[American Institute of Aeronautics and Astronautics 36th AIAA Plasmadynamics and Lasers Conference - Toronto, Ontario, Canada ()] 36th AIAA Plasmadynamics and Lasers Conference - Kinetic

American Institute of Aeronautics and Astronautics

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Kinetic Experiments for an NCl3 Fueled Iodine Laser

Dr William E. McDermott, Dr Robert Coombe, Dr Julanna Gilbert, Dr Boris Nizamov, Zane Lambert University of Denver Research Institute (DRI), 2050 E. Iliff Ave,Denver, CO 80208

Abstract

The NCl-I laser has been demonstrated using HN3 as a fuel with a combustor to produce chlorine atoms. In this paper, we discuss the possibility of constructing a NCl(a1∆)-I transfer laser using NCl3 as a fuel that eliminates the requirement for a combustor. NCl3 more stable than HN3 and can be stored as a liquid. We present kinetic modeling and the results of experiments designed to understand the basic physical processes in this system. We have shown the auto decomposition of NCl3 can be initiated by modest heating of the NCl3 or by simple expansion through a nozzle. The NCl-I laser operates using only gaseous species, eliminating the need for heterogeneous gas liquid reactions such as used in COIL chemical lasers. The lasing species is the same as in COIL, simplifying the scaling process since many optical, tracking and propagation problems have been demonstrated in the ABL program.

I. Introduction The level structure of the NCl molecule is identical to the structure of O2 since the two molecules are isoelectronic. As such, the first excited state of the NCl molecule, NCl(a1∆) is metastable and capable of transferring it's energy to the iodine atom as does O2(1∆) in the chemical oxygen iodine laser. Lasers have been demonstrated using the energy transfer from NCl(a1∆) to iodine atoms [1, 2]. These lasers produce NCl(a1∆) from the reaction of chlorine atoms and hydrogen azide. Chlorine atoms difficult to produce (requiring a combustor) and hydrogen azide is in itself extremely unstable. Our approach is to look at the possibility of using NCl3 as a fuel replacing HN3 and perhaps eliminating the need for a chlorine atom source. At Denver University, we have shown that the combustion of NCl3 by hydrogen atoms produces NCl(a1∆) in high yield [3]. We used a microwave discharge to produce hydrogen atoms in those studies. A more recent study at Physical Sciences, Inc has observed the pumping of iodine atoms by NCl(a1∆) and a resulting decrease in the optical absorption in the H atom initiated system [4]. In previous studies, we looked at the chlorine atom initiated decomposition of NCl3 as a source of NCl(a1∆) [5]. In this research we are investigating the use of the auto decomposition of NCl3 as an energy source.

II. Experimental Setup The time profile apparatus has been described in previous papers [6]. Briefly, we have flexible flow system that allows us to inject reactants in several places and measure the emissions from various species as a function of time from the injection points. Typical operating conditions are given in Table 1.

Species Concentration

(1/cm3) Pressure

(Torr) Mole Fraction Ar 5.62E+16 1.73 73.20% Cl2 1.18E+14 0.00 0.15% Cl 7.86E+13 0.00 0.10% NCl3 4.09E+13 0.00 0.05% HI 8.80E+13 0.00 0.20% H2 2.03E+16 0.63 26.20% Total 7.67E+16 2.37

Table 1 Typical operating conditions.

The flow velocity is about 900 m/s. The NCl3 is produced in a hood under several layers of protection and carried with argon to the experimental apparatus. The concentration of NCl3 is measured by absorption at 254 nm (Hg pen lamp) and recorded using LabView. The amount of argon and NCl3 admitted to the flow system is

36th AIAA Plasmadynamics and Lasers Conference6 - 9 June 2005, Toronto, Ontario Canada

AIAA 2005-5038

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc.The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes.All other rights are reserved by the copyright owner.


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measured by replacement of a small auxiliary flow of argon. That is, the argon make up flow is decreased by the desired flow rate and then argon and NCl3 introduced to return to the original pressure. This procedure allows flow measurement without risking the decomposition of the NCl3 in the flow meter. HI is added from a bulb diluted to about 5% in argon. The total flow is measured with a flow meter and typically results in an HI concentration of about 8 x 1013 cm-3. Decomposition experiments were conducted in a laser cavity flow system. The cross sectional area matches the flow tube study apparatus, so the flow rates were similar (~ 900 m/s). In some experiments, the flow was chocked downstream of the laser cavity reducing the flow velocity and increasing the total pressure.

III. Results A typical time profile of the NCl(a) emission (after subtraction of interfering emission from chlorine

recombination) is shown in Figure 1.

Time Profile of NCl(a) - Cl2 (B - X) Emission121603-1a �

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Figure 1 NCl(a1∆) with Cl2 (B-X) emission subtracted

When HI is added (carried in argon) into the H2 flow; Figure 2 shows that the I* signal is linear in the NCl3 flow rate. Variation of chlorine (Figure 4) and hydrogen (Figure 3) are linear at lower concentrations and reach a peak at higher additions. This indicates that NCl3 is the limiting reactant in these experiments.


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I* vs NCl3062404

y = 0.0902x + 0.001R2 = 0.9455

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I*

Figure 2 Variation of I* signal with NCl3 flow rate

I* signal vs H2 flow062404

y = -2E-06x2 + 0.0004x + 0.0042R2 = 0.9991

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I*

Figure 3 Variation of I* signal with H2 flow rate


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Cl2 flow vs I* signal062204

y = -1E-05x2 + 0.0013x - 0.0025R2 = 0.9907

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Figure 4 Variation of I* signal with Cl2 flow rate We made a small electrical heater out of a coil of nichrome wire and connected it to a variable power supply (Figure 5). The leads were glued into a ¼ inch tube to form a vacuum tight connection and passed through an Ultra-Torr fitting. NCl3 (~2% in argon) was admitted to the flow tube and allowed to flow over the heated wire. With modest heating, the chlorine atom recombination glow could be clearly seen a couple of centimeters downstream of the heater. As the heater power was reduced, the Cl atom recombination glow moved down the flow tube. At low heater powers, the glow moved several meters downstream. This corresponds to a variable decomposition time of about 200 msec. A spectrum of the decomposition flame showed lines characteristic of Cl2(B-X) radiation (Figure 6).

Figure 5 Experimental Setup for NCl3 thermal decomposition experiments


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Figure 6 Spectrum of the autodecomposition of NCl3 (blue) compared with the spectrum of a microwave discharge of 5% Cl2 in argon (red).

Figure 7 shows a photograph of the NCl3 thermal decomposition flame (not optimized) when NCl3 is passed over a heated nichrome wire at a flow rate of ~5 µmole/s. NCl(b1Σ) is produced when hydrogen (H2) is added to the flow, which is directly correlated to the formation of Cl, NCl2 and NCl(a1∆).

Figure 7 A new method of producing electronically excited metastables of NCl for the NCl(a)-I laser. Left: NCl3 decomposition flame. The red afterglow results from three-body Cl atom recombination reactions following NCl3 decomposition. Right: Generation of NCl(b1Σ) from the addition of H2 to the flame. NCl(b1Σ) most likely results from the NCl(a1∆) + NCl(a1∆) pooling reaction.

We believe that the thermal decomposition of NCl3 can be used to produce NCl(a) without the use of a

microwave discharge or a combustor to initiate the reaction. We then replaced the 0.25 m monochrometer and visible detection photomultiplier tube with an IR enhanced optical multichannel analyzer (OMA) to investigate the NCl(a) and I* profiles with enough spectral resolution to identify Cl2 recombination and discharge excited species. This eliminated the ambiguities we noted in the early time NCl(a) profiles. The OMA was on loan from the United States Air Force Academy. Figure 8 and Figure 9 show spectra of NCl(a1∆) and I*(2P1/2) taken with the OMA.

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SPECTRUM OF THE AUTODECOMPOSITION OF NCl3


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Figure 8 Spectrum of NCl(a1∆) in the DU flow system. The production is initiated by Cl atoms.

The I* signal is much brighter than the NCl(a) emission and is consistent with efficient production of I*. The rise time of the NCl(a) emission is somewhat slower than previously observed. The I* emission also falls off considerably faster than in previous experiments. These results suggest a significant wall quenching rate. We replaced the halocarbon coated reaction tube with one coated with 100% phosphoric acid (baked to remove water) to change the wall quenching rate. We observed that the NCl(a) and I* emission were both significantly reduced indicating wall reactions are important in the flow tube. The ratio of NCl(a) to I* emission is 42% assuming the OMA response is flat between the two wavelength ranges and the lifetimes of NCl(a) and I* are 2.7 sec and 0.2 sec [7] .


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Figure 9 Spectrum of iodine 2P1/2 radiation produced by the addition of HI to the NCl3 decomposition flame

Under standard conditions (low chlorine flow) we have obtained time profiles of the emission of both NCl(a) and I* using the OMA. The data is shown in Figure 10. The I* peak (uncorrected for sensitivity changes between 1.08 and 1.3µ) is much larger than the NCl(a) and the peak of the I* intensity trails the peak in the NCl(a) emission. The slow rise time of the NCl(a) signal is likely due to mixing effects and the decay of the I* is most likely due to wall quenching.


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Figure 10 Time Profile of NCl(a) and I* obtained with the OMA

In our initial experiments with the laser cavity flow system (Figure 11), we observed that decomposition could be initiated simply by letting the NCl3 expand through a small nozzle. Metal nozzles (stainless) appeared to quench the decomposition, but using a glass nozzle with 100µ holes we could observe chlorine recombination radiation arising from the auto decomposition of NCl3 when the flow system was choked (~ 300-400 m/s). Total pressure was on the order of 3 Torr (Figure 12).

Figure 11 Laser cavity design

Time Profile NCl(a) and I*Wax Coated - Normal Conditions

9/30/04

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SLOW RISEMIXING LIMITED

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To Vacuum pump

H2 + HI NCl3 + Ar

Optical cavity (1m, Max R mirrors)

Additional laser, diagnostic ports

Gas heater

To Vacuum pump

H2 + HI NCl3 + Ar



Gas heater

To Vacuum pump

H2 + HI NCl3 + Ar



Gas heater


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Figure 12 Chlorine recombination radiation showing the auto-decomposition of NCl3

IV. Laser Modeling Our kinetic model currently includes 29 rate constants. They are detailed in reference [8]. We have updated our model to include the recent work at Emory University [9] which gives new quenching rate constants for H2, HCl, Cl2 and O2. A reinvestigation of these rates were necessary since earlier studies using ClN3 as a source assumed NCl(a1∆) was a primary product of the photodissociation of ClN3. It was recently shown that the primary product channel is fragmentation to Cl+N3 and production of NCl(a1∆) occurs as a subsequent reaction [10]. In all cases, the new quenching rate is significantly slower than the old rate. In our current experimental set up we have a 10 cm2 flow channel and can reach velocities of about 15 m/s. Keeping these parameters constant, we can change the flow channel to a 10 cm by 1 cm laser channel. Since we need high H2 concentrations based on model calculations, we've chosen the bulk of the flow to be H2. Nominally picking 10 Torr as the operating pressure, the required flow rate of H2 (at 15 m/s thru a 10 cm2 duct) is ~200 sccm/s or about 9 mmoles/s. We intend to use the hot H2 (~350K) required to kick off the H2+Cl reaction to also dissociate the NCl3 as we've demonstrated qualitatively in the heater experiments. We ran the kinetic model with a density of 3 x 1014 initial Cl atoms and NCl2 to simulate the complete dissociation of NCl3 by H2. The H2 concentration was 5 x 1017 and the HI was 2.4 x 1014. We iterated the injection time and the HI concentration to estimate a rough optimum. The injection time used for the HI was 0.14 msec. The model also predicts gain at higher temperatures, but the gain is a little lower due to the doppler broadening of the line shape. The calculated gain is shown below (Figure 13). It reaches a maximum of about 0.13%/cm. Note also that the gain doesn't seem to decay over the calculation time of about 5 msec (7.5 cm in our flow system). This raises the possibility of longitudinal flow.


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Gai

n (%

cm

-1)

Figure 13 Calculated gain for a pre dissociated NCl-I laser.

In these calculations, the H2 was mixed with the HI and it was injected downstream of the dissociated NCl3. The species densities for NCl2, Cl, H2 and H are shown in Figure 14.

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NCl2ClHH2

Figure 14 Species Concentrations for pre dissociated NCl2

Another way to look at the excited state development is to assume a premixed model. When starting with dissociated NCl3, the reaction kinetics are fast and the products of interest (I,I*) are long lived. This suggests that the formation kinetics will be dominated by the NCl3 dissociation rate and that the desired products will be formed immediately. The premixed model (Figure 15) under the same conditions as above predicts a slightly higher peak gain of 0.15%/cm. Note also that the gain remains high for over 50 millisec ( 75 cm at our flow rates). This again suggests that a longitudinal flow reactor would be feasible.


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m)

Figure 15 Gain in a premixed system

The premixed calculations were done at higher pressures than the nominal 10 Torr. H2 pressure in the system was about 15 Torr. Flow rates for H2, NCl3(5%)/Ar and HI(5%)/Ar are 12 mmoles/s, 0.16 mmoles/s and 0.16 mmoles/s. These flows are within the capability of our flow system and NCl3 generating capacity.

V. Conclusion Our kinetic studies indicate that an NCl3 fueled NCl(a1∆)-I transfer laser is feasible. Such a laser could operate

without a high temperature combustor increasing the mass flow efficiency of the device and simplifying its design. Since it operates on the iodine atom transition at 1.315µ, optical components and beam control methods developed for the COIL laser will be directly transferable. Unlike the COIL laser, the NCl3 laser uses only gas phase reaction chemistry. Unlike the AGIL laser, it uses a more stable storable fuel. We are currently investigating the stability of NCl3 and developing a high flow NCl3 generator in preparation for a small scale laser demonstration.

Acknowledgments This program is supported by the Air Force Office of Scientific Research through the Multidisciplinary Research

Initiative (MRI) program. The Air Force Program Manager is Dr. Mike Berman. The author credits many useful discussions with other members of the MRI team. They include Dr. Tom Henshaw and Dr. Dave Neumann of Directed Energy Solutions; Dr. Steve Davis of Physical Sciences Incorporated and Dr. Mike Heaven and Dr. Keiji Morokuma of Emory University. The stability studies and generator scale up effort is being supported by the Joint Technology Office and is sponsored by the Airborne Laser Program Office through a contract with Directed Energy Solutions and the Denver Research Institute.

References

1. Henshaw, T.L., et al., A new energy transfer chemical laser at 1.315 µm. Chemical Physics Letters, 2000. 325(5,6): p. 537-544.

2. Henshaw, T.L., et al., A new chemically pumped I (2P1/2- 2P3/2) laser at 1.315 µm. Proceedings of the International Conference on Lasers, 2000. 23rd: p. 127-131.

3. Exton, D.B., J.V. Gilbert, and R.D. Coombe, Generation of Excited NCl by the Reaction of Hydrogen Atoms with NCl3. J. Phys. Chem., 1991. 95: p. 2692.

4. Hunter, A.J.R., et al. Spectroscopic Studies of the NCl3 + H reaction sequence as a candidate laser system. in Gas and Chemical Lasers and Intense Beam Applications III. 2004. San Jose, CA: SPIE.


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5. McDermott, W.E., et al., Progress Towards an NCl3 Fueled Iodine Laser. Journal of Directed Energy, 2005. submitted and accepted.

6. McDermott, W.E., et al. Advanced Iodine Lasers. in Gas and Chemical Lasers and Intense Beam Applications IV. 2003. San Jose, CA: SPIE.

7. Manke, G.C., et al., The Measurement of Gain in a Supersonic, Combustion-Driven Generator for NCl(a1∆). 2004, Air Force Research Laboratory.

8. McDermott, W.E., et al. Flow Tube Studies of NCl3 Reactions. in Gas and Chemical Lasers and Intense Beam Applications III. 2004. San Jose, CA: SPIE.

9. Tinney, S.P., J. Han, and M.C. Heaven. Photodissociation of ClN3 and Quenching of NCl(a) at elevated temperatures. in Gas and Chemical Lasers and Intense Beam Applications III. 2004. San Jose, CA: SPIE.

10. Wodtke, A.M., et al., The Cl to NCl branching ratio in 248-nm photolysis of chlorine azide. Chemical Physics Letters, 2004. 391(4-6): p. 334-337.

Documents

[American Institute of Aeronautics and Astronautics 36th AIAA Plasmadynamics and Lasers Conference - Toronto, Ontario, Canada ()] 36th AIAA Plasmadynamics and Lasers Conference - Kinetic