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Design and optimisation of a pulsed CO 2 laser for laser ultrasonics Presented at CSIR R&I Conference Dr Andrew Forbes Senior Scientist February 2006 A. Forbes 1, L.R. Botha 1, N. du Preez 2 and T. Drake 3 1 CSIR National Laser Centre, 2 SDI (Pty) Ltd., 3 Lockheed Martin Aerospace Company

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Slide 2 © CSIR 2006 www.csir.co.za Contents Laser ultrasonics What is it, and how does it work? Optimising the parameters Choices and consequences Laser chemistry A physicists approach to chemistry Discharge disturbances Bloody hell, not more problems! Final system performance Working at last!

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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

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Slide 4 © CSIR 2006 www.csir.co.za Composite Generation Laser Detection Laser Interferometer Laser ultrasonics – basic principles ScannerOptics

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Optimising the parameters How do you design a laser for this application?

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Slide 6 © CSIR 2006 www.csir.co.za Objective Optimise the parameter set Maximise energy, Maximise repetition rate, Maximise acoustic signal efficiency. Short time pulses

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Slide 7 © CSIR 2006 www.csir.co.za Laser parameters Choices and consequences

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Slide 8 © CSIR 2006 www.csir.co.za Gas influences

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Laser chemistry Impact of gas mix on laser chemistry

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Slide 10 © CSIR 2006 www.csir.co.za 36 kV Heat Exchanger 2 CO 2 2 CO + O 2 2 CO 2 2 CO + O 2 Problem statement

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Slide 11 © CSIR 2006 www.csir.co.za Typical problem

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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!

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Slide 13 © CSIR 2006 www.csir.co.za Basic chemistry (rate laws) oxygen generation (discharge) oxygen recombination (catalysts) Can we explain? Can we predict? Can we improve?

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Slide 14 © CSIR 2006 www.csir.co.za Discharge parameters ~ 90ppm/s @ 400Hz and 22.5kV

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Slide 15 © CSIR 2006 www.csir.co.za A word on mathematics n = 0 or 1Already solved (easy) n = 2Riccatti equation (require a particular solution) Any n?Numerically only BUT, remove constant and you have an easy to solve Bernoulli equation!

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Slide 16 © CSIR 2006 www.csir.co.za With catalysts

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Slide 17 © CSIR 2006 www.csir.co.za Recombination rate

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Slide 18 © CSIR 2006 www.csir.co.za Predictions 1. Oxygen levels will stabilise

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Slide 19 © CSIR 2006 www.csir.co.za Predictions 2. Increasing the initial reactant concentrations decreases the time to equilibrium; increasing the temperature will decrease equilibrium levels.

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Slide 20 © CSIR 2006 www.csir.co.za Predictions 3. The equilibrium values can decrease with increasing power!

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Slide 21 © CSIR 2006 www.csir.co.za Outcome

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Discharge disturbances Acoustics and shocks

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Slide 23 © CSIR 2006 www.csir.co.za Acoustics

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Slide 24 © CSIR 2006 www.csir.co.za Stable discharge

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Final system In operation at Lockheed Martin for the JSF

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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 …

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Thank you and Acknowledgements Co-authors: Dr Lourens Botha (National Laser Centre) Mr Neil du Preez (SDI) Dr Tommy Drake (Lockheed Martin)

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