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.
ICOPS2006_Natalie_01
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).
Iowa State University
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  qe
t
2

i
Iowa State University
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 , pumps )
t



 v 
 NkT     v v        qi N i 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



Iowa State University
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:
[O 2 (1 )  O 2 (1 )]
Y
([O 2 ]  [O 2 (1 )]  [O 2 (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
Iowa State University
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
Iowa State University
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
Iowa State University
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
ICOPS2006_Natalie_21
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
ICOPS2006_Natalie_22