1-D and 3-D Modeling of the All Gas-Phase Iodine Laser (AGIL) (2003)

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.