EXCITATION OF O 2 ( 1 Δ) IN PULSED RADIO FREQUENCY FLOWING PLASMAS FOR CHEMICAL IODINE LASERS...

EXCITATION OF O2(1Δ) IN PULSED RADIO FREQUENCY FLOWING PLASMAS FOR CHEMICAL

IODINE LASERS

Natalia Babaeva, Ramesh Arakoni and Mark J. Kushner

Iowa State UniversityAmes, IA 50011, USA

[email protected] [email protected]@iastate.edu

http://uigelz.ece.iastate.edu

October 2005

* Work supported by Air Force Office of Scientific Research and NSF

Iowa State University

Optical and Discharge Physics

AGENDA

Introduction to eCOILS

Description of the model

O2(1Δ) yield for CW and Spiker-Sustainer Excitation

Optimization with Frequency Summary

GEC_2005_02



OXYGEN-IODINE LASERS

• In chemical oxygen-iodine lasers (COILs), oscillation at 1.315 µm (2P1/2 2P3/2) in atomic iodine is produced by collisional

excitation transfer of O2(1) to I2 and I.

• Plasma production of O2(1) in electrical COILs (eCOILs)

eliminates liquid phase generators.

• Self sustaining Te in eCOILs plasmas (He/O2a few to 10s

Torr) is 2-3 eV. Excitation of O2(1) optimizes at Te = 1-1.5 eV.

• One method to increase system efficiency is lowering Te using

spiker-sustainer (S-S) techniques.

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O2(1∆) KINETICS IN NON-EQUILIBRIUM He/O2 DISCHARGES

• Production of O2(1∆) is by:

•Direct electron impact [0.98 eV]

•Excitation of O2(1Σ) [1.6 eV] with rapid quenching to O2(1∆).

• Self sustaining is Te = 2-3 eV. Optimum conditions are Te = 1-1.2 eV.

• Addition of He typically increases yield by reducing E/N.

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University of Illinois


SPIKER SUSTAINER TO LOWER Te

Spiker-sustainer (S-S) provides in-situ “external ionization.”

Short high power (spiker) pulse is followed by plateau of lower power (sustainer).

Excess ionization in “afterglow” enables operation below self-sustaining Te (E/N).

Te is closer to optimum for exciting O2(1).

Example: He/O2=1/1, 5 Torr, Global kinetics model

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•A computational investigation of eCOILs has been performed with a 2-d plasma hydrodynamics model (nonPDPSIM) to investigate spiker-sustainer methods.

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

j

sjjqN jj

j St

N

jjjj

s Sqt

))(()(

Electron energy equation:

e

ieiie

e qjTNnEjt

n

,

2

5

DESCRIPTION OF the MODEL: CHARGED PARTICLES, SOURCES

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Fluid averaged mass density, momentum and thermal energy density are obtained using unsteady, compressible algorithms.

Individual species are addressed with superimposed diffusive transport.

)pumps,inlets()v(t

iiiiiiii

iii EqmSENqvvkTN

t

v

i i

iiifipp EjHRvPTcvTt

Tc

DESCRIPTION OF the MODEL: NEUTRAL PARTICLE TRANSPORT

SV

T

iTifii SS

N

ttNNDvtNttN

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GEOMETRY FOR CAPACITIVE EXCITATION

• Cylindrical flow tube 6 cm diameter

• Capacitive excitation using ring electrodes.

• He/O2 = 70/30, 3 Torr, 6 slm .

• Yield:

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Flow Flow

])O[5.1O][5.0)](O[)](O[]O([

)](O)(O[

31

21

22

12

12

Y



TYPICAL PLASMA PROPERTIES (13 MHz, CW)

• O2(1Σ) and O densities are maximum near peak power deposition.

• O2(1∆) increases downstream while O2(1Σ) is quenched to O2(1∆).

• 3 Torr, He/O2=0.7/0.3, 6 slm

• Power, [e], O, O2(1Σ) and O2(1∆)

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MIN

MAX

• O2(1∆) yield on Axis



Spiker-sustainer (S-S) consists of pulsed modulated rf excitation.

High power pulses produce excess ionization and allow discharge to operate nearer to optimum Te for O2(1∆) production.

.

SPIKER-SUSTAINER: VOLTAGE WAVEFORM

• 27 MHz, 120 W, 1 MHz Carrier, 20% duty cycle

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SINGLE SPIKER: Te and ELECTRON DENSITY

0 - 2 x 1010 cm-3 0 - 3.1 eV

• Short high power pulse (spiker) is applied , followed by a longer period of lower power.

• Te is low after spiker enabling more efficient production of O2 (1Δ).

• Excess ionization created by the spiker

decays within 10 – 15 µs.

ANIMATION SLIDE

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Te (eV) [e]

• 13 MHz, 40 W Single Spiker• t = 0.5 – 20 s

MIN

MAX



S-S vs CW : PLASMA PROPERTIES

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• O2(1Σ ) is quickly collisionally quenched to O2(1∆) after the plasma zone.

• O2(1∆) is quenched slowly.

• O atom production nearly equals O2(1∆).

• 13 MHz, 40 W, 3 Torr, He/O2=0.7/0.3, 6 slm

• Spiker-Sustainer• CW


Optical and Discharge PhysicsGEC_2005_13

• Dissociation fraction decreases when using S-S.

• Lower Te enabled by S-S reduces rate of dissociation while increasing rate of excitation of O2(1).

S-S vs CW: O2(1) PRODUCTION AND O2 DISSOCIATION


• 13 MHz, 120 W, 3 Torr, He/O2=0.7/0.3, 6 slm



S-S vs CW: ELECTRON TEMPERATURE

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• Increasing power and increasing intra-pulse conductivity enables lowering of Te.

• The effect is more pronounced with S-S.

• 13 MHz, 3 Torr, He/O2=0.7/0.3, 6 slm



S-S vs CW: O2(1∆) YIELD AND PRODUCTION EFFICIENCY

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• S-S raises yields of O2(1∆) by 10-15% at lower powers.

• Efficiency decreases with power due to dissociation.

• Low power produces the highest efficiency with S-S but requires longer residence times to achieve high yield.

• 13 MHz, 3 Torr, He/O2=0.7/0.3, 6 slm

• Efficiency



Intra-pulse Te decreases with increasing rf frequency.

As electron density and conductivity increases with successive pulses, Te decreases.

Average Te with 27 MHz is ≈1 eV, optimum for O2(1∆) production

S-S: ENGINEERING Te FOR YIELD

ANIMATION SLIDE

0 - 2.5 eV

13 MHz 27 MHz

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• t = 2 - 15 µs 0 - 4.1 eV

MIN

MAX

Te (eV)



13 MHz vs 27 MHz : O2(1Δ) YIELD

The efficiency of S-S increases with rf frequency by producing a higher [e] and lower Te.

Reduction in Te shifts operating point closer to optimum value, increasing yield by 10% to 20%.

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• 3 Torr, He/O2=0.7/0.3, 6 slm




GOING TO HIGHER RF FREQUENCIES?

• Increasing frequency above 27 MHz further decreases Te but improvements, if any, are small.

• At sufficiently high frequencies, Te may decrease below that for optimum O2(

1) production (e.g., 40 MHz, Te = 0.5 eV)

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Optimum Te

• 3 Torr, He/O2=0.7/0.3, 6 slm

• 27 MHz vs 40 MHz • Te vs frequency



CONCLUDING REMARKS

S-S method can raise yields of O2(1) compared to CW excitation

by lowering pulse average Te.

The efficiency of S-S methods generally increase with increasing rf frequency by producing

Higher electron density, Lower Te

Going to very high frequencies may reduce Te below the optimum value for O2(

1) production.

GEC_2005_19

EXCITATION OF O 2 ( 1 Δ) IN PULSED RADIO FREQUENCY FLOWING PLASMAS FOR CHEMICAL IODINE LASERS...

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