Preionizing a large volume transversely excited sub-atmospheric-pulsed CO 2 laser for enhanced materials processing

Abstract

Two methods of exciting a trigger wire double discharge electrode system are investigated for the generation of high repetition frequency pulsed discharges in a sub-atmospheric, transversely excited carbon-dioxide laser. Specifically, a 10 microsecond pulse forming network excited, 2 liter, transverse discharge was initiated by a corona discharge generated from trigger wires placed above a planar cathode. The discharge was pumped at pulse repetition frequencies up to 10 kHz and input pulse energies of 10 J. Two electrical systems for exciting the auxiliary preionizing discharge are considered viz. direct capacitive coupling from the anode voltage and separate high voltage pulse excitation. The laser itself was designed to improve materials processing capabilities of carbon-dioxide lasers. This followed from a detailed theoretical analysis of the laser-material interaction process and it was concluded that pulse repetition frequencies of about 5 - 10 kHz were needed with optical pulse lengths of 6 - 8 microseconds and plateau powers of between 20 - 150 kW. A stable single pass resonator was installed and used to assess the performance of both discharge initiation circuits by measuring the laser output pulse energy with a photon drag monitor. For the separately excited preionizer, the delay time between the preionizing pulse and the main discharge pulse was optimized by maximizing the laser output pulse energy as a function of the delay time. The discharge was modelled using SPICE to investigate the effects of the preionizing electron density on the discharge voltage and current.