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
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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.
GEC_2005_03
Iowa State University
Optical and Discharge Physics
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.
GEC_2005_04
University of Illinois
Optical and Discharge Physics
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
GEC_2005_05
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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
GEC_2005_06
Iowa State University
Optical and Discharge Physics
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
GEC_2005_07
Iowa State University
Optical and Discharge Physics
GEOMETRY FOR CAPACITIVE EXCITATION
• Cylindrical flow tube 6 cm diameter
• Capacitive excitation using ring electrodes.
• He/O2 = 70/30, 3 Torr, 6 slm .
• Yield:
GEC_2005_08
Flow Flow
])O[5.1O][5.0)](O[)](O[]O([
)](O)(O[
31
21
22
12
12
Y
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Optical and Discharge Physics
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∆)
GEC_2005_09
MIN
MAX
• O2(1∆) yield on Axis
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Optical and Discharge Physics
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
GEC_2005_10
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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
GEC_2005_11
Te (eV) [e]
• 13 MHz, 40 W Single Spiker• t = 0.5 – 20 s
MIN
MAX
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Optical and Discharge Physics
S-S vs CW : PLASMA PROPERTIES
GEC_2005_12
• 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
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• 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
• Spiker-Sustainer• CW
• 13 MHz, 120 W, 3 Torr, He/O2=0.7/0.3, 6 slm
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Optical and Discharge Physics
S-S vs CW: ELECTRON TEMPERATURE
GEC_2005_14
• 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
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Optical and Discharge Physics
S-S vs CW: O2(1∆) YIELD AND PRODUCTION EFFICIENCY
GEC_2005_15
• 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
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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
GEC_2005_16
• t = 2 - 15 µs 0 - 4.1 eV
MIN
MAX
Te (eV)
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Optical and Discharge Physics
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%.
GEC_2005_17
• 3 Torr, He/O2=0.7/0.3, 6 slm
• Spiker-Sustainer• CW
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Optical and Discharge Physics
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)
GEC_2005_18
Optimum Te
• 3 Torr, He/O2=0.7/0.3, 6 slm
• 27 MHz vs 40 MHz • Te vs frequency
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Optical and Discharge Physics
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