Oxygen assisted iodine atoms production in an RF discharge for a cw oxygen-iodine laser

Transcription

1 Oxygen assisted iodine atoms production in an RF discharge for a cw oxygen-iodine laser Pavel A. Mikheyev* a,b, Andrey V. Demyanov c, Nikolay I. Ufimtsev a, Igor V. Kochetov c, Valeriy N. Azyazov a,b and Anatoly P. Napartovich c a P.N. Lebedev Physical Institute of RAS, Samara branch, 44311, Russia; b Samara State Aerospace University, 44386, Samara, Russia; c SRC RF Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow Province, Russia, ABSTRACT Results of experiments and modeling of CH 3 I dissociation in a 4 MHz RF discharge in a discharge chamber of original design to produce iodine atoms for cw oxygen-iodine laser are presented. In experiments a substantial increase in CH 3 I dissociation efficiency due to addition of oxygen into Ar:CH 3 I mixture was observed. Complete CH 3 I dissociation in Ar:CH 3 I:O 2 mixture occurred at 2 W discharge power. Fraction of discharge power spent on iodine atoms production was equal to 16% at.17 mmol/s CH 3 I flow rate. The rate of carbon atoms production as a function of molecular oxygen and water contents in CH 3 I:Ar mixtures was studied with the help of numerical modeling. It was found that addition of water vapor resulted in increase while addition of molecular oxygen and HI in decrease of the rate of carbon atoms production. Due to diffusion most of carbon atoms had enough time to deposit on the walls of the discharge chamber. However, contrary to the situation in a DC discharge, in the RF discharge accumulation of carbon on the walls of the discharge chamber did not hamper discharge stability and iodine production, as it was observed in our experiments. Keywords: Oxygen-iodine laser, iodine atoms, plasma chemistry, RF discharge, methyl iodide dissociation 1. INTRODUCTION In an oxygen-iodine laser iodine molecules are dissociated by excited oxygen in the singlet delta state O 2 (a 1 Δ), and iodine atoms that are subsequently pumped by O 2 (a 1 Δ) to the I( 2 P 1/2 ) state (I*) are the source of laser radiation. Two energy loss channels are inherent in that iodine dissociation process decreasing laser efficiency. First, about 1% of the O 2 (a 1 Δ) molecules must be expended to dissociate I 2 loosing energy that could be converted to laser radiation. For a discharge-driven system, with its modest singlet oxygen yield, saving O 2 (a 1 Δ) molecules is especially important. Second, I 2 strongly deactivates I*, leading to energy loss, limiting the optimal [I] concentration and, consequently, small-signal gain. In the absence of I 2, the optimal [I] concentration and gain in the active medium could be increased. Therefore, injection of externally produced iodine atoms into a singlet oxygen flow is the obvious way to improve oxygen-iodine laser performance. Moreover, recent experiment 1 had shown that it is possible to increase COIL s intracavity pressure at least twice and operate COIL with large water content by using iodine atoms instead of I 2 molecules. H 2 O molecules are quenchers for both excited I 2 and I*, impeding I 2 dissociation 2 and inducing extra energy loss. Excluding the time needed for I 2 dissociation, loss associated with dissipation of energy through H 2 O molecules can be minimized. This result is most important for a COIL system, because it leads to improvement of its weight per power ratio. Glow discharges proved to be suitable for atomic iodine production out of different iodine containing precursor molecules both in cw and pulsed regimes 3-6. An advantage of using precursors instead of molecular iodine is a small fraction of I 2 at the discharge generator outlet, because it is formed in discharge plasma only through recombination of iodine atoms. For example, methyl iodide (CH 3 I) is a convenient precursor being a non-toxic volatile liquid with 4 Torr of the saturated vapor pressure at room temperature. It dissociates quite easily in discharge plasma, much easier to handle than crystalline iodine, and products of its dissociation and species that appear in subsequent plasma chemical reactions do not deteriorate oxygen-iodine laser medium 7. *mikheyev@fian.smr.ru, paulmikheyev@hotmail.com; phone ; XX International Symposium on High-Power Laser Systems and Applications 214, edited by Chun Tang, Shu Chen, Xiaolin Tang, Proc. of SPIE Vol. 9255, 92552F 215 SPIE CCC code: X/15/$18 doi: / Proc. of SPIE Vol F-1

2 Argon proved to be the best carrier gas for dissociation of iodine precursors, because in mixtures with Ar the highest dissociation efficiency was obtained 4, 8, 9. However, discharge stability becomes a problem with increase of a precursor partial pressure. Also, the products of CH 3 I dissociation include carbon that contaminates electrodes making a diffuse dc discharge impossible after a few minutes of operation. RF discharges easily resolve those problems as they proved to be capable to operate at sufficiently high pressures of tens of Torr and to be immune to contamination of electrodes 1. However, carbon particles may occasionally enter laser active medium and burn there producing inhomogeneity, therefore, it is desirable to reduce their number. Modeling of CH 3 I dissociation in a DC glow discharge in Ar:CH 3 I mixtures 11 revealed that recombination of iodine atoms with methyl radicals I+CH 3 CH 3 I is the main process of iodine atoms loss that leads to increase of specific energy per produced iodine atom. Therefore, addition of species reacting with methyl radical may shift the chemical equilibrium and make iodine atoms production more efficient. It is expected that small amounts of molecular oxygen added into the discharge plasma can reduce CH 3 concentration, because oxygen atoms produced by electron impact dissociation can bound CH 3 radicals efficiently. Also, oxygen atoms provide new channels for iodine atoms production, may reduce carbon atoms number density and destroy iodine molecules formed as a result of recombination. The goal of the present work was to test these ideas by experimental studies and numerical modeling. 5 nm 2. EXPERIMENT 2.1 Experimental setup The sketch of the experimental setup is represented in Figure 1. In experiments we used a discharge chamber of an original design. It consisted of eight parallel quartz tubes of 7 mm inner diameter placed tightly between two plain parallel water cooled 1 12 cm aluminum electrodes with shorter size along the flow. A similar design was exploited 12 for singlet oxygen production with good results. The discharge chamber provided excellent stability of a homogeneous glow discharge in all the range of experimental parameters. To ensure easy ignition of all tubes weak barrier discharges upstream of the main discharge adjacent to aluminum electrodes were sustained with the help of 5 kv microsecond pulses from a 5.7 khz blocking generator. The largest power density in these experiments amounted to 1 W cm MHz generator & matching circuit Figure 1. Sketch of the experimental setup. A 4 MHz power supply and an L-type matching network for the experiments were designed and manufactured in our laboratory. The power supply provided up to 5 W RF output and for measurements of the forward and reflected power we used Daiwa CN-11L power meter. In experiments nearly 1% matching had been achieved, and it was observed that matching parameters depended slightly on the CH 3 I fraction in the gas mixture and discharge power. Ar was used as a carrier gas at a flow rate of 4 mmol s -1 in the pressure range of 2 25 Torr. CH 3 I flow rates varied up to.19 mmol s -1 and O 2 flow rate was fixed at.1 mmol s -1. O 2 and a carrier gas flow rates were measured with the help of Bronkhorst mass flow meters, and CH 3 I flow rate with the help of a recalibrated rotameter. Iodine atoms number density at the discharge outlet was determined by measuring concentration of I 2 molecules formed as a result of recombination of I atoms in a connecting duct on their way to a diagnostic arm. Gas residence time in the duct was about 1 ms. I 2 concentration was measured in the diagnostic arm of 45 cm length using light absorption at Proc. of SPIE Vol F-2

3 5 nm. The diagnostic arm and the connecting duct were heated up to 7 C to prevent iodine deposition on the walls and windows. Gas pressure was measured near the outlet of the discharge and in the diagnostic arm. Pressure difference amounted to a few Torr depending on the gas mixture and was accounted for in iodine number density calculations. 2.2 Experimental results In our earlier experiments with a DC discharge 13, contamination of electrodes by products of CH 3 I dissociation was a serious problem, because the discharge eventually became unstable and the electrodes needed cleaning. An RF discharge with bare planar aluminum electrodes had shown much better results as it turned out to be absolutely insensitive to the contamination of electrodes 1, presumably, because the discharge operated in α-mode. However, planar configuration also suffered from discharge inhomogeneity when mixtures with large CH 3 I content were used. In the present work the RF discharge was sustained trough the dielectric walls of the quartz tubes and the discharge volume was divided between those tubes. These two circumstances provided excellent homogeneity and stability of the discharge plasma and ability to produce large quantities of iodine atoms. In the experiments the discharge behaved as follows. Even at the lowest discharge power of 1 W in pure argon the discharge occupied all the interelectrode space inside the tubes. With addition of CH 3 I the upstream part of the discharge became bluish in color, while the downstream part remained orange-pink, clearly demonstrating the change in chemical composition of the discharged gas mixture. With further increase of the CH 3 I flow all the discharge became bluish in color and its size along the flow decreased. Photographs in figure 2 illustrate this behavior. Figure 2. Appearance of the RF discharge at.6 (left) and.15 mmol s -1 (right) CH 3 I flow rates. When discharge power was increased up to 2 and 3 W no change in discharge size along the flow was observed in all the range of the studied gas mixtures. Addition of.1 mmol s -1 of O 2 did not change the appearance of the discharge. The discharge remained diffused occupying all the volume inside the tubes without visible bright spots or layers indicating its operation in the abnormal α-mode. Atomic iodine number density at the end of the discharge was determined using the measured I 2 number density in the diagnostic arm with account for pressure change across the connecting duct and assuming 34 K temperature. It is represented in figure 3 for Ar carrier as a function of CH 3 I flow rate. Iodine atoms number density achieved in our present experiments turned out to be twice larger than in our previous work 1 when we used the discharge chamber with bare plane aluminum electrodes. As shown in figure 3 for Ar carrier, unlike in 1, addition of oxygen at 1 W discharge power input clearly had an advantageous effect on iodine production increasing [I] up to 35%. In our opinion the reason for the discrepancy of those results is better homogeneity of the discharge in the quartz tubes. With CH 3 I flow rate, discharge size along the flow decreased almost twice at the largest flow rates, shortening the residence time of the gas mixture in the plasma region and [I] exhibited maximum. At 2 W discharge power input, corresponding to power density of 6.5 W cm -1, the discharge did not change its size with CH 3 I flow rate. Addition of.1 mmol s -1 O 2 had a profound effect, increasing [I] up to 25% and leading to almost 1% dissociation of CH 3 I. Further increase in discharge power up to 3 W had almost no effect for mixtures with oxygen, because complete CH 3 I dissociation occurred at 2 W already. At 3 W for mixtures without oxygen larger [I] were observed at large CH 3 I flow rates, than at 2 W, but fraction of discharge power spent on dissociation was smaller. Proc. of SPIE Vol F-3

4 W, 2 W, 3 W; CH,I+Ar+Oz O 1 W, 2 W, A 3 W; CH,I+gr CH3I, mmol s'.15.2 Figure 3. Iodine atoms number density [I] at the discharge chamber outlet as a function of CH 3 I flow rate with RF power input into the discharge as the parameter. Ar carrier flow rate was 4 mmol s -1. O 2 flow rate.1 mmol s -1, the data points for the Ar:O 2 mixtures are darker. Polynomial fits are for 2 W power input. Gas pressure in the discharge chamber varied in the range Torr, [I] is recalculated for T=34 K from [I 2 ] measurements. Experimental uncertainty is of the same order for all sets of data points. Fraction of the discharge power that resulted in I atoms production or discharge efficiency was calculated assuming 234 kj mol -1 CH 3 -I bond strength as the ratio between the power needed to break this bond and the total discharge power. The results for Ar carrier are represented in figure O O.1 Al+1 o.5. _ FQ-I A O 1 W, 1 W, 2 W, 2 W, CH3I, mmol s' 3 W; CH,I+Ar+Oz 3 W; CH,I+gr.2 Figure 4. Discharge efficiency as a function of CH 3 I flow rate and discharge input power as a parameter for mixtures with and without oxygen. Oxygen flow rate was.1 mmol s -1. The points are calculated from the data represented in figure 2. The largest efficiency of 21% was observed at 1 W discharge input power for the Ar:CH 3 I:O 2 gas mixture. At this point about 8% of CH 3 I dissociated. At 2 W in mixtures with oxygen CH 3 I dissociated completely, but efficiency was smaller up to 16%. Proc. of SPIE Vol F-4

5 3. MODELING OF CARBON PRODUCTION 3.1 Model description The numerical model was described in detail in our previous papers 1, 11. For studies of carbon atoms production it was extended by inclusion of electron impact processes with carbon and carbohydrates, ion-molecular, chemical and energy exchange reactions with those species and products that appear in discharge due to plasma chemical reactions. These reactions are listed in table 1. The present model includes about 36 reactions (see ref.) for 8 species (electron, charged, neutral and excited particles). Table 1. List of reactions relative to carbon atoms production. Reaction Rate constant, cm 3 s -1 Reference CH 3 I+ e CH 2 + HI + e EEDF [11] CH 3 I + e CH 3 + I + e EEDF [11] CH 2 + H + M CH 3 + M [14] CH 2 + H CH + H , * [14] CH 3 + OH CH 2 + H 2 O [14] CH + H C + H [14] CH + O CO + H [14] CH 3 +O CH 2 O+H [14] CH 3 +H CH [14] CH 3 +OH CH 3 OH [14] CH 3 +HI CH 4 +I [14] CH 3 I+O CH 3 +IO [14] CH 3 I+H CH 3 +HI [14] I 2 +H HI+I [14] H+HI I+H [14] *Forward and reverse reactions rate constants Carbon appears only in the discharge zone as a result of the reactions chain: CH 3 I+e CH 2 +HI CH 2 +H CH+H 2 CH+H C+H 2. Evidently, to reduce carbon production rate, [CH 2 ], [CH], [H] number density have to be reduced. We had studied influence of addition of H 2 O, O 2 and HI on carbon production. Proc. of SPIE Vol F-5

6 3.2 Modeling results The results of modeling of carbon atoms number density are represented in figure 5. ;E t) Û CH) - - CH31:H2O:O2 -- CH3I:2.'.5CH31:.5H1:2 i. i. i 113 / Time, ms 4 5 Figure 5. Results of modeling of time dependent carbon number density for Ar:CH 3 I:M mixtures. Residence time in discharge is 5.6 ms. Ar flow 4 mmol s -1, iodine-containing molecules flow.13 mmol s -1, O 2 and H 2 O flow.13 mmol s -1. The baseline case is for the mixture Ar:CH 3 I=4:.13 mmol s -1 and it is represented in figure 5 by the solid line. Addition of.13 mmol s -1 of oxygen results in almost twofold decrease of carbon number density, because oxygen favors bounding of hydrogen atoms. However, for the mixture Ar:CH 3 I:O 2 :H 2 O = 4:.13:.13:.13 mmol s -1 carbon number density increases nearly twice in comparison to the baseline case, because water increases hydrogen atoms production. The lowest [C] one order of magnitude lower than for the baseline case is predicted for the mixture Ar:CH 3 I:HI:O 2 :H 2 O = 4:.65:.65:.13:.13 mmol s -1. Excess in HI helps to remove H atoms in the process H+HI I+H 2 with the rate constant cm 3 s -1. However, use of HI in practice is more complicated due to its physical and chemical properties. 4. CONCLUSIONS In our work we have demonstrated in experiment for the first time up to 35% enhancement of atomic iodine production in 4 MHz glow discharge when a few percent of oxygen were added into He:CH 3 I mixture. However, we would like to note that oxygen atoms should not reach oxygen-iodine active medium, because they are quenchers of excited iodine atoms 15. Fraction of the discharge power that resulted in I atoms production up to 21% was observed. The present RF discharge configuration turned out to be absolutely insensitive to electrodes surfaces contamination, which was a serious problem in a DC discharge and exhibited excellent discharge stability. Carbon particles that are a by-product of CH 3 I dissociation may introduce inhomogeneity in the laser medium. Modeling predicts that addition of oxygen and hydrogen iodide suppresses carbon atoms production, while water molecules enhance it. 5. AKNOWLEDGEMENTS We gratefully acknowledge partial support of the work at Samara State Aerospace University by the Science-Educational Center Physics of nonequilibrium open systems, by the Ministry of Education and Science of Russia under grant # Proc. of SPIE Vol F-6

7 /K and at Samara Branch of P.N. Lebedev Physical Institute by the Russian Foundation for Basic Research under grant # REFERENCES [1] Mikheyev, P. A., Azyazov, V. N., Zagidullin, M. V., "Chemical oxygen-iodine laser with external production of iodine atoms in CH 3 I/Ar dc glow discharge," Appl. Phys. B: At. Mol. Phys. 11(1-2), 7-1 (21). [2] Azyazov, V. N., Heaven, M. C., Role of O 2 (b) and I 2 (A,A) in chemical oxygen-iodine laser dissociation process, AIAA Journal 44(7), (26). [3] Azyazov, V. N., Vorob'ev, M. V., Voronov, A. I., Kupryaev, N, V., Mikheyev, P. A., Ufimtsev, N. I., "Parameters of an electric-discharge generator of iodine atoms for a chemical oxygen-iodine laser," Quantum Electron. 39(1), (29). [4] Jirasek, V., Schmiedberger, J., Censky, M. and Kodymova, J., "Production of iodine atoms by RF discharge decomposition of CF 3 I," J. Phys. D: Appl. Phys. 44, (211). [5] Vagin, N. P., Pazyuk, V. S., Yuryshev, N. N., "Pulsed chemical oxygen-iodine laser with bulk formation of iodine atoms by an electric discharge," Quantum Electron. 25(8), (1995). [6] Hicks, A., Bruzzese, J. R., Adamovich, I. V., "Effect of iodine dissociation in an auxiliary discharge on gain in a pulser-sustainer discharge excited oxygen-iodine laser," J. Phys. D: Appl. Phys. 43, 2526 (21). [7] Mikheyev, P. A., Azyazov, V. N., "Properties of O 2 ( 1 Δ)-I( 2 P 1/2 ) laser medium with a dc glow discharge iodine atoms generator," J. Appl. Phys. 14, (28). [8] Mikheyev, P. A., Shepelenko, A. A., Voronov, A. I., Kupryaev, N. V., "Atomic iodine production in a gas flow by decomposing methyl iodide in a dc glow discharge, " Quantum Electron. 32(1), 1-4 (22). [9] Jirasek, V., Schmiedberger, J., Censky, M. and Kodymova, J., "Dissociation of molecular iodine in RF discharge for oxygen-iodine lasers," Eur. Phys. J. D 66(4), (211). [1] Mikheyev, P. A., Ufimtsev, N. I., Demyanov, A. V., Kochetov, I. V., Napartovich, A. P., "Plasma chemistry of iodine atoms production for CW oxygen-iodine laser," Proc. SPIE 8677, 8677А-1 (213). [11] Demyanov, A. V., Kochetov, I. V., Napartovich, A. P., Azyazov, V. N. and Mikheyev, P. A., "Study of iodine atoms production in Ar/CH 3 I dc glow discharge," Plasma Sources Sci. Technol. 19(2), 2517 (21). [12] Woodard, B. S., Day, M. T., Zimmerman, J. W., Benavides, G. F., Palla, A. D., Carroll, D. L., Verdeyen, J. T. and Solomon, W. C., "The influence of radio-frequency discharge geometry on O 2 (a 1 Δ) production," J. Phys. D: Appl. Phys. 44, (211). [13] Mikheyev, P. A., Shepelenko, A. A., Voronov, A. I., Kupryaev, N. V., "Production of iodine atoms by dissociating CH 3 I and HI in a dc glow discharge in the flow of argon," J. Phys. D: Appl. Phys. 37, (24). [14] "NIST Chemical Kinetics database," < [15] Mikheyev, P. A., Postell, D. J. and Heaven, M. C., Temperature dependence of the O+I( 2 P 1/2 ) O+I( 2 P 3/2 ) quenching rate constant, J. Appl. Phys. 15(9), (29). Proc. of SPIE Vol F-7