Transient surface heating of metals by CO2 laser pulses with air-plasma ignition

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The ignition of an air plasma by a laser pulse at the surface of a metal target may yield a large increase in the heat deposition in the target. The target surface temperature during the pulse is determined by the thermal flux from the plasma, rather than the laser-pulse parameters. The plasma prevents direct observation of the surface temperature. We show how the surface temperature can be calculated using plasma thermal-coupling models based on thermal conduction from a radially expanding plasma (due to Robin) and on thermal emission from a axially propagating plasma. The two unknown parameters of the models, the thermal-coupling coefficient and the propagation or expansion speed, can be evaluated with simple measurements of the total heat and spot-center thermal fluence depositions. The model calculations are applied to an experimental example using a 137-J 30-μs CO2 laser pulse. No discrepancy between the calculated peak surface temperatures and the observed absence of surface melting is found. The calculated values of plasma propagation and expansion speed are in fair agreement with the observed plasma development early in the pulse. The models therefore appear to be reliable for calculation of the surface thermal transient.