Photodetachment neutralizer development: Laser window design study: Volume 1, Summary: Final report

: Solid-state lasers convert electrical energy into light using optical excitation. Although efficient, scientists continue research into scaling these devices for high-power applications. Chemical lasers efficiently convert energy derived from chemical bonds and reactions into laser photons. Scientists use chemical lasers for applications that require high-power, lightweight, and self-contained sources of laser radiation because of their inherent high efficiency and scalability. The two classes of chemical lasers are the hydrogen fluoride (HF) laser and the chemical oxygen-iodine laser (COIL).

The recent demonstration of gain and continuous wave power extraction on the electronic P1/2 to P3/2 transition of the I atom using NCl(a1∆) as the pumping species proving the viability of the all gas-phase iodine laser (AGIL), has brought with it the need for an increased theoretical understanding of this complex system. Multiple gas phase chemical processes occur in this system: the generation of Cl atoms with F atoms, the generation of I atoms with Cl atoms, the production of NCl(a1∆) with Cl atoms and HN3, and the pumping of I to the I(P1/2) state with NCl(a1∆). Each of these processes may be composed of multiple individual chemical reactions describing production and loss processes. And finally, this set of processes occurs within a flowing gas, with the exoand endothermicity of the reactions complicating the coupling of the chemical reactions to the fluid dynamics and vice-versa. The goal of this paper is to provide an overview of the current theoretical understanding of this complex system with respect to the numerous measured finite-rate chemical processes and the coupled fluid dynamic-diffusional mixing processes. 1-D explorations of the AGIL finite-rate chemistry mechanism including sensitivity studies are shown. 3-D NavierStokes simulation studies of reactant mixing are examined, and predictions made for conditions to improve mixing and reaction in AGIL device hardware. Confirmation of these predictions in results from recent AGIL power extraction experiments is shown. * Member AIAA. Introduction The development of chemical lasers begins with the demonstration of the first iodine laser by Casper and Pimentel who in 1964 demonstrated lasing on the electronic P1/2 to P3/2 transition of atomic iodine at 1.315μm by photodissociating alkyl iodide compounds. The high energy and high atmospheric transmission of this wavelength developed considerable interest in the chemical generation of the I(P1/2) state. With no known chemical reactions directly producing high concentrations of I(P1/2), interest shifted to the use of energy transfer reactions to generate this state. Derwent, Kearns, and Thrush postulated the use of the first electronic excited state of oxygen, O2(a 1∆), as the transfer agent. O2(a∆) being both metastable and near-resonant with I(P1/2) would provide a logical energy transfer species. McDermott et al provided proof of concept for this hypothesis, demonstrating a laser based on the O2(a 1∆) to I(P3/2) energy transfer and producing the first chemical oxygen-iodine laser (COIL). With the advent of COIL also came the realization of one of its’ primary drawbacks, namely the 2 phase chemistry required to produce O2(a 1∆). The basic hydrogen peroxide solution used during O2(a 1∆) generation constitutes a large fraction of the overall weight of the system and adds water vapor, a relatively strong deactivator of I(P1/2). In order to circumvent the deleterious effects of the 2 phase COIL chemistry, energy donors for I were sought that could be generated in the gas phase at high number densities. The nitrene metastables were recognized as possible replacements for O2(a 1∆) as energy donors, primarily due to their visibility from investigations into their use as agents for generating short wavelength chemical lasers. The isovalent electronic structures of NCl(a1∆) and NF(a1∆) with respect to O2(a 1∆), long lifetimes, and the ability to generate large number densities in the gas phase made NCl(a1∆) and NF(a1∆) likely candidates for iodine energy donors. Yang et al were the first to investigate NCl(a1∆) as an iodine energy donor, measuring the transfer rate and key deactivation rates subsequently followed by a measurement of gain on the I(P1/2) to I( P3/2) transition. Similarly, Ray and Coombe measured the transfer rate followed by a pulsed laser demonstration via pulsed photolysis of ClN3 and CH2I2. With the early work of Yang et al, and Ray and Coombe establishing the viability of an NCl(a1∆) based transfer laser, significant effort was spent on measurement of the key rates constituting the complex chemistry of supporting these experiments. Henshaw measured the temperature dependence of NCl(a1∆) quenching by I(P3/2), as well as rates for chemical generation of NCl(a1∆).20 Setser with co-workers 34th AIAA Plasmadynamics and Lasers Conference 23-26 June 2003, Orlando, Florida AIAA 2003-4310 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

The spectroscopically measured lineshape of an atomic transition provides a wealth of useful information relative to diagnosing the state of a gas. The center of the lineshape is specific to a particular transition of a specific atom. The width of the lineshape indicates the amount of broadening of the transition, due to the effects of both collisions with other particles in the gas and Doppler shift due to the movement of the atom. Since the Doppler shift is proportional to velocity, the width of the transition can be used to estimate the degree of random molecular motion in the gas, expressed macroscopically as temperature. A Doppler shift to the frequencies in the transition can also occur through the bulk motion of the gas, and this can be used to examine the velocity field of the gas. The astronomy community was the first to recognize the utility of these concepts in practical applications, stemming to the early 1930s.1,2 In 1934 Stuve and Elvey3 showed that by including a bulk gas velocity Doppler broadening term in the Voigt equation for the transition lineshape in addition to the random thermal motion term, it was possible to estimate whether a stellar atmosphere was ‘turbulent’ or not based upon the fit of the Voigt equation to the measured transition lineshapes for that atmosphere. Taking the theory a step further, using estimated optical paths or length scales for the stellar atmospheres that they were measuring, they were able to estimate median gas velocities and correlate increasing velocity magnitude with increasing temperature of the stellar atmosphere. More recently, continuously tunable diode lasers have been applied to lineshape measurement of transitions within species in the chemical oxygen-iodine laser (COIL) flowfield as a means to determine number density and laser gain on the I 2P1/2→2P3/2 transition. Davis and Allen et al4,5,6,7 applied lineshape measurement and Voigt fitting methods to various COIL species as a mechanism to determine concentration and translational temperature in the COIL flowfield as an experiment diagnostic. While the effect of the bulk gas velocity was not taken into account in these investigations, the diagnostics developed by these investigations provided the means to do so. Nikolaev et al,8 applied the same type of continuously tunable diode laser to the I 2P1/2→2P3/2 transition to investigate the influence of mean gas velocity through a COIL device upon the laser gain. They showed that by varying the angle at which the diode laser beam passes through the COIL flowfield, the line center gain can be varied through bulk velocity induced Doppler broadening. Using this mechanism, they were able to determine the mean flow velocity for the COIL and convert the laser gain

Simulation of chemical lasers such as the chemical oxygen-iodine laser (COIL) is of timely interest due to the recent acceleration of the airborne laser military development program and ongoing commercial development programs. As a contribution to these efforts, a 3-D COIL simulation model was developed based on the Computational Fluid Dynamics (CFD) code GASP which solves the conservative, finite-volume formulation of the full Navier-Stokes equations coupled to a new finite-rate, non-equilibrium chemistry model and a conservative, multicomponent molecular diffusion model. A 3-D simulation of the COIL flowfield was performed and compared to detailed species distributions measurements from experiment for the purpose of validation using a unique averaging technique developed to mimic the actual physics of the experimental gain measurement. These detailed comparisons demonstrate that the simulation model accurately predicts the experimentally measured distributions, a significant result in 3-D simulation of reacting flows. Additional results from the validated simulation include: development of new finite-rate COIL chemistry mechanism, evidence indicating the presence of H2O condensation in the COIL mixing nozzle, and evidence suggesting that the physical processes in the flow from the O2('A) generator are not completely understood.

A chemical method of atomic iodine generation for the chemical oxygen-iodine laser (COIL) was studied. This method is based on fast reaction of gaseous hydrogen iodide with chemically produced fluorine atoms formed by a preceding reaction of molecular fluorine with nitrogen oxide. Iodine at oms were generated in special reactors and injected into the supersonic flow of nitrogen in the COIL cavity. Concentration profiles of atomic iodine both along and perpendicular to the primary gas flow were measured in dependence on flow rates of the reaction gases. Very high concentration of atomic iodine (up to 3.2x10 15 cm -3 ) was measured in the laser cavity with a good yield related to both F 2 (up to nearly 100 %) and HI (up to 60%). An important advantage of this method is using of commercially available reactive gases. Some drawback of this method for its application in the COIL is a rather high gas temperature (250 – 400 K). Keywords: atomic iodine, atomic fluorine, Chemical Oxygen-Iodine Laser, COIL

The paper describes a study of pulse-periodic operation of a chemical oxygen-iodine laser. The generator in use was of the jet type with high chlorine utilization degree (• 97%). Atomic iodine was released by dissociating methyl-iodide ( KG 3 I) in volumetric electric discharge. Steady lasing was achieved at a repetition rate up to 30 Hz. The emission energy attained per individual pulse in a train was 1.1 J at a pulse energy repeatability of (3÷ 5) % and a specific energy extraction from the active medium of 1.7 J/L. The lasing pulse duration depended on the concentra-tion of methyl-iodide and the energy deposited into discharge. The minimal half-height duration of pulses was achieved as 10 µ m at a concentration of atomic iodine in the laser cavity ~1*10 15 cm -3 . Key words: chemical oxygen-iodine laser, pulse-periodic, methyl-iodide, volumetric electric discharge The pulse-periodic chemical oxygen-iodine laser (COIL) is considered by us as a likely applicant to technologies requiring the conditions that are unattainable so far with the aid of solid-state pulse-periodic lasers. It means the range of average lasing power from 1 to 10 kW at a per-pulse lasing energy up to 10 J. There are at least two approaches possible to get COIL in pulse-periodic mode of operation. The first consists of Q-switching in the laser cavity or gain modulation in the active medium. Along with this, the active medium is produced by mixing singlet oxygen with molecular iodine just as it takes place in cw lasing case. Application of this method to formation of large-volume active media has well-known restrictions applied to relaxation processes occurred during the interaction between singlet oxygen and iodine and we’d not describe it here. Another method proposed by scientists of the Lebedev Institution and confirmed by specialists in China assumes using an initially stable mix of singlet oxygen and iodine dissociating by one or another method upon filling the laser cavity by appropriate gases [1, 2, 3].

Great success has been obtained in the R&D of a chemical oxygen-iodine laser (COIL) operating on the electronic transition of the iodine atom, which gets an excitation from the energy donor -singlet delta oxygen (SDO). The latter is normally produced in a chemical SDO generator using very toxic and dangerous chemicals, which puts a limit for civilian applications of COIL that is still a very unique apparatus. Totally new non-chemical SDO generator is needed to allow oxygen-iodine laser to achieve its full potential as a non-hazardous efficient source of high-power laser radiation. There was interest in producing SDO in electric discharge plasma since the 50's long before COIL appearing. The idea of using SDO as a donor for iodine laser was formulated in the 70's. However, the injection of iodine molecules into a low- pressure self-sustained discharge did not result in iodine lasing. One of the main factors that could prevent from lasing in many experiments is a rather high threshold yield approximately 15 percent at 300K, which is needed for obtaining an inversion population. An analysis of different attempts of producing SDO in different kinds of electric discharge plasma has been done which demonstrates that high yield at gas pressure of practical interest for modern COIL technology can be obtained only in non-self sustained electric discharge plasma. The reason is that the value of relatively low reduced electrical field strength E/N approximately 1E-16 V.cm 2 , which is an order of magnitude less than that for the self-sustained discharge, is extremely important for the efficient SDO production. Although different kinds of non-self sustained discharges can be used for SDO production, we got started experiments with e-beam sustained discharge in gas mixtures containing oxygen. High specific input energy up to approximately 3 - 5 kJ/ has been experimentally obtained. Theoretical calculations have been done for different experimental conditions indicating a feasibility of reasonable SDO yield. Experimental and theoretical research of self-sustained electric discharge in SDO produced in a chemical generator, which is very important for getting plasma-chemical kinetic data needed for an estimation of SDO yield, is also discussed.

The key results gathered in the COIL Laboratory of the Institute of Physics AS in the Czech Republic since 1985 to date on the experimental and theoretical investigation of Chemical Oxygen-Iodine Laser (COIL), and related problems are reviewed in a certain context of historical perspectives of the COIL research and development.

Three dimensional transient analysis of bubble dynamics in centrifugal bubble singlet oxygen generator for scalable COIL

The US Air Force Research Laboratory has actively pursued the development of the Chemical Oxygen-Iodine Laser (COIL) since its invention in 19771. The power scaling potential of the COIL was verified in the late 1980's on the RotoCOlL device, which was later decommissioned. A smaller COIL teststand (ReCOIL) was then upgraded with a rotating-disk oxygen generator and a variable-height slit nozzle to become the principal testbed for continued COIL development. The modified device was named the Research assessment evice Iodine chemical Laser (RADICL). RADICL has supported several major test campaigns during the past 9 years, many of which have been published. These include 2-D gain maps2, iodine dissociation studies3, magnetic gainswitching demonstrations4, mode-locking5, metal cutting/fiber delivery demonstrations, and more recently measurements of water concentration and oxygen yield . Areview of the major results of these test programs and previously unpublished data are presented.

The lineshape of the I 2 P 1/2 → 2 P 3/2 transition provides a means to ascertain a variety of useful information regarding the performance of the chemical oxygen-iodine laser (COIL). The value at the center of the lineshape, commonly referred to as the 'line center,' is proportional to the laser amplification on the I 2 P 1/2 → 2 P 3/2 transition. The infinite integral of the lineshape is proportional to the number density of the ground and excited states of atomic iodine in the gas, indicating the degree of I 2 dissociation. And the width of the lineshape indicates the amount of broadening of the transition, both due to collisional and Doppler shift effects. As the Doppler shift is proportional to velocity, the width of the transition can be used to estimate the degree of random molecular motion in the gas, expressed in macroscopic terms as temperature. A Doppler shift to the frequencies in the transition can also occur through the straight-line, bulk motion of the gas, and this can be used to examine the velocity field of the gas. However, the flow may experience rotation through the presence of eddies carried within the gas, and these too may contribute to the Doppler shift of the lineshape frequencies. Given that eddies by virtue of their positive and negative velocity components can induce positive and negative Doppler shift, the widening of the lineshape is similar to thermal motion which also includes positive and negative velocities. Thus, when interpreting transition lineshapes, if some account is not made for both thermal and rotational motion, the effect of either physical process will be over-estimated. The work discussed here is oriented toward examining the interplay between the gas dynamics and the lineshape of the I 2 P 1/2 → 2 P 3/2 transition, and in turn determine the ramifications for the use of spectroscopic lineshape based diagnostics and interpretation of their data. These efforts in turn are directly linked to efforts improve the understanding of the physical processes underlying chemical lasers, as excursions outside the traditional operational parameter space become increasingly necessary.

Derivative couplings near a conical intersection and spin-orbit couplings between different spin states are known to facilitate nonadiabatic transitions in molecular systems. Here, we investigate a prototypical electronic energy transfer process, I(2P3/2) + O2(a1Δg) → I(2P1/2) + O2(X3Σg-), which is of great importance for the chemical oxygen-iodine laser. To understand the nonadiabatic dynamics, this multistate process is investigated in full dimensionality with quantum wave packets using diabatic potential energy surfaces coupled by both derivative and spin-orbit couplings, all determined from first principles. A near quantitative agreement with structural, energetic, and kinetic measurements is achieved. Detailed analyses suggest that the nonadiabatic dynamics is largely controlled by derivative coupling near conical intersections, which leads to a small effective barrier and hence a slightly positive temperature dependence of the rate coefficient. The new results should extend our understanding of energy transfer, provide a quantitative basis for numerical simulations of the chemical oxygen-iodine laser, and have important implications in other electronic energy transfer processes.

Chemical oxygen iodine laser (COIL) is the only laser expected to be applied for thick plate cutting, for instance, by high laser power over 5 kW transferred to the processing fields by the optical silica fiber. As a result of the first stage of industrial COIL development, a 1 kW subsonic COIL was delivered as a commercial processing device first in the world in 1992. The supersonic COIL development was carried out at the second stage and the output power of over 5 kW was obtained in 1995. Using this high power beam, superiority of the COIL as an industrial laser was studied by processing metals.

Chemical oxygen-iodine laser (COIL) has a great potential for applications such as decommissioning and dismantlement (D&D) of nuclear reactor, rock destruction and removal and extraction of a natural resource (Methane hydrate) because of the unique characteristics such as power scalability, high optical beam quality and optical fiber beam. Five-kilowatt Chemical oxygen-iodine laser (COIL) test facility has been developed. The chemical efficiency of 27% has been demonstrated with a moderate beam quality for optical fiber coupling. Our research program contains conventional/ejector-COIL scheme, Jet-SOG/Mist-SOG optimization, fiber delivery and long-term operation.

A simple geometric-optics model is developed that describes the power extraction in chemical oxygen-iodine lasers (COILs) with unstable resonators. The positive and negative branch unstable resonators with cylindrical mirrors that were recently used in COILs are studied theoretically. The optical extraction efficiency, spatial distributions of the intracavity radiation intensity in the flow direction, and the intensity in the far field are calculated for both kinds of resonator as a function of both the resonator and the COIL parameters. The optimal resonator magnifications that correspond to the maximum intensity in the far field are found.