O2(1∆) PRODUCTION AND OXYGEN-IODINE KINETICS IN FLOWING AFTERGLOWS FOR ELECTRICALLY EXCITED CHEMICAL-OXYGEN-IODINE LASERS*

Ramesh Arakoni, Natalia Y. Babaeva, and Mark J. Kushner

Iowa State UniversityAmes, IA 50011, USA

arakoni@iastate.edu natalie5@iastate.edumjk@iastate.edu

http://uigelz.ece.iastate.edu

October 2006

* Work supported by Air Force Office of Scientific Research and National Science Foundation.

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AGENDA

• Introduction to eCOIL

• Description of the model

• Oxygen-iodine kinetics mechanism

• NO/NO2 addition, I2 dissociation

• Concluding Remarks

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ELECTRICALLY EXCITED OXYGEN-IODINE LASERS• In chemical oxygen-iodine lasers (COILs), oscillation at 1.315 µm

I(2P1/2)→ I(2P3/2) occurs by excitation transfer of O2(1∆) to I2 and I.

• Plasma production of O2(1∆) in electrical COILs (eCOILs) eliminates liquid phase generators.

• I2 injection and supersonic expansion (required to lower Tgas for inversion) occurs downstream of the plasma zone.

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• Ref: CU Aerospace

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NO AND NO2 INJECTION

• Excitation of O2(1∆) optimizes at Te = 1 eV whereas self sustaining discharges require Te = 2-3 eV.

• NO additive (lower ionization potential) to inlet flow may lower Te to a more optimum value.

• Significant electron impact dissociation of O2 produces large fluxes of O atoms which:

• Quench the upper laser level → Increases O2(1∆) for oscillation.• Dissociate I2 → Decreases O2(1∆) required to produce I atoms.

• NO2 injection may be used to control O atom inventory

NO2 + O → O2 + NO

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GEOMETRY FOR CAPACITIVE EXCITATION

• Cylindrical flow tube 6 cm diameter

• Capacitive excitation (10s MHz) using ring electrodes.

• Ring injection nozzles

• Typical Conditions: He/O2=70/30, 3 Torr 10s to 100 W

• Outflow: O2(1∆)/O2 = 0.15 - 0.25

He/NO2Injection

He/I2InjectionPrimary inflow

He/O2/NO

NO2

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Outflow

• Electron impact [0.9 eV] and excitation of O2(1Σ) with quenching to O2(1∆) are the main channels of O2(1∆) production.

• O atom and O3 production result in quenching and I2-oxygen chemistry downstream. Iowa State University

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O2(1∆) KINETICS IN He/O2 DISCHARGES

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• I2 is rapidly dissociated by atomic oxygen and O2(1∆, 1Σ).

• Population inversion by excitation transfer of O2(1∆) to I(2P3/2).

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OXYGEN-IODINE AND NOx KINETICS

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• NO/NO2 recycling chain scavenges O atoms.

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THE ROLE OF ADDITIVES

• The roles of additives (NO, NO2), their synergy with I2 injection and production of I(2P1/2) in eCOILS were computationally investigated.

• Global modeling: Basic kinetics and scaling

• 2-d modeling: Hydrodynamics and injection strategies.

• What are tradeoffs in using additives to optimize I(2P1/2)?

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• Poisson’s equation, continuity equations and surface charge are simultaneously solved using a Newton iteration technique.

• Electron energy equation:

∑ +=Φ∇⋅∇−j

sjjqN ρε

jjj StN

+⋅−∇=∂

∂φr

∑ Φ−∇⋅∇−+⋅∇−=∂∂

jjjj

s Sqt

))(()( σφρ r

( )e

ieiie

e qjTNnEjtn φλεϕκ∂ε∂ rrrr

=⎟⎠⎞

⎜⎝⎛ ∇−⋅∇−−⋅= ∑ ,25

DESCRIPTION OF 2d-MODEL: CHARGED PARTICLES, SOURCES

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• Fluid averaged values of mass density, mass momentum and thermal energy density obtained using unsteady algorithms.

• Individual fluid species diffuse in the bulk fluid.

)pumps,inlets()v(t

+⋅−∇=rρ

∂ρ∂

( ) ( ) ( ) ∑+⋅∇−⋅∇−∇=i

iii ENqvvNkTtv vrrr

µρ∂ρ∂

( ) ( ) ∑ ∑ ⋅+−⋅∇++∇−−∇=i i

iiifipp EjHRvPTcvTtTc rrr ∆ρκ

∂ρ∂

( ) ( ) ( )SV

T

iTifii SS

NttNNDvtNttN ++⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛ +∇−⋅∇−=+

∆∆ r

DESCRIPTION OF 2d-MODEL: NEUTRAL PARTICLE TRANSPORT

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[NO] INLET ADDITIVE

• The effect of NO on electron density is small.

• O atoms are rapidly depleted by NO.

• Global model captures trends.

• He/O2/NO= 70/30/0-3

3 Torr, 40W, 25 MHz, 6000 sccm

• Global Model • 2d Model

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BASE CASE PARAMETERS

• Peak Te above that for optimum O2(1∆) production.

• Electron density localized due to rapid attachment.

• O2(1∆) yield is 15%.

• O atoms consumed primarily by O3production.

• He/O2=70/30, 3 Torr, 40W, 25MHz, 6000 sccm

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• He/O2=70/30, 6000 sccm 3 Torr, 40W, 25 MHz

He/O2

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0.2 sccm NO2 AND I2INJECTION

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• Injection: He/NO2=0.995/0.005, 36 sccm

• Injection: He/I2=0.995/0.005, 36 sccm

• Atomic O nominally depleted by NO2

• Excess atomic oxygen totally dissociates small amount of injected iodine.

He/O2 He/NO2 He/I2

MIN MAX

• He/O2=70/30, 6000 sccm 3 Torr, 40W, 25 MHz

He/NO2 He/I2He/O2

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• Injection: He/NO2=0.9/0.1, 36 sccm

• Injection: He/I2=0.9/0.1, 36 sccm

MIN MAX

• Atomic oxygen is almost completely scavenged by NO2

• O2(1∆) is rapidly depleted by I2 in pumping reaction.

• Only fraction of injected I2is dissociated.

• I* peaks near inlet.

• He/O2=70/30, 6000 sccm 3 Torr, 40W, 25 MHz

3.6 sccm NO2 AND I2INJECTION

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EFFECT OF ADDITIVES ON GAS TEMPERATURE

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• He/NO2=0.995/0.005• He/I2=0.995/0.005

He/NO2 He/I2 or He/I (36 sccm)

• Gas temperature increases due to exothermicity of scavenging and dissociation reactions

NO2 + O → O2 + NO O + I2 → IO + I• Injection of I atoms reduces downstream Tgas.

• He/NO2=0.9/0.1• He/I2=0.9/0.1

• He/NO2=0.9/0.1• He/I=0.9/0.1

Predissociated iodine

• He/O2=70/30, 6000 sccm 3 Torr, 40W, 25 MHz

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• Injection:•He/NO2=0.9/xxx, 36 sccm•He/I2=0.995/0.005, 36 sccm

• Injection:•He/NO2=0.9/xxx, 36 sccm•He/I2=0.9/0.1, 36 sccm

• NO2 injection has little effect on O2(1∆). • I2 and I quenching (laser pumping reactions) rapidly deplete O2(1∆).

O2(1∆) vs ADDITIVES

• He/O2=70/30, 6000 sccm 3 Torr, 40W, 25 MHz

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ATOMIC OXYGEN vs ADDITIVES

• Injection:•He/NO2=0.9/xxx, 36 sccm•He/I2=0.995/0.005, 36 sccm

• Injection:•He/NO2=0.9/xxx, 36 sccm•He/I2=0.9/0.1, 36 sccm

• Optimum NO2 flow rate scavenges excess O atoms leaving enough atoms to dissociate injected I2.

• He/O2=70/30, 6000 sccm 3 Torr, 40W, 25 MHz

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• Injection:•He/NO2=0.995/0.005, 36 sccm•He/I2=0.995/0.005, 36 sccm

• Injection:•He/NO2=0.9/0.1, 36 sccm•He/I2=0.9/0.1, 36 sccm

• Optimum NO2 injection will optimize density of I* for a given O2(1∆) production.

IODINE SPECIES vs ADDITIVES

• He/O2=70/30, 6000 sccm 3 Torr, 40W, 25 MHz

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OPTIMIZING I* PRODUCTION

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• Injection:•He/NO2=0.9/xxx, 36 sccm•He/I2=0.995/0.005, 36 sccm

• Injection:•He/NO2=0.9/xxx, 36 sccm•He/I2=0.9/0.1, 36 sccm

• Predissociation of I2 lessens the need to have a small O atom flow for dissociation of I2.

• Optimum NO2 completely scavenges O atoms.

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CONCLUDING REMARKS

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• Oxygen-iodine kinetics in flowing afterglows for electrically excited chemical-oxygen-iodine lasers has been computationally investigated.

• NO2 injection scavenges O atoms.

• Reduces amount of quenching of I*.

• Also reduces the amount of dissociation of I2.

• End result is delicate balance is required.

• Injection of pre-dissociated I2 eliminates competition between these two processes and more easily optimizes I*.