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Design and optimisation of a pulsed CO2 laser for laser ultrasonics
Presented at
CSIR R&I Conference
Dr Andrew Forbes
Senior Scientist
February 2006
A. Forbes1, L.R. Botha1, N. du Preez2 and T. Drake3
1CSIR National Laser Centre, 2SDI (Pty) Ltd., 3Lockheed Martin Aerospace Company
Slide 2 © CSIR 2006 www.csir.co.za
Contents
• Laser ultrasonicsWhat is it, and how does it work?
• Optimising the parametersChoices and consequences
• Laser chemistryA physicists approach to chemistry
• Discharge disturbancesBloody hell, not more problems!
• Final system performanceWorking at last!
Slide 3 © CSIR 2006 www.csir.co.za
Laser ultrasonics – basic principles
• Optical absorption leads to thermal expansion
• Thermal expansion causes ultrasonic waves in sample
• Long pulse laser, coupled to an interferometer detects mechanical displacements produced by ultrasonic waves
• Ultrasound can be detected optically using an optical heterodyne technique
Slide 4 © CSIR 2006 www.csir.co.za
Composite
Generation Laser
Detection Laser
Interferometer
LaserUT Signal
-1.0
-0.5
0.0
0.5
1.0
-2.0 0.0 2.0 4.0 6.0 8.0 10.0
Time [us]
Am
plitu
de [n
orm
aliz
ed]
Laser ultrasonics – basic principles
Scanner Optics
Optimising the parameters
How do you design a laser for this application?
Slide 6 © CSIR 2006 www.csir.co.za
Objective
• Optimise the parameter set• Maximise energy,• Maximise repetition rate,• Maximise acoustic signal efficiency.
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0 10 20 30
Time (ms)
Inte
nsi
ty (
a.u
.)
0
0.001
0.002
0.003
0.004
0.005
0.006
0 0.5 1 1.5 2 2.5
Time (ms)
Inte
nsi
ty (
a.u
.)
Short time pulses
Slide 7 © CSIR 2006 www.csir.co.za
Pulse Envelope Factors affecting this are:
OC Reflectivity HV N2 CO2 Energy Factors affecting this are: OC Reflectivity HV N2 CO2 Stability Factors affecting this are: Electrodes HV Acoustics Pre-ionisation Catalysts Lifetime Factors affecting this are: OC Reflectivity HV Catalysts Gas Mix
Laser parameters
• Choices and consequences
Slide 8 © CSIR 2006 www.csir.co.za
Gas influences
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
0 200 400 600 800 1000
Energy (mJ)
Eff
icie
ncy
(V
/J)
70%
55%
50%
36%
0.2 0.4 0.6 0.8 1CO2 fraction
0.2
0.4
0.6
0.8
1e
Hnoitcarf
Laser chemistry
Impact of gas mix on laser chemistry
Slide 10 © CSIR 2006 www.csir.co.za
36 kVHeat Exchanger
2 CO2 2 CO + O2
2 CO2 2 CO + O2
Problem statement
Slide 11 © CSIR 2006 www.csir.co.za
Typical problem
0
0.5
1
1.5
2
2.5
3
0 100 200 300 400 500 600 700 800
Time (s)
Oxyg
en
(%
)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
No
rmalised
En
erg
y
Oxygen (%)
Energy
Slide 12 © CSIR 2006 www.csir.co.za
Approach
Take a guess
Compute the consequences
Compare to experiment
But, cannot determine rate laws from reaction stoichiometry!
Slide 13 © CSIR 2006 www.csir.co.za
Basic chemistry (rate laws)
dqp
Tmn
gc kCOOkCOOMkt
O
][][][][][
222
oxygen generation (discharge)oxygen recombination (catalysts)
• Can we explain?• Can we predict?• Can we improve?
Slide 14 © CSIR 2006 www.csir.co.za
Discharge parameters
O2 Production Rate (Kd)
y = 19.31x
0
20
40
60
80
100
120
140
0 1 2 3 4 5 6 7
Power Input (kW)
Oxy
gen
Pro
du
ctio
n (
pp
m/s
)
no catalysts
with catalysts
~ 90ppm/s @ 400Hz and 22.5kV
Slide 15 © CSIR 2006 www.csir.co.za
A word on mathematics
)()()( txMktxkktx ngcTd
n = 0 or 1 Already solved (easy)
n = 2 Riccatti equation (require a particular solution)
Any n? Numerically only
BUT, remove constant and you have an easy to solve Bernoulli equation!
Slide 16 © CSIR 2006 www.csir.co.za
With catalysts
gcT
dogcT
gcT
do Mkk
kttMkk
Mkk
kxtx
)))((exp()()()()( txMkkktx gcTd
0 1000 2000 3000 4000 5000 6000 7000Times
12000
14000
16000
18000
20000
22000
24000
negyxOmpp
Slide 17 © CSIR 2006 www.csir.co.za
Recombination rate
Catalyst ActivityEa ~ 38kJ/mol
0.E+00
1.E-03
2.E-03
3.E-03
4.E-03
5.E-03
6.E-03
7.E-03
8.E-03
280 290 300 310 320 330 340 350
Temperature (K)
Mg
.kc
RT
EATk a
c exp)( PldvCP
RTTPTT
v0
000 ),(
Slide 18 © CSIR 2006 www.csir.co.za
Predictions
gc
deq Mk
kxtx )(
1. Oxygen levels will stabilise
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80
Time (min)
Ox
yg
en
(%
)
Slide 19 © CSIR 2006 www.csir.co.za
Predictions2. Increasing the initial reactant concentrations decreases the time to equilibrium; increasing the temperature will decrease equilibrium levels.
Oxygen Decay, ~2:2:6 mix
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
Time (min)
Ox
yge
n %
298K, 0% CO298K, 1% CO298K, 2% CO308K, 2% CO
Slide 20 © CSIR 2006 www.csir.co.za
Predictions
R
RTEERTE
RT
ldvCPP aaav
2
)4(2 00
0
0*
3. The equilibrium values can decrease with increasing power!
0.00
0.50
1.00
1.50
2.00
2.50
3.00
0 200 400 600 800 1000
Repetition Rate (Hz)
Ox
yg
en
(%
)
Model
Exp. data
Slide 21 © CSIR 2006 www.csir.co.za
Outcome
0
1
2
3
4
5
6
7
8
9
10
0 10 20 30 40 50 60 70
Time (min)
En
erg
y Ji
tter
(st
d d
ev)
Jitter (after)
Jitter (before)
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80
Time (min)
No
rmal
ised
Po
wer
power (after)
power (before)
Discharge disturbances
Acoustics and shocks
Slide 23 © CSIR 2006 www.csir.co.za
0
10
20
30
40
50
60
70
80
300 350 400 450
Repetition Rate (Hz)
Std
Dev
iatio
n (%
)weak+ series
strong+ parallel
strong+series
strong+ series+damping
Acoustics
Slide 24 © CSIR 2006 www.csir.co.za
Stable discharge
Final system
In operation at Lockheed Martin for the JSF
Slide 26 © CSIR 2006 www.csir.co.za
Final laser
• 6x improvement in efficiency
• 2x improvement in energy
• 2x improvement in production throughput
With some very interesting science along the way …
600 800 1000 1200 1400 1600 1800 2000Time in ns
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
edutilpm
A
Thank you and Acknowledgements
Co-authors:
Dr Lourens Botha (National Laser Centre)
Mr Neil du Preez (SDI)
Dr Tommy Drake (Lockheed Martin)
