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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 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 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 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 Objective Optimise the parameter set Maximise energy, Maximise repetition rate, Maximise acoustic signal efficiency. Short time pulses

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Slide 7 © CSIR Laser parameters Choices and consequences

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Slide 8 © CSIR Gas influences

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Laser chemistry Impact of gas mix on laser chemistry

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Slide 10 © CSIR 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 Typical problem

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Slide 12 © CSIR 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 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 Discharge parameters ~ 400Hz and 22.5kV

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Slide 15 © CSIR 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 With catalysts

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Slide 17 © CSIR Recombination rate

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Slide 18 © CSIR Predictions 1. Oxygen levels will stabilise

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Slide 19 © CSIR 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 Predictions 3. The equilibrium values can decrease with increasing power!

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Slide 21 © CSIR Outcome

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Discharge disturbances Acoustics and shocks

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Slide 23 © CSIR Acoustics

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Slide 24 © CSIR Stable discharge

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Final system In operation at Lockheed Martin for the JSF

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Slide 26 © CSIR 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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