*Skobeltsyn Institute of Nuclear Physics, Moscow State University, Vorob'evy Gory, Moscow 119992, Russia. E-mail: [email protected]
1. Introduction For development of discharge oxygen-iodine la-
ser, it is required to develop electro-discharge Singlet Oxygen Generator (SOG) with high efficiency. There are a number of recent publications [Carrol, 2003; Savin, 2003; Rakhimova, 2003; Fujii, 2003; Frolov, 2003] on problems of Singlet Oxygen Generator (DSOG) development. However, in spite of a certain success, DSOG developmental level has not yet reached a practical level. Still, to get the required DSOG, several problems of general nature in the area of gas plasma science and technology should be ad-dressed.
As shown in [Carrol, 2003; Savin, 2003; Rakhi-mova, 2003; Fujii, 2003], the high-frequency and mi-crowave discharges could be perspective methods for singlet oxygen (SO) excitation. We used a traveling microwave discharge in a steady–state mode, in order to efficiently accumulate the molecules O2(a1∆g) in oxygen containing gas flow. The discharge is formed in the standing wave non-uniform field of the cavity MW-resonator. The structure of such discharge is de-termined by peculiarities of origination and spreading of surface wave along the plasma-dielectric interfaces to sustain dense plasma. Below there are presented the experimental data and results of simulation in traveling MW discharge (TMWD) in pure oxygen and he-lium/oxygen mixtures. The oxygen pressures in the plasma reactor region were P = 1 - 4 Torr. The total gas pressure of mixtures (O2:He=1:1;O2:He=1:2) were P = 2 - 4 Torr and gas flow rates were 30 – 100 m/s.
It was shown that SO yield is increased with in-put power and measured SO yield reaches maximal values 22% and 30% for total gas pressure P=2 Torr in pure oxygen and O2:He=1:1 mixture, respectively.
Two-dimensional (r,z) model for calculations of plasma-chemical kinetics, heat and mass transfer in pure oxygen and argon/oxygen mixtures has been de-veloped and used for simulation of TMW discharge processes. The model includes neutral and charged species kinetics, electron energy distribution calcula-tion, species transport, gas heating and thermal con-ductivity.
Simulation allows to analyze different mecha-nisms of SO production and loss in TMWD plasma and afterglow. Some disagreement of calculated and experimental SO yields is observed at low power in-puts: calculated SO yields are lower than experimental yields.
2. Experimental set-up
A schematic of the experimental facility “MW
– DSOG – 1” (VNIIEF) is shown in fig. 1. It was ar-ranged on the basis of a continuous-action microwave magnetron operating at a frequency of f0 = 2450 MHz ± 2% (λ = 12.24 cm). A resistive load was a coaxial-type quarter-wave cavity resonator 3 where gas dis-charge is burned. The power was supplied from the magnetron to the resonator via a coaxial cable through a directional coupler 7. Using a wattmeter (type M3-46) this arrangement allowed us to monitor the micro-wave emission power at the load input and the micro-wave emission power reflected by the resonator. The highest input powers introduced into the plasma were no more than 50W.
Our experiments were carried out in pure oxy-gen O2 and helium/oxygen mixtures He:O2 = 1:1, 2:1 and 4:1. The gas flow rate in the resonator was from 30 to 100 m/s, while in the monitoring chamber 6 it was no more than 1-3 m/s. The gas pressure was varied from 1 to 10 Torr and the gas flow rates from 0.7⋅10–4 mol/s to 4.5⋅10–4 mol/s depending on the experiment. The total gas flow rate through the plasma reactor was determined with the aid of a flowmeter 1 accommodated at the resonator input. The diameter of the entrance orifice of the input flowmeter was 2 mm. The flow dynamics of the plasma species and gas pres-sures in the resonator volume and in the monitoring chamber was dependent on the flow-control orifice (d = 4.2 mm) 2 installed in the end part of the gas-dynamic channel of the facility.
Upon passing the plasma torch (narrow quartz tube) the decaying plasma products enter the monitor-ing chamber 6 (wide quartz tube, inner diameter 43 mm and length Lmonit = 265 mm).
35th AIAA Plasmadynamics and Lasers Conference28 June - 1 July 2004, Portland, Oregon
The absolute concentrations of molecular oxygen in metastable excited states a1∆g and b1Σ+
g were defined with the aid of emission spectroscopy techniques. In most experiments the absolute concentration of mole-cules O2(a1∆g) was monitored in the entrance part of the monitoring chamber as it is shown in fig.1. In this case a receiver 11 was a photodetector, type New Fo-cus, Model 2153 equipped at its input with a narrow-band interference filter (λ0 = 1270 nm, ∆λ = 55 nm). In some experimental series investigating the dependency of the SO yield in plasma flow direction, the photode-tector 11 together with the filter was located transversally to the discharge tube. The absolute con-centration of molecules О2(b1Σ+
g) was monitored by us at the plasma reactor output (in the plasma region) with the aid of a spectrometer, type SD 2000 (Ocean Optics Co., λwork=355÷950 nm). All photodetectors used in our experiments were carefully calibrated using a stan-dard lamp, type SIRSH 8.5-200-1, with a tungsten filament at a 2840 K temperature. These measurements were duplicated using a mercury lamp, type LS – 1- CAL (Ocean Optics), and single-mode diode laser, type Vortex 6025 (λ = 1315.273 nm), purchased from the New Focus Inc.
The emission spectrum in the range from 700
to 1400 nm was recorded by a monochromator, type MDR-23, based on a 600-lines/mm diffraction grating (blazing angle in a 1000-nm range, linear dispersion - 2.6 nm/mm). Small-level emission was recorded by a mechanical modulator 13 in combination with differ-ential amplifiers 3, 8. The temperatures of gas flows at the plasma rector output and in the monitoring cham-ber were determined by the spectral procedures using the spectral measurements in the range of P – branch of atmospheric molecular oxygen band (λ0 = 762 nm).
In order to determine the singlet oxygen yield ([O2(а1∆g)/([O2(X3Σg
-)] + [O2(а1∆g)])), the concentra-tion of atomic oxygen [O(3P)] was measured in the plasma region (using the actinometry technique with argon atoms). The mixtures O2 : Ar = 9:1 and O2:Ar = 99:1 have been used in these experiments. The spec-trum of the atomic oxygen transitions O(3p – 3s) 844 nm, O(5p – 5s) 777 nm and Ar (2p1 – 1s2) 750 nm were recorded using SD2000 spectrometer or MDR-23 monochromator jointly with New Focus Model 2151 photodetector.
As it was claimed in a few works [Pagnon, 1995; Ivanov, 2000a], the cross section of electronic-impact excitation and collisional relaxation velocities are well known for the transitions O(5P) → O(5S) ( λ= 777 nm) and Ar(2p1) → Ar(1s2) (λ= 750 nm). This is the reason allowing application of the 777-nm and 750-nm emission lines to determination of the atomic oxygen density at pressures typical for our experi-ments. As it was shown in the present work, up to 10% of argon in the active mixture wouldn’t change essen-tially the processes of SO production in our discharge.
In order to shed light on mechanisms of MW surface wave interaction with oxygen plasma flow, we have carried out experimental measurements of radial distribution of the temperature and atomic and molecu-lar oxygen radiation in different lines (bands). This was made by using an additional quartz window placed for observation on the tube axis (d = 8.5 mm) and a spherical lens (f = 35 mm) with the aid of which an image of the in-resonator discharge was built on a spe-cial screen. Radial scanning of this image with a quartz waveguide (din = 200 µm) attached to a spectrometer, type SD2000, provided us the radial intensities of atomic and molecular oxygen lines (bands) from plasma re-gion and, consequently, the radial distribution of the oxygen rotational temperature. A photo of a cross-sectional discharge image taken with the aid of the spontaneous emission from the in-resonator field is shown in fig. 3.
3. Experimental results The experimental data on maximally achieved
SO density ([O2(a1∆)]), SO yield [O2(a1∆)]/([O2(X3Σg)] + [O2(a1∆)])) and energy effi-ciency of O2(a1∆) generation in pure oxygen at pres-sure p = 2 Torr are presented in Fig.2 as function of the deposited energy.
14 17 8 10
6 18
13
Pdisch
Tektronix
TDS 3012B
Pump
2
4-MW generator
Wattmeter 3 7
12
Spectrometer SD2000
Pobs
1
15 Optical fibers
16
5
911
3 American Institute of Aeronautics and Astronautics
Fig. 2. Pure O2, water cooling, Pdisch= 2 Torr, Pmonit = 1.0-1.2 Torr, Tmonit = 370 - 390 K, Lmonitor = 16.5cm, g = (1.21-1.44)⋅10-4 mol/s
Energy efficiency of SO generation is meas-ured as the ratio eabs = (W∆)/(Wabs), where (W∆) is the specific energy content of SO in the plasma decay products and (Wabs) is the discharge specific energy deposited into the plasma. The experimental data on W∆ were obtained from the measurements of the rela-tive SO concentrations at the SOG output and of the total gas flow rate used in a particular experiment as it was described above. The energy of singlet oxygen was here assumed to be equal to 0.98 eV/molecule. The specific discharge energy deposition into the plasma is (Wabs) = Pabs/Gplasma, where Pabs is the micro-wave emission power absorbed in the plasma and Gplasma is the total flowrate in the plasma reactor.
Fig. 3. O2 : He =1:1, water cooling, Pdisch = 4 Torr, Pobserv = 2.5-2.7 Torr, Tobserv = 360-380K, Lobserv = 16.5cm, g = (4.14-4.36)⋅10-4 mol/s
SO excitation efficiency is decreased from 20% to 5% with specific energy input increase from 2 J/(cm3 atm) to 16 J/(cm3 atm).
Fig. 2 shows saturated specific energy input dependence of SO density and SO yield at pressure of 2 Torr The specific energy deposition optimal for SO density and SO yield values are increased from about 8 to 30 J/(cm3 atm). The highest SO yield ~22% was observed at 2 Torr of oxygen pressure. It should be noted that the measurements of SO density and SO yield took place at a distance of 16.5 cm from the resonator in these experimental series. Experimental data obtained on maximally achieved SO density, SO yield and energetic efficiency of O2(a1∆) generation in oxygen-helium mixtures de-pending on the energy deposition are presented in Figs. 3-5. The first two experimental series investigated the mixture O2:He = 1:1 at a 4- and 2-Torr total pressure at
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.04.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Concentration SO
Yield SO
%1015cm-3
Efficiency
eabs, J/(cm-3 atm)
0.0
0
3
6
9
12
15
18
21
24
2 3 4 5 6 7 8 9 10 11 12 131
2
3
4
5
6
7
SO density
SO yield
%1015cm-3
Efficiency
eabs, J/(cm-3 atm)
0
0
5
10
15
20
25
30
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.00
1
2
3
4
5
6
SO density
SO yield
%1015cm-3
Efficiency
eabs, J/(cm-3 atm)
0
0
5
10
15
20
25
30
2 4 6 8 10 12 14 162
3
4
5
6
7
8
SO density SO yield
%1015cm-3
Efficiency
eabs, J/(cm-3 atm)
0
4
8
12
16
20
24
4 American Institute of Aeronautics and Astronautics
the gas-discharge tube input (Figs. 3, 4). The third ex-perimental series used the mixture O2:He = 1:2 at a 3 Torr pressure.
Fig. 6. Radial distribution of atomic oxygen radiation at λ = 777 nm, measured at different downstream axial distances L from the resonator (L=0 ~ z0 + 2.5 cm).
It is seen from figs. 3 – 5 that in general the dependencies observed kept the same character as it was measured previously for pure oxygen. It should be noted that in the presence of helium additives in oxy-gen the discharge energy deposition into the plasma went down to a level of 1.5-12.5 J/cm3 atm. Along with this, the optimal values of eabs corresponding to our observations of the highest SO density and SO yield decreased as well. It is also seen from figs. 4 - 5 that the highest SO yield observed in our experiments with helium achieved about 30%. Allowing for the SO loss in the quartz tube and the presence of atomic oxy-gen in the plasma decay products, the ratio O2(a1∆)]/[O2] directly in the discharge was estimated to be at a level no less than 27-30%.
Fig. 6 shows radial distribution of atomic oxygen ra-diation at λ = 777 nm, measured at different axial dis-tances L from the resonator. These profiles indicate that plasma parameters and effective reduced field is not uniform. As it seen main radical production and power deposition occur in the near wall region r>R/2, R=0.425 cm is the internal tube radius.
4. Two-dimensional model Developed 2D cylindrical (r,z) model for cal-
culations of plasma-chemical kinetics, heat and mass transfer was modified and used for simulation of the processes in TMW discharge. The model includes non-stationary neutral and charged species conserva-tion equations and enthalpy conservation equation for gas temperature distribution calculation in the frame of fixed pressure approach and radial gas flow veloc-ity distribution v(r)=2⋅v0⋅(1-r2/R2), v0 is mean flow velocity, R is the tube radius. Plasma-chemical reac-
tion mechanism involves 18 species and about 160 reactions in pure oxygen. Electron energy distribution (EED), electron temperature and electron-particle re-action rate coefficients are recalculated by solving kinetic equation in two-term approach at each spatial grid points for current mixture composition. Cross-sections of electron processes and plasma-chemical reaction mechanism are presented in [Ivanov, 1999; Ivanov, 2000a]. Radial diffusion and drift of charged species in ambipolar fields has been taken into ac-count.
The previous non-self-consistent method of electric field profile and plasma length determination has been replaced by the solution of power absorption equation in axial (z) direction. Radially average power flux W(z) upstream and downstream from mw power input zone center z0 could be determined from follow-ing equation:
)(zWdz
dW ⋅−= α (1)
Here α is a coefficient of electromagnetic wave ab-sorption. Symmetric relation W(z=z0)=0.5⋅W0 was used as a boundary condition. For given total input power W0 and known function α(z) a length of the plasma region is determined automatically from solu-tion of Eq. (1). In [Nowakowska, 2001; Naidis, 2001], it was shown that analytical expression for α could be de-rived in the case ω << ν << ωp
2/ω. (2) Here ω is circular frequency of electromagnetic wave, ν is a electron collision frequency, ωp is a plasma fre-quency. Knowing α, it is possible to find from (1) power flux distribution W(z) and then reduced electric field (E/N)eff from Eq. (2):
∫⋅
=
R
eeff
rdrrNrNE
dzdW
0
22
)()(2 σπ , (3)
Here
22 ωνσν
+><⋅=
uE
NE
eff
,
σe(r) is electron conductivity, N(r) is a total gas con-centration. Here we assumed that (E/N)eff is radially uniform. Real radial distribution of (E/N)eff is not uni-form and we have studied an effect of different radial profiles of (E/N)eff and found that calculated results are not of crucial dependence from (E/N)eff non-uniformity.
Radial distributions of intensity of oxygen atoms radiation λ= 777 nm (PO2 = 3 Torr, W abs = 4.1 J/cm3*atm )
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6
R mm
I, re
l.un.
L = 0 mm L = 20 mm L = 40 mm
Wall of a quartz tube
5 American Institute of Aeronautics and Astronautics
5. Calculation results and discussion
For our discharge tube of small diameter 8.5 mm, it was reveled experimentally an effect that pro-vides additional problems for numerical simulation in the frame of constant gas pressure assumption. Namely, significant pressure drop between plasma upstream and downstream zones is measured. For example, for downstream pressure Pout = 1.2 Torr (pressure in the measurements chamber) the upstream pressure was 2 Torr and higher for our typical input powers 30-50 W. To study this effect special 2D gas flow calculations with solution of conservation equa-tion for mass, radial and axial momentums and energy for given uniform (for simplicity) power input in dis-charge zone were carried out. Experimental oxygen flow rates and room temperature are set at left (up-stream) boundary of the numerical grid, given ex-perimental downstream pressures (measured in the measurements chamber) are set at the right boundary. Calculated steady state axial profiles of gas pressures and radially average gas temperature for typical ex-perimental flow rates and input powers are shown in Fig.7.
Fig.7. Calculated axial profiles of gas pressures and radially average gas temperature for typical experi-mental flow rates and input powers. Bars correspond to measured upstream pressures.
Corresponding experimental local upstream pressure measurements are also shown. Location of the input power zone for W0 =27W is shown, the length of this zone for W0 = 42W is assumed to be two times longer. As it seen, for all regimes upstream pressure increase is observed as a result of flow disturbances in plasma region and evolution of these disturbances in upstream and downstream direction. Upstream disturbances are enhanced because of reflection from left boundary, dividing high pressure region and discharge tube. The obtained pressure profiles were taken into account in plasma calculations. Further, characteristic upstream pressures will be used for indication of discharge pres-sures.
Because of our simplified assumption on ra-dial uniformity of (E/N)eff, we don’t lay special em-phasis on realistic description of discharge parameters radial profiles. So all presented further results will be
radially averaged. Fig. 8 and 9 show calculated spe-cies profiles and local measurements in our TMW discharge system for gas pressure P = 2 Torr, flow rates of 0.144 mmol/s and 0.115 mmol/s and input power of 27 W and 42 W, respectively. Full consump-tion of the power introduced at z0 = 5 cm occurs on the length of 8 cm and 10 cm for W0 = 27 and 42 W respectively.
0.E+00
1.E+15
2.E+15
3.E+15
4.E+15
5.E+15
6.E+15
0 2 4 6 8 10 12 14 16 18 20 22z, cm
N, 1
/ccm
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
O/N
, SO
Yie
ld
O2(a)O2(b)OO/NSO yield, experimentSO yield
Fig. 8. Calculated axial distributions of O2(a1∆g), O2(b1Σg
+) and O densities, atomic oxygen mole frac-tion O/N and singlet oxygen yield SO for gas pressure P = 2 Torr, flow rates of 0.144 mmol/s and input power of 27 W. Measured SO yield is also shown.
0.E+00
1.E+15
2.E+15
3.E+15
4.E+15
5.E+15
6.E+15
7.E+15
8.E+15
9.E+15
0 2 4 6 8 10 12 14 16 18 20 22z, cm
N, 1
/ccm
0
0.05
0.1
0.15
0.2
O/N
, SO
yie
ld
O2(a)O2(b)OO/NSO yield, experimentSO yield
Fig. 9. Calculated axial distributions of O2(a1∆g), O2(b1Σg
+) and O densities, atomic oxygen mole frac-tion O/N and singlet oxygen yield SO for gas pressure P = 2 Torr, flow rate of 0.115 mmol/s and input power of 42 W. Measured SO yield is also shown. Calculated SO yields Y=[O2(а1∆g)/([O2]+ [O2(а1∆g)]) were 11-12% and 20% for input power 27W and 42W respectively. Note that calculated input power depend-ence of SO yields describes well the experimental observation, but calculated yields Y are lower than experimental ones which are Y=18-19% and 22%, respectively.
6 American Institute of Aeronautics and Astronautics
1.E+15
1.E+16
1.E+17
1.E+18
1.E+19
0 2 4 6 8 10 12 14 16 18 20z, cm
Rea
ctio
n ra
tes,
1/(c
cm s
) .O2+e => O2(a)+e
O2+e => O2(b)+e
O2(a)+e => O2+e
O(1D)+O2 => O+O2(b)
O2(b)+O =>O2(a)+O
O2(a)+O => O2+O
O2(a)+O+M =>O2+O+M
Fig. 10. Axial distributions of important O2(а1∆g) and O2(b1Σ g
Fig. 11. Axial distributions of important O2(а1∆g) and O2(b1Σ g
+ ) production and loss reactions for the same
conditions as in Fig. 9. Corresponding O2(а1∆g) concentrations reach
the values (5-6)⋅1015 cm-3. It should be noted that SO concentrations are saturated more slowly than in the experiments. To study the possible reasons of this dif-ference consider the axial profiles of main reaction rates of O2(а1∆g) and O2(b1Σ g
+ ) production and loss.
These profiles for Р = 2 Torr and input powers W0 = 26 W and 42 W are shown in Fig. 10 and 11. In plasma region, main production of O2(а1∆g) mole-cules occurs by electron impact: e + O2 → e + O2(а1∆g) (R1) The rates of reaction (R1) for W0 = 26 W and 42 W are equal to (2-3)⋅1018 см-3/с, as it seen from Fig. 10 and 11. However, it should be noted that cascade deac-tivation of second singlet state O2(b1Σ g
+ ) contributes
significantly in O2(а1∆g) excitation: O2(b1Σ g
+ ) + O(3P) → O2(а1∆g) + O(3P) (R2)
As it seen from Fig. 10, the rates of processes (R1) and
(R2) became comparable at the end of plasma zone. Note that plasma length and thus residence time are increased with input power.
Specific feature of O2(b1Σ g+ ) excitation in
TMW discharges is a dominance of the energy transfer process from excited state O(1D) of atomic oxygen in reaction (R3): O(1D) + O2 → O(3P) + O2(b1Σ g
+ ), (R3)
in comparison with direct electron impact excitation of O2(b1Σ g
+ ). This is a consequence of high rates of
O(1D) production in dissociation processes (R4): e + O2 → O(3P) + O(1D) (R4) e + O2 → O(3P) + O(3P) (R5) as well as in direct electron impact excitation: e + O(3P) → e + O(1D) (R6) For relatively high electron temperatures (Te = 2.5 - 3 eV) in TMW discharges the rate of O2 dissociation by electron impact (R4,R5) is high and leads to high atomic oxygen concentrations, e.g. O(3P) concentration at the downstream end of plasma zone is about 7⋅1015 cm-3 and 1016 cm-3 for input power 26 W and 42 W, respectively. Maximal dissociation degrees of O2 in the plasma region are equal to 16% and 22%, respec-tively. These values are slightly higher than experi-mentally measured by actinometry technique using small addition of Ar atoms. It should be emphasized once again, that signifi-cant contribution of reaction (R6) in O(1D) production deal with sufficiently high oxygen dissociation degree in the discharge. Note that cross-section of O(1D) ex-citation in reaction (R6) is not well known and ob-served differences in experimental and theoretical in-put power dependences of SO excitation could be deal with this inexact cross-section.
Moreover, main process of O2(b1Σ g+ ) quench-
ing is heterogeneous deactivation on tube walls: O2(b1Σ g
+ ) + wall → O2 + wall (R7)
with loss probability γ ≅ 0.1 [Ivanov, 1999; Ivanov, 2000b]. For P = 2 Torr characteristic frequency of O2(b1Σ g
+ ) heterogeneous loss reaches (2.5-3)⋅103 s-1.
In particular this process compensates high production rate of O2(b1Σ g
+ ) in reaction (R3). We supposed that
molecules O2(а1∆g) are not produced in process (R7). There are no experimental data on the pathways of O2(b1Σ g
+ ) heterogeneous quenching. However if for
example 10% of reaction (R7) leads to O2(а1∆g) pro-duction, it will result in increase of SO concentrations
7 American Institute of Aeronautics and Astronautics
and energetic efficiency of SO generation. Calculations show that O2(а1∆g) production
exceeds quenching in plasma region. It means that SO concentration is far from saturation and its absolute value at the end of the plasma zone is determined mainly by mean electron temperature and input power.
As it seen from Fig. 10 and 11 main loss of O2(а1∆g) molecules is due to electron deactivation and reactions with atomic oxygen in plasma and off plasma regions, respectively. Quenching of SO states on ozone is not important in our conditions because O3 concen-trations are too low: [O3] ~ (2-3)⋅1011 cm-3 .
The role of reactions with O3 and three body process O2(а1∆g) + O(3P) + O2 → O2 + O(3P) + O2 (R8) in O2(а1∆g) quenching will be increased with gas pres-sure [Klopovsky, 2004, Rakhimova, 2004].
Conclusion
Experimental and theoretical study of singlet oxygen O2(а1∆g) generation in traveling mw discharge for oxygen pressures P = 1 – 4 Torr, flow rates of 20-30 m/s and specific energy inputs of 2.5-17 J/(cm3 atm).
It was shown that SO yield is increased with input power and measured SO yield reaches maximal values 22% and 27-30% for total gas pressure P = 2 Torr in pure oxygen and O2 : He =1:1 mixture, respec-tively. Experimental SO generation efficiency is in-creased in several times with specific energy decrease from 17 down to 2.5 J/(cm3 atm) reaching the maximal values ~20%.
Developed 2D cylindrical (r,z) model for cal-culations of plasma-chemical kinetics, heat and mass transfer, effective reduced electric field and ambipolar radial field was used for simulation of the processes in TMW discharges. Calculated and experimental SO yields are in good agreement at high input powers. However, SO number density increase with input power is more intensive in the experiments than in the calculations. Simulation allows to study different mechanisms of SO production and loss in plasma and afterglow. Main mechanism of O2(а1∆g) production in plasma zone is direct electron excitation. In addition, cascade deactivation of high electronic states of O2, in particular O2(b1Σ g
+ ) quenching by atomic oxygen,
could contribute significantly in O2(а1∆g) excitation. The most part of O2(b1Σ g
+ ) are quenched on the tube
walls. In this case, contrary to quenching by oxygen atoms, O2(а1∆g) molecules are not produced in signifi-cant amount. Changing the relations of heterogeneous and bulk processes of O2(b1Σ g
+ ) are quenching –
enhancing bulk loss, e.g., using a tube of larger diameter - it could increase the efficiency of O2(а1∆g) generation.
Acknowledgements
The work is partially funded from the ISTC, Project #1581. TR, YuM and NP are grateful for Key Science Schools (grant № 1713.2003.2) for support and partial funding.
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