rus
Issue #8/2020
A. V. Avdeev, A. S. Boreisho, I. A. Kiselev, A. V. Morozov, A. E. Orlov
Supersonic Gas and Chemical Lasers: Technology Development
Supersonic Gas and Chemical Lasers: Technology Development
DOI: 10.22184/1993-7296.FRos.2020.14.8.648.661
Supersonic gas and chemical lasers are sources of powerful continuous radiation with high optical quality. These qualities allow the use of such lasers as part of autonomous mobile complexes. A brief overview of work on the creation and improvement of lasers, the formation of high-quality radiation, the development of advanced storage and preparation systems for the working fluid, and pressure recovery systems is presented.
Supersonic gas and chemical lasers are sources of powerful continuous radiation with high optical quality. These qualities allow the use of such lasers as part of autonomous mobile complexes. A brief overview of work on the creation and improvement of lasers, the formation of high-quality radiation, the development of advanced storage and preparation systems for the working fluid, and pressure recovery systems is presented.
Теги: chemical oxygen-iodine lasers (coil) continuous-wave chemical lasers based on hf and df molecules (hf gas-dynamic co2 lasers (gdl) газодинамические со2‑лазеры (гдл) непрерывные химические лазеры на молекулах hf и df (hf / df-нхл) химические кислород-йодные лазеры (хкил)
Supersonic Gas and Chemical Lasers: Technology Development
A. V. Avdeev 3, A. S. Boreisho 1,2, I. A. Kiselev 1,2, A. V. Morozov 1,2, A. E. Orlov 2
Baltic State Technical University “VOENMEH” D. F. Ustinov, St. Petersburg, Russia
Laser Systems JSC, St. Petersburg, Russia
Moscow Aviation Institute (National Research University), Moscow, Russia
Supersonic gas and chemical lasers are sources of powerful continuous radiation with high optical quality. These qualities allow the use of such lasers as part of autonomous mobile complexes. A brief overview of work on the creation and improvement of lasers, the formation of high-quality radiation, the development of advanced storage and preparation systems for the working fluid, and pressure recovery systems is presented.
Keywords: gas-dynamic CO2 lasers (GDL), continuous-wave chemical lasers based on HF and DF molecules (HF / DF-CWCL), chemical oxygen-iodine lasers (COIL)
Received on: 24.11.2020
Accepted on: 04.12.2020
Introduction
Since the advent of optical quantum generators, the most powerful systems have always attracted particular interest, and today the question of obtaining high power densities of light energy at a large distance for a sufficiently long exposure time remains topical. Currently, one of the most powerful sources of CW laser radiation are supersonic gas and chemical lasers (SGCL). These lasers include gas-dynamic CO2 lasers (GDL), continuous-wave chemical lasers based on HF and DF molecules (HF / DF-CWCL), and chemical oxygen-iodine lasers (COIL). SGCL differ from each other in physical principles of operation, the composition of the active medium, and gas-dynamic parameters. However, they have a common design scheme, and the workflow from the point of view of gas dynamics has much in common: the issues of mixing supersonic reacting flows, flow behind multi-nozzle blocks, pressure recovery in rectangular channels, etc. are common.
The possibility of obtaining high radiation power and transmitting it over long distances, long operating time, the absence of external energy sources, high energy efficiency and good optical quality of the beam all this makes the use of SGCL in autonomous mobile systems for various applications relevant today [1].
Gas-dynamic CO2 lasers
Combustion-driven gas-dynamic CO2 lasers were developed back in the 70s‑80s of the last century. Laboratory GDLs of megawatt class, developed in the USSR and the USA, as well as a 400 kW American laser of the same type, together with a beam control system, installed at the flying laboratory ALL (Fig. 1) [2], for the first time made it possible to practically check and evaluate the scale and complexity of technological problems of creating laser weapons.
CO2-GDL is a molecular laser that operates on forced transitions between the vibrational levels of a carbon dioxide (CO2) molecule. The conversion of the accumulated energy of the active medium into laser radiation occurs as a result of the exothermic reaction of combustion of hydrocarbon fuels with oxygen-containing oxidants and the acceleration of the resulting combustion products to supersonic speeds using nozzle blocks.
The scheme of combustion-driven gas-dynamic CO2 laser is shown in Fig. 2. CO2-GDL consists of a components storage and supply system (CSSS), a gas generator, where a gas mixture with a high temperature and pressure is produced, containing molecules of CO2, N2 and H2O, a nozzle block consisting of a large number of flat nozzles, where there is a rapid cooling of the mixture during gas-dynamic expansion, a laser chamber, an optical resonator and a supersonic diffuser, which ensures the exhaust of the spent medium into the environment.
However, the long wavelength (10.6 μm) and, consequently, the high divergence of the laser beam, the not sufficiently high energy efficiency of laser sources, as well as the need to store on board all the components of the fuel for the laser, leads to the fact that systems based on CO2-GDL capable of solving problems of forceful action on distant objects become too bulky, which does not allow them to be placed on mobile carriers.
Nevertheless, there are variants of the GDL, where air supplied from the environment by the compressor of an aircraft gas turbine engine (GTE) is used as an oxidizer, where the main fuel component (more than 95%) is taken from the atmosphere. This allows you to significantly reduce the storage system of components, since the transportable stock includes only fuel, the share of which in the total consumption does not exceed 5%. Such a laser seems to be quite promising for placing it on aircraft.
The ideas of combining GDL with gas turbine engines have existed for a long time, both in our country and abroad. There are domestic and foreign patents for the introduction of the GDL into an aircraft engine [3–5]. In the late 70s, the world’s first gasdynamic laser with air extraction from the R‑38–300 aircraft engine with a power of about 180 kW was created under the leadership of domestic academician O. N. Favorsky, which continuously operated for a long time [6]. It was a cumbersome ground installation.
Over the entire period of development of aviation GTEs from the 1950s to the 2010s, their characteristics have significantly improved, affecting the output parameters of the GDL, such as the pressure behind the compressor and the operating temperature in the combustion chamber. There is a steady increase in the degree of pressure increase to values of 30–35 atm. and the temperature level of workability of materials up to 1200–1250K, as well as an increase in the operating temperature of the gas up to 2000K. All this makes it possible to provide acceptable energy characteristics of the laser and create a CO2-GDL placed on board an aircraft.
Figure 3 shows the output energy characteristics of the GDL based on the “kerosene-air” fuel composition based on two aircraft engines AL‑31F [7]. It can be seen that when using two engines with a high degree of pressure increase and a high flow rate, even when only 5% of the air is taken off, it is possible to obtain the laser power in the uncooled version of the nozzle unit (the temperature in the laser prechamber is not more than 1500K) of about 80 kW. Using cooled nozzle blades and a pre-chamber temperature of about 2000K, the power can be increased to 135 kW. The general view of such a laser is shown in Fig. 4.
Continuous-wave chemical lasers based on HF and DF molecules
Interest in HF(DF)-CWCL is associated with the possibility of efficient conversion of the internal chemical energy of substances into coherent radiation, bypassing other stages of conversion. The chemical efficiency of CWCL significantly exceeds the efficiency of other lasers, which makes it possible to obtain high radiation power. Moreover, the use of HF-CWCL is possible only for space conditions, since its wavelength is strongly absorbed in the atmosphere, and DF-CWCL, with the wavelength falling into the atmospheric transparency window, can be used under ground conditions.
The schematic diagram of a laser facility based on HF(DF)-CWCL is shown in Fig. 5. Autonomous HF(DF)-CWCL consists of the following main parts: an active medium generator (AMG); optical resonator; supersonic diffuser; a heat exchanger to reduce the temperature of the laser exhaust gases; an ejector installed when there is a need to provide exhaust to the atmosphere.
HF-CWCL for space applications
One of the promising options for using HF-CWCL is the ability to clean near-earth space from space debris (SD) using a multipurpose space laser facility (MSLF) [8]. Moreover, the MSLF can be used as a source of sounding radiation for analyzing the content of hydrocarbons in the atmosphere.
The MSLF is located in the unpressurized compartment of the spacecraft and includes: a radiation generation system consisting of generators of the active medium and an optical system; storage and delivery system for laser fuel components; forming optical system (FOS) and laser ranging system.
The layout diagram of the MSLF on board the spacecraft using large-size flat-block AMG, the prototypes of which were developed at NPO Energomash, is shown in Fig. 6. On the generator of the active medium with a flat nozzle grating AMG, a master generator of repetitively pulsed radiation with a serially connected preliminary radiation power amplifier is assembled. Neutralization of jet thrust during operation of AMG is provided by an exhaust tract containing a supersonic diffuser and two symmetrical oppositely directed exhaust pipes. The operation of AMG can be carried out both in the DF-CWCL mode for laser sensing of the Earth’s atmosphere, and in the HF-CWCL mode in order to combat space debris. The optical system of radiation generation system (RGS) is mounted on a separate optical frame.
The system for supplying laser fuel components in a gaseous state to the active medium generators of the MSLF RGS includes cylinders and elements of the supply system: pipelines, valves, throttles, as well as a frame and units of the general assembly. The system provides 10 s operation in the HF-CWCL radiation mode and 100 s in the DF-CWCL mode. The storage system for laser fuel components consists of cryogenic cylindrical tanks with elliptical bottoms for storing hydrogen, deuterium, helium and nitrogen trifluoride.
FOS consists of a cylindrical matching telescope with a section (200 × 200) mm. The forming telescope contains mirrors with an aspherical surface and two optical hinges, providing guidance of radiation in two mutually perpendicular planes in an angular range of ±7.5° and a drive for moving the counter-reflector mirror. The laser ranging system includes a frequency-doubling yttrium-aluminum garnet laser with a radiation energy of ~ 0.5 J, a laser locator telescope with a main mirror diameter of 120 mm and an optical system for combining the power and location channels.
The spacecraft with MSLF on board has an active lifetime in orbit of 180 days. The total mass of the spacecraft with MSLF is ~ 19,700 kg and it can be placed under the fairing of the Proton-M launch vehicle. In this case, the total duration of operation is 30 minutes with the exposure time to the space debris ~ 1 s in the mode of repetitively pulsed radiation on HF molecules with an energy of 0.8 J per pulse and 180 minutes in the mode of radiation on DF molecules with an energy in the pulse of ~ 5 mJ.
Chemical oxygen-iodine lasers
Since the nineties of the last century, another CWCL began to actively develop, the chemical oxygen-iodine laser (COIL). The COIL radiation wavelength (λ = 1.315 μm) falls within the atmospheric transparency window and also corresponds to the operating range of fiber optics. This means that there are no restrictions on the use of lasers of this type in various atmospheric and extra-atmospheric conditions. A short wavelength ensures a decrease in the diffraction limit, and a low density of the active medium in the resonator cavity provides a high optical quality of the laser beam.
It was on the basis of the oxygen-iodine laser that the most powerful mobile complex ABL (Airborne Laser, USA) [9] of the megawatt class, located on board the Boeing 747–400F aircraft, and designed to destroy ballistic missiles at distances up to 400 km, was created.
The principle of operation of a chemical oxygen-iodine laser is based on the conversion of chemical energy into radiation energy by mixing flows of singlet oxygen – O2 ( 1Δ) (SO) and gaseous molecular iodine. A schematic diagram of a chemical oxygen-iodine laser is shown in Fig. 7.
A supersonic COIL consists of the following main elements: a singlet oxygen generator (SOG), in which singlet oxygen is produced during the gas-liquid reaction of gaseous chlorine (Cl2) with an basic hydrogen peroxide (BHP), an iodine evaporator, a nozzle bank (NB), where streams of SO and gaseous iodine are accelerated and mixed of the, the laser chamber (LC), where the jets of the SO oxidizer and gaseous iodine mix and react, resulting in the formation of excited atoms I*, an optical resonator and a pressure recovery system (PRS), which provides exhaust products into the atmosphere.
In the development of COIL technologies, the main attention was paid to finding ways to increase the efficiency of SOG (increasing the output of SO and utilization of chlorine, reducing the ratio of solution consumption to chlorine consumption, increasing pressure while minimizing SO losses), optimizing the mixing scheme of reacting components in the nozzle block, as well as improving operational characteristics.
One of the most difficult tasks is to ensure the exhaust of the spent active medium of the laser while ensuring optimal conditions for generating radiation. The complexity of the problem is due to the low pressure of the flow in the laser chamber, which leads to high requirements for the compression ratio of the PRS, as well as the presence of chemical and kinetic processes along the entire length of the laser path from the SOG to the exhaust diffuser, which lead to a noticeable release of heat in the flow. Due to the complexity of the processes occurring in the gas-dynamic path of the COIL, it is not possible to carry out model experimental studies in full. Therefore, it is necessary to have full-scale installations that allow the testing of all elements of the COIL.
Fig. 8 shows a laboratory COIL with an output power of up to 15 kW (Laser Systems JSC) and a laboratory demonstration COIL complex developed by specialists of Laser Systems JSC, scientists of the D. F. Ustinov BSTU “VOENMEH” and the Samara branch of the Physics Institute of the Russian Academy of Sciences [10], which includes the latest achievements in the development of COIL and is intended for testing and optimizing the entire process chain from storage of initial components to supplying radiation to the place of use and disposal of the spent active medium.
The laser complex allows to work for up to 30 seconds with a power of more than 300 W with a stable resonator. The centrifugal bubbling singlet oxygen generator provides the ratio of the solution flow rate to the chlorine flow rate up to 1 l / mol, a high chlorine utilization rate (more than 95%) and the singlet oxygen yield (more than 60%). The laser nozzle unit is a single profiled slotted nozzle with an iodine injector in the transonic region. The optical cavity is multimode with an intracavity diaphragm and an output aperture of 50 mm.
The exhaust system of the spent active medium of the laboratory complex is a two-mode system, which includes a two-stage mechanical pump and a cryosorption pumping system. The laser can operate both on a mechanical pump and on a cryosorption system. The installation includes a fiber-optic system that allows transporting laser radiation at a distance of up to 80 meters.
Pressure recovery systems for continuous-wave chemical lasers
In SGCL in the cavity of the resonator, it is necessary to have a low pressure Р ≈ 30 Torr – for СО2-GDL, Р = 12–15 Torr for HF / DF-CWCL, and Р = 4–6 Torr for COIL. To ensure the operation of SGCL in ground conditions, the Pressure Recovery Systems (PRS) are used, which in the general case include a exhaust supersonic diffuser (SD) and an ejector (EJ) (Fig. 9). CO2-GDL can function using conventional “passive” diffusers. The HF / DF-CWCL requires the use of a single-stage ejector with a compression ratio of 10–12 in the PRS, and a two-stage ejector with a total compression ratio of the order of 50–60 is required for a SGCL [11].
The development of such complex technical systems as PRS for SGCL requires coordination of the gas dynamics of the entire channel, from the laser nozzle block to the last stage of the ejector. Moreover, the flow in the channel is complicated by the fact that the flow has low Re numbers (104–105 for HF / DF-CWCL, 103–104 for COIL) and is accompanied by chemical and kinetic processes. In addition, the channel of a real installation has a complex geometry, which leads to a significant complication of the flow and the appearance of a launch feature of such a channel.
SCL exhaust supersonic diffusers
The deceleration of the supersonic flow in the SD channels occurs in a system of oblique shock waves that arise due to the growth of the boundary layer on the channel walls. The recovery pressure behind such a system of jumps at large Re numbers is close to the pressure behind the direct jump at the corresponding length of the SD [11]. This becomes incorrect for chemical lasers due to the low Re numbers and heat release in the supersonic flow.
Integrally, the change in the velocity W in the flow under external action describes the well-known relationship:
,
where dF is the change in the channel cross-sectional area, dQ is the heat input, and dFfr is the friction force. When heat is supplied to a supersonic flow in a channel of constant cross section, the flow is inhibited. Thus, in chemical lasers, a mechanism related to heat release is added to the gas-dynamic mechanism of flow deceleration, which leads to a reduction in the length of the deceleration zone. Moreover, its length depends on many design and operating parameters. Therefore, NB, LC and SD in a SCL should be designed as a whole. A particularly strong influence of complicating factors – low Re number of the flow and heat release on the flow is observed in COIL. Figure 10 shows the experimental data on the change in pressure in the resonator versus the back pressure. The pressure in the laser chamber increases with increasing back pressure, which is associated with the presence of thick boundary layers, along which disturbances from the subsonic zone are transmitted towards the flow to the nozzle.
To achieve effective SD operation, it is necessary to control the growth of the boundary layer. For large-scale installations, this can be done by blowing high-pressure gas from small nozzles into the boundary layer along the channel walls.
In PRS for COIL, the ejector must have two stages with a compression ratio ε ≈ 40–50. Therefore, it is possible to combine the blowing in the diffuser with the first stage of the ejector, i. e. implement an active diffuser (AD) [12]. Fig. 11 shows a diagram of the inlet part of an active diffuser and a general view of an PRS with an AD, developed at Laser Systems JSC for COIL with an active medium flow rate of 0.2 kg / sec.
Experiments have shown [12] that an active diffuser allows not only to significantly increase the total ejection coefficient of the PRS, but also to improve the optical quality of COIL radiation due to the suction of the boundary layer from the walls of the laser chamber, which prevents the appearance of shock waves in the cavity.
EJECTORS
An ejector (EJ) is a device where the energy of a high-pressure active flow is transferred to a low-pressure passive gas during mixing, thereby increasing its total pressure. The classical EJ scheme is of two types – central, with one axisymmetric active gas nozzle, or peripheral, with an annular nozzle. The problem of increasing the ejection coefficient (the ratio of passive and active gas flow rates) at a given compression ratio is important from the point of view of reducing the mass-dimensional characteristics of the PRS.
In SCL PRS, the active and passive gases are different, in this case, the ejection coefficient can be defined as (where T is the temperature, μ is the molar mass, and n00 is the ejection coefficient for the same gases). Hence, it can be seen that to increase the total n, it is necessary to increase the temperature and decrease the molar mass of the active gas, or decrease the temperature of the passive gas. Therefore, as an active gas in the PRS, a vapor-gas mixture is used, with a low molar mass in comparison with the combustion products and the maximum allowable temperature. Heat exchangers are used to reduce the temperature of the passive gas.
It is possible to increase the ejection coefficient in comparison with traditional schemes by intensifying the process of mixing flows with the help of vortex-forming elements installed on the active gas nozzle. When using such elements, it is possible to increase the n × ε values by 12–15% [13]. When using a distributed mixing scheme – the injection of an active gas through a multitude of medium-sized nozzles, the ejection coefficient can be increased by 1.5 times [14].
PRS for COIL, built on the basis of AD as the first stage of the ejector and the second stage, using mixing intensifiers and multi-nozzle active gas supply, allows not only to reduce the size of the laser module in comparison with the traditional layout and to reduce the required active gas consumption, but also to improve the optical quality of radiation [14].
Conclusion
Despite the long and not always successful history of supersonic gas and chemical lasers, they still remain unsurpassed in the power of continuous radiation with high optical quality. Another indisputable advantage of such lasers is their energy “self-sufficiency”, which allows them to be used as part of autonomous mobile complexes.
During the decades that have passed since their inception, interest in them has changed, and not always in a positive direction, however, work on the study and improvement of the lasers themselves, the formation of high-quality radiation, the development of promising storage systems and preparation of the working fluid, pressure recovery systems continue, which we wanted to show in this article.
The authors hope that all the lasers discussed above, including multi-kilowatt CO2-GDL based on GTJE with practically unlimited time of continuous radiation, sufficient for solving many practical problems on the ground and in the air, and highly efficient multi-megawatt continuous chemical lasers capable of solving unique problems in deep space, will also attract the attention of potential users and customers in the near future.
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Patent RU 2587509. Method of operating aircraft gas turbine engine and device therefor / Vovk M. YU., Marchukov E.YU., Petrienko V. G., Perelshtejn B. KH. 2015.
Gubarev V. Samaya vygodnaya energetika v Rossii budet na gaze. V mire nauki. 2017; 11: 110–115.
Boreisho A. S. et al. Combustion-driven gas-dynamic CO2‑laser on the basis of modern aviation engines. Journal of Physics: Conference Series. 2020; 1565.
Avdeev A. V. et al. About possibilities of clearing near-Earth space from dangerous debris by a spaceborne laser system with an autonomous cw chemical HF laser Quantum Electronics (2011),41(7): 669–674. DOI:10.1070/QE2011v041n07ABEH014534.
Trusdell K. A. Recent Airborne Laser –laser results. Proc. SPIE. 2006; 6343: 6346 1L‑1-6346 1L‑16.
Boreysho A.S, Evdokimov I. M., Kiselev I. A., M. Konyaev, V. Skorniakov, Zagidullin M. V., Khvatov N. A. Chemical Oxygen Iodine Laser Technologies Laboratory Complex Demonstrator. Photonics Russia (Fotonica). 2015; № 4:92–101.
Boreysho A. S., Malkov V. M. et al. Pressure recovery systems for high-power gas and chemical lasers. Thermophysics and Aeromechanics. 2001; 8(4): 605–623.
Borejsho A. S. et al. Himicheskij kislorod-jodnyj lazer: aerooptika i gazodinamika. Inzhenerno-fizicheskij zhurnal. 2011; 84(1): 57–73.
Boreysho A. S., Druzhinin S. L., Malkov V. M. et al. Pressure recovery system for a high-power HF / DF laser: implementation practice. Thermophysics and Aeromechanics. 2007; 14(4): 591–607.
Malkov V. M., Kiselev I. A., Shatalov I. V., Duk A. A., Emelyanova A. V. Ejectors for pressure recovery systems of supersonic chemical lasers. Thermophysics and Aeromechanics. 2017; 24(3): 443–459. DOI: 10.1134/S0869864317030118.
About authors
Avdeev A. V., Ph. D, Moscow Aviation Institute (National Research University), Moscow
ORCID: 0000-0003-2643-6622
Boreysho A. S., Dr. Sc., Professor, BSTU “VOENMEH” D. F. Ustinov, St. Petersburg.
ORCID: 0000-0002-3245-9321
Kiselev I. A., Ph. D, BSTU “VOENMEH” D. F. Ustinov, St. Petersburg.
ORCID: 0000-0002-8092-1648
Morozov A. V., Ph. D, BSTU “VOENMEH” D. F. Ustinov, St. Petersburg.
ORCID: 0000-0002-0948-7367
Orlov A. E., Ph. D, Laser Systems JSC, St.Petersburg.
ORCID: 0000-0001-9515-8107
Contribution by the members
of the team of authors
The article was prepared on the basis of many years of work by all members of the team of authors.
Conflict of interest
The authors claim that they have no conflict of interest.
A. V. Avdeev 3, A. S. Boreisho 1,2, I. A. Kiselev 1,2, A. V. Morozov 1,2, A. E. Orlov 2
Baltic State Technical University “VOENMEH” D. F. Ustinov, St. Petersburg, Russia
Laser Systems JSC, St. Petersburg, Russia
Moscow Aviation Institute (National Research University), Moscow, Russia
Supersonic gas and chemical lasers are sources of powerful continuous radiation with high optical quality. These qualities allow the use of such lasers as part of autonomous mobile complexes. A brief overview of work on the creation and improvement of lasers, the formation of high-quality radiation, the development of advanced storage and preparation systems for the working fluid, and pressure recovery systems is presented.
Keywords: gas-dynamic CO2 lasers (GDL), continuous-wave chemical lasers based on HF and DF molecules (HF / DF-CWCL), chemical oxygen-iodine lasers (COIL)
Received on: 24.11.2020
Accepted on: 04.12.2020
Introduction
Since the advent of optical quantum generators, the most powerful systems have always attracted particular interest, and today the question of obtaining high power densities of light energy at a large distance for a sufficiently long exposure time remains topical. Currently, one of the most powerful sources of CW laser radiation are supersonic gas and chemical lasers (SGCL). These lasers include gas-dynamic CO2 lasers (GDL), continuous-wave chemical lasers based on HF and DF molecules (HF / DF-CWCL), and chemical oxygen-iodine lasers (COIL). SGCL differ from each other in physical principles of operation, the composition of the active medium, and gas-dynamic parameters. However, they have a common design scheme, and the workflow from the point of view of gas dynamics has much in common: the issues of mixing supersonic reacting flows, flow behind multi-nozzle blocks, pressure recovery in rectangular channels, etc. are common.
The possibility of obtaining high radiation power and transmitting it over long distances, long operating time, the absence of external energy sources, high energy efficiency and good optical quality of the beam all this makes the use of SGCL in autonomous mobile systems for various applications relevant today [1].
Gas-dynamic CO2 lasers
Combustion-driven gas-dynamic CO2 lasers were developed back in the 70s‑80s of the last century. Laboratory GDLs of megawatt class, developed in the USSR and the USA, as well as a 400 kW American laser of the same type, together with a beam control system, installed at the flying laboratory ALL (Fig. 1) [2], for the first time made it possible to practically check and evaluate the scale and complexity of technological problems of creating laser weapons.
CO2-GDL is a molecular laser that operates on forced transitions between the vibrational levels of a carbon dioxide (CO2) molecule. The conversion of the accumulated energy of the active medium into laser radiation occurs as a result of the exothermic reaction of combustion of hydrocarbon fuels with oxygen-containing oxidants and the acceleration of the resulting combustion products to supersonic speeds using nozzle blocks.
The scheme of combustion-driven gas-dynamic CO2 laser is shown in Fig. 2. CO2-GDL consists of a components storage and supply system (CSSS), a gas generator, where a gas mixture with a high temperature and pressure is produced, containing molecules of CO2, N2 and H2O, a nozzle block consisting of a large number of flat nozzles, where there is a rapid cooling of the mixture during gas-dynamic expansion, a laser chamber, an optical resonator and a supersonic diffuser, which ensures the exhaust of the spent medium into the environment.
However, the long wavelength (10.6 μm) and, consequently, the high divergence of the laser beam, the not sufficiently high energy efficiency of laser sources, as well as the need to store on board all the components of the fuel for the laser, leads to the fact that systems based on CO2-GDL capable of solving problems of forceful action on distant objects become too bulky, which does not allow them to be placed on mobile carriers.
Nevertheless, there are variants of the GDL, where air supplied from the environment by the compressor of an aircraft gas turbine engine (GTE) is used as an oxidizer, where the main fuel component (more than 95%) is taken from the atmosphere. This allows you to significantly reduce the storage system of components, since the transportable stock includes only fuel, the share of which in the total consumption does not exceed 5%. Such a laser seems to be quite promising for placing it on aircraft.
The ideas of combining GDL with gas turbine engines have existed for a long time, both in our country and abroad. There are domestic and foreign patents for the introduction of the GDL into an aircraft engine [3–5]. In the late 70s, the world’s first gasdynamic laser with air extraction from the R‑38–300 aircraft engine with a power of about 180 kW was created under the leadership of domestic academician O. N. Favorsky, which continuously operated for a long time [6]. It was a cumbersome ground installation.
Over the entire period of development of aviation GTEs from the 1950s to the 2010s, their characteristics have significantly improved, affecting the output parameters of the GDL, such as the pressure behind the compressor and the operating temperature in the combustion chamber. There is a steady increase in the degree of pressure increase to values of 30–35 atm. and the temperature level of workability of materials up to 1200–1250K, as well as an increase in the operating temperature of the gas up to 2000K. All this makes it possible to provide acceptable energy characteristics of the laser and create a CO2-GDL placed on board an aircraft.
Figure 3 shows the output energy characteristics of the GDL based on the “kerosene-air” fuel composition based on two aircraft engines AL‑31F [7]. It can be seen that when using two engines with a high degree of pressure increase and a high flow rate, even when only 5% of the air is taken off, it is possible to obtain the laser power in the uncooled version of the nozzle unit (the temperature in the laser prechamber is not more than 1500K) of about 80 kW. Using cooled nozzle blades and a pre-chamber temperature of about 2000K, the power can be increased to 135 kW. The general view of such a laser is shown in Fig. 4.
Continuous-wave chemical lasers based on HF and DF molecules
Interest in HF(DF)-CWCL is associated with the possibility of efficient conversion of the internal chemical energy of substances into coherent radiation, bypassing other stages of conversion. The chemical efficiency of CWCL significantly exceeds the efficiency of other lasers, which makes it possible to obtain high radiation power. Moreover, the use of HF-CWCL is possible only for space conditions, since its wavelength is strongly absorbed in the atmosphere, and DF-CWCL, with the wavelength falling into the atmospheric transparency window, can be used under ground conditions.
The schematic diagram of a laser facility based on HF(DF)-CWCL is shown in Fig. 5. Autonomous HF(DF)-CWCL consists of the following main parts: an active medium generator (AMG); optical resonator; supersonic diffuser; a heat exchanger to reduce the temperature of the laser exhaust gases; an ejector installed when there is a need to provide exhaust to the atmosphere.
HF-CWCL for space applications
One of the promising options for using HF-CWCL is the ability to clean near-earth space from space debris (SD) using a multipurpose space laser facility (MSLF) [8]. Moreover, the MSLF can be used as a source of sounding radiation for analyzing the content of hydrocarbons in the atmosphere.
The MSLF is located in the unpressurized compartment of the spacecraft and includes: a radiation generation system consisting of generators of the active medium and an optical system; storage and delivery system for laser fuel components; forming optical system (FOS) and laser ranging system.
The layout diagram of the MSLF on board the spacecraft using large-size flat-block AMG, the prototypes of which were developed at NPO Energomash, is shown in Fig. 6. On the generator of the active medium with a flat nozzle grating AMG, a master generator of repetitively pulsed radiation with a serially connected preliminary radiation power amplifier is assembled. Neutralization of jet thrust during operation of AMG is provided by an exhaust tract containing a supersonic diffuser and two symmetrical oppositely directed exhaust pipes. The operation of AMG can be carried out both in the DF-CWCL mode for laser sensing of the Earth’s atmosphere, and in the HF-CWCL mode in order to combat space debris. The optical system of radiation generation system (RGS) is mounted on a separate optical frame.
The system for supplying laser fuel components in a gaseous state to the active medium generators of the MSLF RGS includes cylinders and elements of the supply system: pipelines, valves, throttles, as well as a frame and units of the general assembly. The system provides 10 s operation in the HF-CWCL radiation mode and 100 s in the DF-CWCL mode. The storage system for laser fuel components consists of cryogenic cylindrical tanks with elliptical bottoms for storing hydrogen, deuterium, helium and nitrogen trifluoride.
FOS consists of a cylindrical matching telescope with a section (200 × 200) mm. The forming telescope contains mirrors with an aspherical surface and two optical hinges, providing guidance of radiation in two mutually perpendicular planes in an angular range of ±7.5° and a drive for moving the counter-reflector mirror. The laser ranging system includes a frequency-doubling yttrium-aluminum garnet laser with a radiation energy of ~ 0.5 J, a laser locator telescope with a main mirror diameter of 120 mm and an optical system for combining the power and location channels.
The spacecraft with MSLF on board has an active lifetime in orbit of 180 days. The total mass of the spacecraft with MSLF is ~ 19,700 kg and it can be placed under the fairing of the Proton-M launch vehicle. In this case, the total duration of operation is 30 minutes with the exposure time to the space debris ~ 1 s in the mode of repetitively pulsed radiation on HF molecules with an energy of 0.8 J per pulse and 180 minutes in the mode of radiation on DF molecules with an energy in the pulse of ~ 5 mJ.
Chemical oxygen-iodine lasers
Since the nineties of the last century, another CWCL began to actively develop, the chemical oxygen-iodine laser (COIL). The COIL radiation wavelength (λ = 1.315 μm) falls within the atmospheric transparency window and also corresponds to the operating range of fiber optics. This means that there are no restrictions on the use of lasers of this type in various atmospheric and extra-atmospheric conditions. A short wavelength ensures a decrease in the diffraction limit, and a low density of the active medium in the resonator cavity provides a high optical quality of the laser beam.
It was on the basis of the oxygen-iodine laser that the most powerful mobile complex ABL (Airborne Laser, USA) [9] of the megawatt class, located on board the Boeing 747–400F aircraft, and designed to destroy ballistic missiles at distances up to 400 km, was created.
The principle of operation of a chemical oxygen-iodine laser is based on the conversion of chemical energy into radiation energy by mixing flows of singlet oxygen – O2 ( 1Δ) (SO) and gaseous molecular iodine. A schematic diagram of a chemical oxygen-iodine laser is shown in Fig. 7.
A supersonic COIL consists of the following main elements: a singlet oxygen generator (SOG), in which singlet oxygen is produced during the gas-liquid reaction of gaseous chlorine (Cl2) with an basic hydrogen peroxide (BHP), an iodine evaporator, a nozzle bank (NB), where streams of SO and gaseous iodine are accelerated and mixed of the, the laser chamber (LC), where the jets of the SO oxidizer and gaseous iodine mix and react, resulting in the formation of excited atoms I*, an optical resonator and a pressure recovery system (PRS), which provides exhaust products into the atmosphere.
In the development of COIL technologies, the main attention was paid to finding ways to increase the efficiency of SOG (increasing the output of SO and utilization of chlorine, reducing the ratio of solution consumption to chlorine consumption, increasing pressure while minimizing SO losses), optimizing the mixing scheme of reacting components in the nozzle block, as well as improving operational characteristics.
One of the most difficult tasks is to ensure the exhaust of the spent active medium of the laser while ensuring optimal conditions for generating radiation. The complexity of the problem is due to the low pressure of the flow in the laser chamber, which leads to high requirements for the compression ratio of the PRS, as well as the presence of chemical and kinetic processes along the entire length of the laser path from the SOG to the exhaust diffuser, which lead to a noticeable release of heat in the flow. Due to the complexity of the processes occurring in the gas-dynamic path of the COIL, it is not possible to carry out model experimental studies in full. Therefore, it is necessary to have full-scale installations that allow the testing of all elements of the COIL.
Fig. 8 shows a laboratory COIL with an output power of up to 15 kW (Laser Systems JSC) and a laboratory demonstration COIL complex developed by specialists of Laser Systems JSC, scientists of the D. F. Ustinov BSTU “VOENMEH” and the Samara branch of the Physics Institute of the Russian Academy of Sciences [10], which includes the latest achievements in the development of COIL and is intended for testing and optimizing the entire process chain from storage of initial components to supplying radiation to the place of use and disposal of the spent active medium.
The laser complex allows to work for up to 30 seconds with a power of more than 300 W with a stable resonator. The centrifugal bubbling singlet oxygen generator provides the ratio of the solution flow rate to the chlorine flow rate up to 1 l / mol, a high chlorine utilization rate (more than 95%) and the singlet oxygen yield (more than 60%). The laser nozzle unit is a single profiled slotted nozzle with an iodine injector in the transonic region. The optical cavity is multimode with an intracavity diaphragm and an output aperture of 50 mm.
The exhaust system of the spent active medium of the laboratory complex is a two-mode system, which includes a two-stage mechanical pump and a cryosorption pumping system. The laser can operate both on a mechanical pump and on a cryosorption system. The installation includes a fiber-optic system that allows transporting laser radiation at a distance of up to 80 meters.
Pressure recovery systems for continuous-wave chemical lasers
In SGCL in the cavity of the resonator, it is necessary to have a low pressure Р ≈ 30 Torr – for СО2-GDL, Р = 12–15 Torr for HF / DF-CWCL, and Р = 4–6 Torr for COIL. To ensure the operation of SGCL in ground conditions, the Pressure Recovery Systems (PRS) are used, which in the general case include a exhaust supersonic diffuser (SD) and an ejector (EJ) (Fig. 9). CO2-GDL can function using conventional “passive” diffusers. The HF / DF-CWCL requires the use of a single-stage ejector with a compression ratio of 10–12 in the PRS, and a two-stage ejector with a total compression ratio of the order of 50–60 is required for a SGCL [11].
The development of such complex technical systems as PRS for SGCL requires coordination of the gas dynamics of the entire channel, from the laser nozzle block to the last stage of the ejector. Moreover, the flow in the channel is complicated by the fact that the flow has low Re numbers (104–105 for HF / DF-CWCL, 103–104 for COIL) and is accompanied by chemical and kinetic processes. In addition, the channel of a real installation has a complex geometry, which leads to a significant complication of the flow and the appearance of a launch feature of such a channel.
SCL exhaust supersonic diffusers
The deceleration of the supersonic flow in the SD channels occurs in a system of oblique shock waves that arise due to the growth of the boundary layer on the channel walls. The recovery pressure behind such a system of jumps at large Re numbers is close to the pressure behind the direct jump at the corresponding length of the SD [11]. This becomes incorrect for chemical lasers due to the low Re numbers and heat release in the supersonic flow.
Integrally, the change in the velocity W in the flow under external action describes the well-known relationship:
,
where dF is the change in the channel cross-sectional area, dQ is the heat input, and dFfr is the friction force. When heat is supplied to a supersonic flow in a channel of constant cross section, the flow is inhibited. Thus, in chemical lasers, a mechanism related to heat release is added to the gas-dynamic mechanism of flow deceleration, which leads to a reduction in the length of the deceleration zone. Moreover, its length depends on many design and operating parameters. Therefore, NB, LC and SD in a SCL should be designed as a whole. A particularly strong influence of complicating factors – low Re number of the flow and heat release on the flow is observed in COIL. Figure 10 shows the experimental data on the change in pressure in the resonator versus the back pressure. The pressure in the laser chamber increases with increasing back pressure, which is associated with the presence of thick boundary layers, along which disturbances from the subsonic zone are transmitted towards the flow to the nozzle.
To achieve effective SD operation, it is necessary to control the growth of the boundary layer. For large-scale installations, this can be done by blowing high-pressure gas from small nozzles into the boundary layer along the channel walls.
In PRS for COIL, the ejector must have two stages with a compression ratio ε ≈ 40–50. Therefore, it is possible to combine the blowing in the diffuser with the first stage of the ejector, i. e. implement an active diffuser (AD) [12]. Fig. 11 shows a diagram of the inlet part of an active diffuser and a general view of an PRS with an AD, developed at Laser Systems JSC for COIL with an active medium flow rate of 0.2 kg / sec.
Experiments have shown [12] that an active diffuser allows not only to significantly increase the total ejection coefficient of the PRS, but also to improve the optical quality of COIL radiation due to the suction of the boundary layer from the walls of the laser chamber, which prevents the appearance of shock waves in the cavity.
EJECTORS
An ejector (EJ) is a device where the energy of a high-pressure active flow is transferred to a low-pressure passive gas during mixing, thereby increasing its total pressure. The classical EJ scheme is of two types – central, with one axisymmetric active gas nozzle, or peripheral, with an annular nozzle. The problem of increasing the ejection coefficient (the ratio of passive and active gas flow rates) at a given compression ratio is important from the point of view of reducing the mass-dimensional characteristics of the PRS.
In SCL PRS, the active and passive gases are different, in this case, the ejection coefficient can be defined as (where T is the temperature, μ is the molar mass, and n00 is the ejection coefficient for the same gases). Hence, it can be seen that to increase the total n, it is necessary to increase the temperature and decrease the molar mass of the active gas, or decrease the temperature of the passive gas. Therefore, as an active gas in the PRS, a vapor-gas mixture is used, with a low molar mass in comparison with the combustion products and the maximum allowable temperature. Heat exchangers are used to reduce the temperature of the passive gas.
It is possible to increase the ejection coefficient in comparison with traditional schemes by intensifying the process of mixing flows with the help of vortex-forming elements installed on the active gas nozzle. When using such elements, it is possible to increase the n × ε values by 12–15% [13]. When using a distributed mixing scheme – the injection of an active gas through a multitude of medium-sized nozzles, the ejection coefficient can be increased by 1.5 times [14].
PRS for COIL, built on the basis of AD as the first stage of the ejector and the second stage, using mixing intensifiers and multi-nozzle active gas supply, allows not only to reduce the size of the laser module in comparison with the traditional layout and to reduce the required active gas consumption, but also to improve the optical quality of radiation [14].
Conclusion
Despite the long and not always successful history of supersonic gas and chemical lasers, they still remain unsurpassed in the power of continuous radiation with high optical quality. Another indisputable advantage of such lasers is their energy “self-sufficiency”, which allows them to be used as part of autonomous mobile complexes.
During the decades that have passed since their inception, interest in them has changed, and not always in a positive direction, however, work on the study and improvement of the lasers themselves, the formation of high-quality radiation, the development of promising storage systems and preparation of the working fluid, pressure recovery systems continue, which we wanted to show in this article.
The authors hope that all the lasers discussed above, including multi-kilowatt CO2-GDL based on GTJE with practically unlimited time of continuous radiation, sufficient for solving many practical problems on the ground and in the air, and highly efficient multi-megawatt continuous chemical lasers capable of solving unique problems in deep space, will also attract the attention of potential users and customers in the near future.
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About authors
Avdeev A. V., Ph. D, Moscow Aviation Institute (National Research University), Moscow
ORCID: 0000-0003-2643-6622
Boreysho A. S., Dr. Sc., Professor, BSTU “VOENMEH” D. F. Ustinov, St. Petersburg.
ORCID: 0000-0002-3245-9321
Kiselev I. A., Ph. D, BSTU “VOENMEH” D. F. Ustinov, St. Petersburg.
ORCID: 0000-0002-8092-1648
Morozov A. V., Ph. D, BSTU “VOENMEH” D. F. Ustinov, St. Petersburg.
ORCID: 0000-0002-0948-7367
Orlov A. E., Ph. D, Laser Systems JSC, St.Petersburg.
ORCID: 0000-0001-9515-8107
Contribution by the members
of the team of authors
The article was prepared on the basis of many years of work by all members of the team of authors.
Conflict of interest
The authors claim that they have no conflict of interest.
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