OPTIMIZATION OF O2(1?) YIELDS IN PULSED RF FLOWING PLASMAS FOR CHEMICAL OXYGEN IODINE LASERS*

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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
• natalie5_at_iastate.edu arakoni_at_iastate.edu
• mjk_at_iastate.edu
• 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

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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(1D) to I2 and I.
• Plasma production of O2(1D) 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(1D)
  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.

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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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O2(1?) KINETICS IN He/O2 DISCHARGES

• Main channels of O2(1?) production
• Direct electron impact 0.9 eV.
• Excitation of O2(1S) with rapid quenching to
  O2(1?).
• Self sustaining is Te2-3 eV. Optimum condition
  for O2(1?) production is Te1-1.2 eV.
• Significant power can be channeled into
  excitation of O2(1?).

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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/O21/1, 5 Torr, Global kinetics model

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DESCRIPTION OF THE MODEL CHARGED PARTICLES,
SOURCES

• Poissons equation, continuity equations and
  surface charge are simultaneously solved using a
  Newton iteration technique.
• Electron energy equation

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DESCRIPTION OF MODEL NEUTRAL PARTICLE TRANSPORT

• Fluid averaged values of mass density, mass
  momentum and thermal energy density obtained
  using unsteady algorithms.
• Individual fluid species diffuse in the bulk
  fluid.

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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

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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.

t 2 - 15 µs
0 - 2.5 eV

• 27 MHz, He/O2 70/30, 3 Torr

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ANIMATION SLIDE
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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(1D) larger.
• VSS/VCW2.5, 20 duty cycle, 13.56 MHz/1 MHz

Iowa State University Optical and Discharge
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• 3 Torr, He/O20.7/0.3, 6 slm

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CW vs SS CARRIER FREQUENCY

• Increasing carrier frequency improves efficiency
  of O2(1D).
• 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/O270/30, 3 Torr
• VSS/VCW2.5, 20 dc, 1 MHz-SS

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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/O270/30, 3 Torr, 40 W
• 20 dc, 27 MHz/1 MHz-SS

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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/O270/30, 3 Torr, 40 W, 20 dc

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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/O270/30, 3 Torr
• VSS/VCW2.5, 20 dc, 13.56 MHz/1 MHz

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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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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.
• gt30-40 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/O270/30, 25 MHz

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DISCHARGE STABILITY WITH PRESSURE
FLOW
Te (eV)
e 1010cm-3

• 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/O270/30, 25 MHz

ANIMATION SLIDE
50 Torr, 670 W
3 Torr, 40 W
50 Torr, 670 W
3 Torr, 40 W
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CONCLUDING REMARKS

• Spiker-sustainer strategies can be effective in
  lowering Te into more optimum regime for exciting
  O2(1D).
• 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(1D) at high yields with careful
  management of
• Ozone density
• Gas temperature
• Stability

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