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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 UniversityAmes, IA 50011, USA
natalie5@iastate.edu arakoni@iastate.edumjk@iastate.edu
http://uigelz.ece.iastate.edu
June 2006
* Work supported by Air Force Office of Scientific Research and NSF.
ICOPS2006_Natalie_01
Iowa State University
Optical and Discharge Physics
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
ICOPS2006_Natalie_02
Iowa State University
Optical and Discharge Physics
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/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.
In this talk, S-S techniques will be computationally investigated.
ICOPS2006_Natalie_03
Iowa State University
Optical and Discharge Physics
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.
ICOPS2006_Natalie_04
Ref: CU Aerospace
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).
ICOPS2006_Natalie_05
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(1Δ).
Example: He/O2=1/1, 5 Torr, Global kinetics model
ICOPS2006_Natalie_06
Iowa State University
Optical and Discharge Physics
Poisson’s equation, continuity equations and surface charge are simultaneously solved using a Newton iteration technique.
Electron energy equation:
j
sjjqN
jjj St
N
jjjj
s Sqt
))(()(
e
ieiie
e qjTNnEjt
n
,
2
5
DESCRIPTION OF THE MODEL: CHARGED PARTICLES, SOURCES
ICOPS2006_Natalie_07
Iowa State University
Optical and Discharge Physics
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
i
iii ENqvvNkTt
v
i i
iiifipp EjHRvPTcvTt
Tc
SV
T
iTifii SS
N
ttNNDvtNttN
DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT
ICOPS2006_Natalie_08
Iowa State University
Optical and Discharge Physics
2D-GEOMETRY FOR CAPACITIVE EXCITATION
Cylindrical flow tube 6 cm diameter
Capacitive excitation using ring electrodes.
Base case: He/O2 = 70/30, 3 Torr, 6 slm .
Yield:
Flow Flow
])O[5.1O][5.0)](O[)](O[]O([
)](O)(O[
31
21
22
12
12
Y
ICOPS2006_Natalie_09
Iowa State University
Optical and Discharge Physics
NON-SELF SUSTAINED DISCHARGES: SPIKER SUSTAINER
27 MHz, He/O2 = 70/30, 3 Torr
Te (eV)
MIN
MAX
• t = 2 - 15 µs
ANIMATION SLIDE
0 - 2.5 eV
ICOPS2006_Natalie_10
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.
Iowa State University
Optical and Discharge Physics
CW vs SPIKER SUSTAINER EXCITATION
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
3 Torr, He/O2=0.7/0.3, 6 slmMIN
MAX
CW Spiker-Sustainer
ICOPS2006_Natalie_11
Flow
Iowa State University
Optical and Discharge Physics
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
CW vs SS: CARRIER FREQUENCY
ICOPS2006_Natalie_15
Iowa State University
Optical and Discharge Physics
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
SS FORMAT: VSS/VCW
ICOPS2006_Natalie_16
Iowa State University
Optical and Discharge Physics
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
SS FORMAT: SPIKER AND SUSTAINER PULSE LENGTH
ICOPS2006_Natalie_17
Iowa State University
Optical and Discharge Physics
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
CW vs SS: POWER DEPOSITION
ICOPS2006_Natalie_14
Iowa State University
Optical and Discharge Physics
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.
ICOPS2006_Natalie_18
Iowa State University
Optical and Discharge Physics
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
ICOPS2006_Natalie_19
DISCHARGE STABILITYWITH PRESSURE
Iowa State UniversityOptical and Discharge Physics
Operating at higher pressures often encounter discharge stability issues.
Constriction of discharge occurs due to smaller mean-free-paths.
Asymmetry in plasma begins to occur due to downstream rarefaction being greater.
He/O2=70/30, 25 MHz
FLOW
[e] 1010cm-3 Te (eV)
3 Torr, 40 W
50 Torr, 670 W
MAX0
3 Torr, 40 W
50 Torr, 670 W
ICOPS2006_Natalie_21
ANIMATION SLIDE
Iowa State University
Optical and Discharge Physics
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
CONCLUDING REMARKS
ICOPS2006_Natalie_22
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