Metal Processing at Culham

Abstract

The Laser Applications Group at Culham utilizes a wide range of commercially-available and in-house CO₂ laser systems, covering the power range 5–15,000 Watts (continuous). The sub-kilowatt systems utilize stable optical resonators, providing Gaussian-mode outputs, and some of these have high mean-power pulsing facilities at repetition rates up to 1 kHz. The highest power in routine use is the 5–6kW unstable-cavity (M=2 ~3) output from the transverse-flow CL5 laser, discussed in the first lecture. Detailed numerical simulations of the behaviour of this laser have been undertaken: Figure 1 illustrates the good agreement obtained between the computed small-signal gain α_o (full curve) and that measured experimentally as a function of gas flow velocity and the distance z downstream of the electrode (Armandillo and Kaye, 1980). To obtain this agreement CO₂ dissociation of ~50% has been assumed. Lower dissociation (and higher gain) are observed immediately after switching on; here the Culham code agrees with the numerical predictions and experimental observations reported by Yoder et al (1978). Representative near and far-field burns from this laser are shown in Figure 2. (More quantitative measurements are made using high-speed IR cameras on irradiated targets. With f/4 spherical mirror optics not less than 80% of the power is focussed through an aperture of diameter 0.3mm onto a calorimetric detector such as a ‘Joule Stick’).