Transcript Document

EXCITATION OF O2(1Δ) IN PULSED RADIO
FREQUENCY FLOWING PLASMAS FOR CHEMICAL
IODINE LASERS
Natalia 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
October 2005
* Work supported by Air Force Office of Scientific Research and NSF
AGENDA
 Introduction to eCOILS
 Description of the model
 O2(1Δ) yield for CW and Spiker-Sustainer Excitation
 Optimization with Frequency
 Summary
GEC_2005_02
Iowa State University
Optical and Discharge Physics
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 eCOILs 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.
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 = 11.2 eV.
• Addition of He typically
increases yield by reducing
E/N.
GEC_2005_04
Iowa State University
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(1D).
 Example: He/O2=1/1, 5 Torr,
Global kinetics model
GEC_2005_05
University of Illinois
Optical and Discharge Physics
DESCRIPTION OF the MODEL:
CHARGED PARTICLES, SOURCES
• 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.
       N j q j  s
j
N j
t

    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
GEC_2005_06
Iowa State University
Optical and Discharge Physics
DESCRIPTION OF the MODEL:
NEUTRAL PARTICLE TRANSPORT
 Fluid averaged mass density, momentum and thermal energy
density are obtained using unsteady, compressible algorithms.


   ( v )  ( inlets, pum ps)
t


 v 



   N i kTi     v v        qi N i Ei  S i mi  i qi E
t
i
 i



 
 c pT 

  T  v c pT   Pi   v f   Ri DH i   ji  E
t
i
i
 Individual species are addressed with superimposed diffusive
transport.

 N i t  Dt   
   SV  S S
N i t  Dt   N i t      v f  Di NT 

N
T




GEC_2005_07
Iowa State University
Optical and Discharge Physics
GEOMETRY FOR CAPACITIVE EXCITATION
Flow
Flow
• Cylindrical flow tube 6 cm diameter
• Capacitive excitation using ring
electrodes.
• He/O2 = 70/30, 3 Torr, 6 slm .
• Yield:
[O2 (1 D)  O2 (1 )]
Y
([O2 ]  [O2 (1 D)]  [O2 (1 )]  0.5[O]  1.5[O3 ])
GEC_2005_08
Iowa State University
Optical and Discharge Physics
TYPICAL PLASMA PROPERTIES (13 MHz, CW)
• O2(1∆) yield on Axis
• Power, [e], O, O2(1Σ) and O2(1∆)
• O2(1Σ) and O densities are maximum near peak power deposition.
• O2(1∆) increases downstream while O2(1Σ) is quenched to O2(1∆).
MIN
MAX
• 3 Torr, He/O2=0.7/0.3, 6 slm
GEC_2005_09
Iowa State University
Optical and Discharge Physics
SPIKER-SUSTAINER: VOLTAGE WAVEFORM
.
 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.
• 27 MHz, 120 W, 1 MHz Carrier,
20% duty cycle
GEC_2005_10
Iowa State University
Optical and Discharge Physics
Te (eV)
[e]
SINGLE SPIKER: Te and ELECTRON DENSITY
• 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.
• 13 MHz, 40 W Single Spiker
• t = 0.5 – 20 s
ANIMATION SLIDE
MIN
0 - 3.1 eV 0 - 2 x 1010 cm-3
GEC_2005_11
MAX
Iowa State University
Optical and Discharge Physics
S-S vs CW : PLASMA PROPERTIES
• CW
• Spiker-Sustainer
•
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
GEC_2005_12
Iowa State University
Optical and Discharge Physics
S-S vs CW: O2(1D) PRODUCTION AND O2 DISSOCIATION
• CW
•
•
• Spiker-Sustainer
Dissociation fraction decreases when using S-S.
Lower Te enabled by S-S reduces rate of dissociation while
increasing rate of excitation of O2(1D).
Iowa State University
• 13 MHz, 120 W, 3 Torr, He/O2=0.7/0.3, 6 slm Optical
and Discharge Physics
GEC_2005_13
S-S vs CW: ELECTRON TEMPERATURE
•
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
Iowa State University
Optical and Discharge Physics
GEC_2005_14
S-S vs CW: O2(1∆) YIELD AND PRODUCTION EFFICIENCY
• Efficiency
• 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
GEC_2005_15
Iowa State University
Optical and Discharge Physics
13 MHz
27 MHz
S-S: ENGINEERING Te FOR YIELD
Te (eV)
 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
ANIMATION SLIDE
MIN
0 - 4.1 eV
GEC_2005_16
0 - 2.5 eV
• t = 2 - 15 µs
MAX
Iowa State University
Optical and Discharge Physics
13 MHz vs 27 MHz : O2(1Δ) YIELD
• CW
• Spiker-Sustainer
 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%.
• 3 Torr, He/O2=0.7/0.3, 6 slm
GEC_2005_17
Iowa State University
Optical and Discharge Physics
GOING TO HIGHER RF FREQUENCIES?
Optimum Te
• 27 MHz vs 40 MHz
• Te vs frequency
• 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(1D) production (e.g., 40 MHz, Te = 0.5 eV)
• 3 Torr, He/O2=0.7/0.3, 6 slm
GEC_2005_18
Iowa State University
Optical and Discharge Physics
CONCLUDING REMARKS
 S-S method can raise yields of O2(1D) 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(1D) production.
Iowa State University
Optical and Discharge Physics
GEC_2005_19