plasma chemistry of iodine atoms production for cw oxygen-iodine laser
TRANSCRIPT
Plasma chemistry of iodine atoms production for CW oxygen-iodine laser
P.A. Mikheyev1a, N. I. Ufimtseva, A.V. Demyanovb, I. V. Kochetovb, A. P. Napartovichb
a Samara branch of P. N. Lebedev Institute, 221 Novo-Sadovaya st., Samara, Russia, 443021; b SRC RF Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow Province, Russia,
142190
ABSTRACT
Usage of iodine atoms instead of molecules in oxygen-iodine laser permits to expand its range of operation parameters and improve the weight per power ratio. Results of experiments and modeling of plasma chemistry processes resulting in CH3I dissociation in a planar 40 MHz discharge are presented, showing that addition of oxygen or water into Ar:He:CH3I mixtures can lead to a noticeable increase in CH3I dissociation rate and inhibit iodine recombination during transport.
Keywords: Atomic iodine, oxygen-iodine laser, RF discharge, plasma chemistry
1. INTRODUCTION Usage of iodine atoms instead of molecules in a CW oxygen-iodine laser permits to expand its range of operation parameters improving weight per power ratio and simplifying logistics issues1. A method of iodine atoms production in sufficient amounts for an oxygen-iodine laser is dissociation of precursor molecules in glow discharge plasma. Methyl iodide (CH3I) is a suitable precursor, because it is dissociated quite easily and products of its dissociation and species that appear in subsequent plasma chemical reactions do not deteriorate oxygen-iodine laser medium2. Molecular iodine consumes singlet oxygen molecules during dissociation. An advantage of using CH3I as the precursor of iodine atoms is a low fraction of I2, which is formed in discharge only due to recombination of iodine atoms. Besides, CH3I is a non-toxic volatile liquid with 400 Torr of the saturated vapor pressure at room temperature making it much easier to handle than crystalline iodine.
Modeling of CH3I dissociation in a DC glow discharge in Ar:CH3I mixtures revealed that recombination of iodine atoms with methyl radicals I+CH3 → CH3I is the main process of iodine atoms’ loss that leads to increase of specific energy per produced iodine atom3. 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 and/or water added into the discharge plasma can reduce CH3 concentration, because oxygen and hydrogen atoms produced by electron impact dissociation can bound CH3 radicals efficiently. Also, oxygen and hydrogen atoms provide new channels for iodine atoms production and destroy iodine molecules formed as a result of recombination. The goal of the present work was to test these ideas by numerical modeling and experimental studies.
2. MODELING Model description
Dissociation of CH3I in glow discharge plasma in a mixture with a noble buffer gas at a pressure of tens of Torr proceeds mainly through electron impact: CH3I+e → CH3+I+e with a rate constant determined by electron energy distribution function (EEDF). The main channel of iodine atoms’ loss is recombination with methyl radical I+CH3 → CH3I (k = 1.2×10-11 cm3s-1), while methyl radical is removed by self recombination CH3+CH3 → C2H6 (k = 6×10-11 cm3 s-1). Modeling based on this reactions’ subset successfully describes methyl iodide dissociation in a DC discharge plasma3.
For the present work the model3 was extended by inclusion of electron impact processes with He5, H2O6, O27, H2
5 and HI, ion-molecular, chemical and energy exchange reactions with those species and products that appear in discharge due
XIX International Symposium on High-Power Laser Systems and Applications 2012,edited by Kerim R. Allakhverdiev, Proc. of SPIE Vol. 8677, 86770A · © 2013 SPIE
CCC code: 0277-786/13/$18 · doi: 10.1117/12.2010462
Proc. of SPIE Vol. 8677 86770A-1
to plasma chemical reactions. The present model includes about 360 reactions (see ref. 3, 5-7) for 80 species (electron, charged, neutral and excited particles). The system of differential equations for species is solved with the help of the GEAR method8, specially developed for stiff systems. The rates of electron induced processes were calculated by solving the steady-state Boltzmann equation in two-term approximation for the electron energy distribution function (EEDF).
An RF discharge is modeled in the present work unlike in3 where a DC discharge was considered. At 40 MHz the angular frequency of the RF field ω≈2.5×108s-1 is much less than electron elastic collision frequency (~1011s-1), while close to or a little larger than the inelastic collision frequency (5.4×107s-1 – 3.6×108s-1). It means that the core of EEDF does not follow the field oscillations and is determined by the effective electric field strength9 = √ , where Ea is the electric field amplitude. The effective electric field strength can be defined with help of the discharge power density W
(which is known from experiments) by the following formula: = ∙ ∙ , where e is the electron charge, μe and ne
are electron mobility and number density, respectively. The rates of electron induced processes are mainly dependant on the effective electric field strength effE . The EEDF has to be recalculated when change in electron number density exceeds 2% providing reasonable calculations’ time and accuracy.
A case of uniform power distribution and constant temperature along the flow is considered. For 40 MHz RF field and power W < 300 W for mixtures and flow rates realized in experiments (see later), the voltage drop near the electrodes is ≈ ∙ ∙∙ ∙ < 20 V, where ve is the electron drift velocity, ε is vacuum permittivity. This voltage drop value is an order of magnitude lower than rms voltage applied and is realized provided an α-mode of RF discharge exists. In this case, more power fraction is spent on CH3I dissociation as compared to a DC discharge.
The reactions of oxygen and hydrogen atoms and OH radicals that provide CH3 bounding speed up CH3I dissociation and destroy iodine molecules are listed in Table 1.
Table 1. List of reactions relative to enhancement of I atoms production.
Along with the main channel of I production, which is electron impact CH3I dissociation, a noticeable contribution is given by dissociation of CH3I and CH2I in collisions with argon metastables4 Ar*. CH3+I recombination restricts I atoms production.
Modeling results
The calculations were performed for planar electrodes of 50 cm2 area, 10 cm along the flow in a channel of 0.5 cm height bounded by electrodes. The discharge power density was in the range (4-12) W cm-3. Buffer gas flow rate was 4 mmol s-1 at ~20 Torr pressure and 340 K, gas residence time in the discharge was ~5.6 ms.
Reaction Rate constant, cm3 s-1 Reference
CH3+O → CH2O+H 1.4×10-10 [10]
CH3+H → CH4 3.2×10-11 Bimolecular in the considered pressure range [4]
CH3+OH → CH3OH 1.4×10-10 Bimolecular in the considered pressure range [4]
CH3+HI → CH4+I 7.2×10-12 [10]
CH3I+O → CH3+IO 1.9×10-11 [10]
CH3I+H → CH3+HI 6.1×10-12 [10]
I2+O → I+IO 1.4×10-10 [10]
IO+O → I+O2 1.4×10-10 [10]
I2+H → HI+I 6.2×10-10 [10]
H+HI → I+H2 2.8×10-11 [10]
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As seen in figure 1, the model predicts that addition of oxygen into the gas mixture in amounts comparable to that of CH3I increases I concentration at the discharge exit by ~30%. In the presence of oxygen, input power can be increased until total CH3I dissociation is achieved without concern for increased iodine recombination. Figure 2 shows that for discharge power density W = 12 W cm-3 most part of CH3I is dissociated to the end of the discharge and the concentration of oxygen atoms grows rapidly. Downstream of the discharge O atoms keep destroying iodine molecules sustaining maximum iodine atoms concentration during at least 4 ms, providing a very useful opportunity to transport them to the singlet oxygen flow intact.
Figure 1. Calculated temporal dependence of iodine atoms concentration in the discharge plasma (t<5.6 ms) and in the afterglow in the mixtures with and without O2. CH3I and O2 flow rates are 0.12 mmol s-1, corresponding to initial concentration of 1.6×1016 cm-3. RF power density is 4 and 12 W cm-3. Buffer gas is Ar with 4 mmol s-1 flow rate, pressure in the discharge chamber 20 Torr, T=340 K.
Figure 2. Calculated time dependence of iodine molecules and oxygen atoms concentrations in the discharge plasma (t<5.6 ms) and in the afterglow for the same conditions as in figure 1.
The model permits us to determine the energy required to produce one iodine atom taking into account all the processes incorporated into the model. For Ar:CH3I mixtures considered in this work and discharge power densities of 4 and 12 W cm-3 the required energy amounts to ~31 eV per atom. Addition of oxygen lowers that value 30% down to
1E+11
1E+12
1E+13
1E+14
1E+15
1E+16
0 2 4 6 8 10 12 14
[N],
cm
-3
Time, ms
Plasma boundary
[I2]
[O] 12 W cm-34 W cm-3
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0,4
0,6
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[I], 1
016cm
-3
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12 W cm-3
4 W cm-3
With O2
With O2
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~22 eV. Considering the CH3 – I bond strength of 2.4 eV the model predicts the energy efficiency of CH3I dissociation of 7.7% without oxygen and 11% with oxygen.
3. EXPERIMENT Experimental setup
The sketch of the experimental setup is represented in Figure 3.In experiments a planar 100 W 40 MHz discharge was used. Discharge chamber was 10 cm along the flow and 5 cm across with water cooled aluminum electrodes. Distance between electrodes was 5 mm providing power density 4 W cm-3 for a uniform discharge located between the electrodes. Ar, He and their mixtures were used as buffer gases at a flow rate of 4 mmol s-1 in the pressure range 15 – 30 Torr. CH3I flow rates varied up to 0.13 mmol s-1 and O2 flow rates up to 0.2 mmol s-1. Concentration of iodine atoms produced in discharge was determined by measuring concentration of I2 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 100 ms. I2 concentration was measured in the diagnostic arm of 45 cm length using light absorption at the wavelength 500 nm. The diagnostic arm and the connecting duct were heated up to 70 °C to prevent iodine deposition on the surfaces of 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.
Figure 3. Sketch of the experimental setup.
A Pi filter with tunable capacitors and inductance was used as a matching network. This design permitted us to achieve perfect matching of an RF generator with the discharge. However, matching network usually needed to be tuned for each methyl iodide flow rate, indicating noticeable dependence of the discharge parameters on CH3I concentration. Fine tuning was also needed if the RF power input changed.
3.2. Experimental results
In our earlier experiments with a DC discharge1-4, contamination of electrodes by products of CH3I dissociation was a serious problem, because the discharge eventually became unstable and the electrodes needed cleaning. The present RF discharge configuration turned out to be absolutely insensitive to electrodes’ surface contamination. Black substance (presumably carbon and iodine) deposited on the electrodes’ surface did not affect the discharge operation mode and the measured iodine concentration at all.
In experiments, concentration of iodine molecules was measured in the diagnostic arm while other parameters varied. In Figure 4 an effect of addition of 0.1 mmol s-1 of O2 into He carrier is demonstrated.
Gas
40 MHz
To a pump
500 nm
Pressure
Pressure
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Figure 4. Iodine atoms concentration at the outlet of the discharge chamber as a function of CH3I flow rate at a constant RF power input 100 W. Carrier gases are He and Ar:He mixture in equal amounts at 4 mmol s-1 flow rate. Oxygen flow rate in the mixture with He is 0.1 mmol s-1. Gas pressure in the discharge chamber varied in the range 24-29 Torr, T=340 K. Experimental uncertainty is of the same order for all sets of data points.
As seen in Figure 4, addition of small amounts of oxygen into He carrier gas resulted in ~30% increase in atomic iodine yield in good agreement with the model predictions. Addition of oxygen and different amounts of CH3I into the carrier gas noticeably changed viscosity of the gas mixture and resulted in slightly different pressures in the discharge chamber and different pressure drops across the connecting duct.
An unexpected result was obtained when Ar:He mixtures or pure Ar were used as the carrier gas. With addition of Ar, efficiency of iodine production increased by ~30%, but at the same time addition of oxygen ceased to substantially affect the iodine production, as illustrated in Figure 5.
Figure 5. Iodine atoms concentration at the outlet of the discharge chamber as a function of O2 flow rate at a constant CH3I flow rate and constant RF power input 100 W. 4 mmol s-1 flow rate of carrier gas. Gas pressure in the discharge chamber varied in the range 22-26 Torr. Experimental uncertainty is of the same order for all sets of data points.
A linear dependence of iodine concentration on O2 flow rate was observed in He carrier. However, in mixtures with Ar the dependence on O2 flow rate was almost negligible.
0
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0,00 0,05 0,10 0,15
[I], 1
016cm
-3
CH3I, mmol s-1
He
He:O2=4:0.1
Ar:He=1:1
0,6
0,8
1
1,2
0 0,05 0,1 0,15 0,2 0,25
[I],
1016
cm-3
O2, mmol s-1
CH3I 0.12 mmol s-1
He:Ar=1:3
He:Ar=1:1
He:Ar=1:0
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Figure 6. Efficiency of dissociation and dissociated fraction of CH3I in He:O2=4:0.1 mmol s-1 carrier mixture as a function of CH3I flow rate and constant RF power input of 100 W. Gas pressure in the discharge chamber varied in the range 25-27.5 Torr.
In figure 6 the dependencies of dissociated fraction and efficiency of dissociation on CH3I flow rate are represented. Dissociated fraction was defined as the ratio of the iodine atoms flow rate, derived from the concentration measurements, to the initial CH3I flow rate. Efficiency of dissociation was defined as a product of the CH3 – I bond strength (234 KJ mol-1) and I atoms flow rate divided by the discharge power (100 W) and amounted to ~12%. As seen, a very good agreement with the model predictions (11% efficiency) and with experiments11 is observed.
Figure 7. Comparison between model calculations and experiment in the conditions like in figure 4. Points – experiment, lines – zero-dimensional model, dashed line – no oxygen, solid line – oxygen 0.1 mmol s-1 of O2 is added.
The comparison between model calculations and experiment is represented in figure 7, showing qualitative and rather good quantitative agreement. Of course, one shouldn’t expect better result out of a zero-dimensional model, where
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Effic
ienc
y
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CH3I, mmol s-1
He:O2=4:0.1 mmol s-1
Dissociated fraction
Efficiency
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He:O2=4:0.1He
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spatial effects are not accounted for. The Reynolds number in our experiments is estimated to be less than 300 so the flow is laminar with Poiseuille parabolic velocity profile. Therefore, gas residence time in the discharge for the gas that flows near the electrodes and in the center is quite different. Near the electrodes methyl iodide have time to dissociate completely, while at the center a considerable amount of CH3I may flow through the discharge intact. Modeling shows that the reduced electric field strength E/N and consequently local power density depends on CH3I concentration. Hence in the regions with higher CH3I concentration power density should be higher resulting in more efficient CH3I dissociation. In our opinion those spatial effects has led to more efficient iodine production observed in experiments compared to model predictions.
4. CONCLUSIONS In our work we have demonstrated in experiment for the first time up to 30% enhancement of atomic iodine production in 40 MHz glow discharge when a few percent of oxygen were added into He:CH3I mixture. An increase in iodine atoms production caused by oxygen atoms was observed in He, but not in Ar, or in Ar rich mixtures. The origin of this effect is unclear at the moment and requires further investigation to be done.
Dissociation efficiency in an RF discharge up to 12% was observed which is ~4 times larger than in a DC discharge11. Estimations show that in our experiments the RF discharge operated in the α-mode and the absence of the cathode fall regions is responsible for the substantially increased efficiency.
Modeling predicts that at a sufficiently high power density in discharge plasma O atoms can eliminate I2 in the discharge and downstream during transport for several milliseconds.
The present RF discharge configuration turned out to be absolutely insensitive to electrodes’ surfaces contamination, which was a serious problem in a DC discharge.
5. ACKNOWLEDGMENTS This work was supported by the RFBR grant # 11-02-00613-а.
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