Abstract

Pulsed CO₂ lasers with suitable output characteristics can efficiently process high reflectivity and/or refractory materials. Following a combined simulation and experimental research programme on both the laser and the interaction process, a transversely excited, high PRF, CO₂ laser was constructed.^1,2 Fig. 1 shows the pumping circuit configuration. To enter the highly absorbing interaction regime, an incident flux intensity of 10¹¹W m^-2 is required with a pulse duration between 6–8µs and, to avoid the deleterious melt-freeze process of conventional pulse modulated CW sources, a pulse repetition frequency (PRF) of 5–10 kHz is necessary. The specific energy density of the pumping medium to generate such optical pulses is approximately 12 J 1^-1atmosphere^-1. Consequently, the gas discharge operates in a substantially different regime to that of TEA lasers, and instabilities due to gas shock wave effects are reduced. Conventional trigger wires were utilised to generate an auxiliary preionising discharge to form stable, uniform glow discharges in a 3-litre cavity over a range of operating conditions, i.e., gas pressure and mixture composition. Analysis of the spatio-temporal development of the electron gain indicates that owing to the low applied E/N the discharge breaks down via the Townsend mechanism. A gas circulation system convectively clears the heated gas from the discharge region. Experimental results are presented for the maximum PRFs achieved with this simple technique of preionisation both with planar-planar electrodes using different laser cavity flow shaping designs (Fig. 3) and for a prototype segmented, ballasted cathode (Fig. 4). Large, gain-switched spikes in the laser output pulse generate intensities sufficiently high to form a plasma above the workpiece. Excessive plasma growth decouples the beam from the workpiece, severely degrading the efficiency of the interaction process. Consequently, the magnitude of the gain-switched spike must be controlled. This can be achieved by optimising the design parameters of the optical resonator and the composition of the laser gas mixture.³ Results are presented that clearly demonstrate the effect of the gas composition on the spike power relative to the total power in the laser output pulse (Fig. 5).

© 1994 IEEE