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OPTIMIZATION OF O2(1) YIELDS IN PULSED RF
FLOWING PLASMAS FOR CHEMICAL OXYGEN IODINE
LASERS*
Natalia Y. Babaeva, Ramesh Arakoni and Mark J. Kushner
Iowa State University
Ames, IA 50011, USA
[email protected] [email protected]
[email protected]
http://uigelz.ece.iastate.edu
June 2006
* Work supported by Air Force Office of Scientific Research and NSF.
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AGENDA
Introduction to eCOIL
Description of the model
Spiker Sustainer excitation vs CW for improving yield
Optimization of O2(1) yields in Spiker Sustainer excitation:
Power
Carrier frequency
Spiker frequency
Duty cycle
Higher pressure operation
Concluding remarks
Iowa State University
Optical and Discharge Physics
ICOPS2006_Natalie_02
ELECTRICALLY EXCITED 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 eCOIL plasmas (He/O2, a 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.
In this talk, S-S techniques will be computationally investigated.
Iowa State University
Optical and Discharge Physics
ICOPS2006_Natalie_03
TYPICAL EXPERIMENTAL CONDITIONS
Laser oscillation has been achieved using He/O2 flowing plasmas to
produce O2(1) using capacitively coupled rf discharges.
I2 injection and supersonic expansion (required to lower Tg for
inversion) occurs downstream of the plasma zone.
Ref: CU Aerospace
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Iowa State University
Optical and Discharge Physics
O2(1∆) KINETICS IN
He/O2 DISCHARGES
Main channels of O2(1Δ)
production:
Direct electron impact [0.9
eV].
Excitation of O2(1Σ) with
rapid quenching to O2(1Δ).
Self sustaining is Te=2-3 eV.
Optimum condition for O2(1Δ)
production is Te=1-1.2 eV.
Significant power can be
channeled into excitation of
O2(1Δ).
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Optical and Discharge Physics
ICOPS2006_Natalie_05
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 selfsustaining Te (E/N).
Te is closer to optimum for
exciting O2(1Δ).
Example: He/O2=1/1, 5 Torr,
Global kinetics model
University of Illinois
Optical and Discharge Physics
ICOPS2006_Natalie_06
DESCRIPTION OF THE MODEL:
CHARGED PARTICLES, SOURCES
Poisson’s equation, continuity equations and surface charge are
simultaneously solved using a Newton iteration technique.
N j q j s
N j
t
j
j S j
s
q j ( j S j ) ( ())
t
j
Electron energy equation:
ne
5
j E ne Ni i Te , j qe
t
2
i
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Optical and Discharge Physics
ICOPS2006_Natalie_07
DESCRIPTION OF MODEL:
NEUTRAL PARTICLE TRANSPORT
Fluid averaged values of mass density, mass momentum and
thermal energy density obtained using unsteady algorithms.
( v ) ( inlets, pum ps)
t
v
NkT v v qi Ni Ei
t
i
c pT
T v c pT Pi v f Ri H i ji E
t
i
i
Individual fluid species diffuse in the bulk fluid.
N i t t
SV S S
N i t t N i t v f Di NT
N
T
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Optical and Discharge Physics
ICOPS2006_Natalie_08
2D-GEOMETRY FOR CAPACITIVE EXCITATION
Flow
Flow
Cylindrical flow tube 6 cm diameter
Capacitive excitation using ring
electrodes.
Base case: He/O2 = 70/30, 3 Torr, 6 slm .
Yield:
[O2 (1 ) O2 (1 )]
Y
([O2 ] [O2 (1 )] [O2 (1 )] 0.5[O] 1.5[O3 ])
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Optical and Discharge Physics
ICOPS2006_Natalie_09
NON-SELF SUSTAINED DISCHARGES: SPIKER SUSTAINER
Te (eV)
Spiker sustainer consists of
modulated rf excitation.
Te decreases during low
power sustainer as there is
excess ionization.
During startup transient, as
electron density and
conductivity increase with
successive pulses, Te
decreases.
0 - 2.5 eV
27 MHz, He/O2 = 70/30, 3 Torr
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ANIMATION SLIDE
MIN
• t = 2 - 15 µs
MAX
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Optical and Discharge Physics
CW vs SPIKER SUSTAINER EXCITATION
Flow
CW
Spiker-Sustainer
Te in bulk plasma is reduced from 2.7 to 2.0 eV with factor of two
larger ne; Dissociation is lower, O2(1) larger.
VSS/VCW=2.5, 20% duty cycle, 13.56 MHz/1 MHz
MIN
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MAX
3 Torr, He/O2=0.7/0.3, 6 slm
Iowa State University
Optical and Discharge Physics
CW vs SS:
CARRIER FREQUENCY
Increasing carrier frequency
improves efficiency of O2(1).
Higher ionization efficiency at
high frequency enables lower
Te.
CW: Lowering Te towards Te-opt
is generally a benefit
SS: Decreasing Te below Te-opt
lowers total excitation
efficiency.
He/O2=70/30, 3 Torr
VSS/VCW=2.5, 20% dc, 1 MHz-SS
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Optical and Discharge Physics
ICOPS2006_Natalie_15
SS FORMAT: VSS/VCW
Pulse power format is critical in
determining efficiency for a
given power deposition.
Larger VSS/VCW shifts power
into ionization, allowing lower
Te during sustainer.
Too large VSS/VCW produces too
much ionization, lowering Te
below Te-opt.
He/O2=70/30, 3 Torr, 40 W
20% dc, 27 MHz/1 MHz-SS
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Optical and Discharge Physics
ICOPS2006_Natalie_16
SS FORMAT: SPIKER AND
SUSTAINER PULSE LENGTH
Ideal spiker is a delta-function
producing instant ionization at
high efficiency.
With fixed VSS/VCW, lower power
in spiker may reduce efficiency.
Increasing sustainer pulse
length provides better
utilization of low Te.
Too long a sustainer allows Te
to increase towards self
sustaining value.
He/O2=70/30, 3 Torr, 40 W, 20%
dc
Iowa State University
Optical and Discharge Physics
ICOPS2006_Natalie_17
CW vs SS:
POWER DEPOSITION
Yield for SS is larger than CW;
both increasing with power.
CW: Decrease in Te from above
Te-opt to near Te-opt improves
efficiency.
SS: Decrease in Te from near
Te-opt to below Te-opt decreases
efficiency.
CW and SS converge at high
power.
He/O2=70/30, 3 Torr
VSS/VCW=2.5, 20% dc, 13.56
MHz/1 MHz
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Optical and Discharge Physics
ICOPS2006_Natalie_14
OPERATING AT HIGHER PRESSURES: GLOBAL MODEL
Many system issues motivate
operating eCOILs at higher
pressures.
If quenching is not important,
[O2(1)] pressure for constant
eV/molecule.
Significantly sub-linear scaling
results in decrease in yield with
increasing pressure.
O3 is a major quencher.
Gas heating at high pressure
reduces O3 production and
increases O3 destruction.
O3 kinetics and Tg control are very
important.
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Optical and Discharge Physics
ICOPS2006_Natalie_18
OPERATING AT HIGHER PRESSURES: FULL 2D HYDRO
Large yields can be obtained at the
edge of the plasma zone.
Up to 20-30 Torr, O3 formation and
quenching decrease yield.
>30-40 Torr, gas heating and
constriction produce locally high
yield that is rapidly quenched.
Reduction in yield is progressively
determined by:
O3 quenching
Gas heating
Discharge stability
He/O2=70/30, 25 MHz
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Optical and Discharge Physics
ICOPS2006_Natalie_19
FLOW
[e] 1010cm-3
Te (eV)
DISCHARGE STABILITY
WITH PRESSURE
Operating at higher
pressures often encounter
discharge stability issues.
Constriction of discharge
occurs due to smaller meanfree-paths.
Asymmetry in plasma begins
to occur due to downstream
rarefaction being greater.
He/O2=70/30, 25 MHz
ANIMATION SLIDE
3 Torr,
40 W
50 Torr,
670 W
0
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3 Torr,
40 W
MAX
50 Torr,
670 W
Iowa State University
Optical and Discharge Physics
CONCLUDING REMARKS
Spiker-sustainer strategies can be effective in lowering Te
into more optimum regime for exciting O2(1).
Higher carrier frequencies (either CW or SS) produce larger
ne and lower Te and so are beneficial.
Advantage of SS is marginal at higher powers due to Te
being naturally lower.
High pressure operation can produce larger densities of
O2(1) at high yields with careful management of
Ozone density
Gas temperature
Stability
Iowa State University
Optical and Discharge Physics
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