injection of the additional hydrogen jets into the oxidant gas stream intensified mixing of the reactants, increased the specific output energy by 10%, and improved homogeneity of the gasdynamic parameters of the active medium.