Optimisation of the Gaseous Discharge and Optical Coupling of a Pulsed CO2 Laser

Watson, Ian Alistair (1993) Optimisation of the Gaseous Discharge and Optical Coupling of a Pulsed CO2 Laser. PhD thesis, University of Glasgow.

Full text available as:
Download (9MB) | Preview


Pulsed, high frequency lasers offer enhanced material processing capabilities when compared to continuous wave sources. To achieve high quality machining at the workpiece, a CO2 laser is developed which can operate at 10 kHz with laser pulse lengths of approximately 6 ?s. The device is specifically designed to deliver the optimum laser pulse for the material workpiece interaction process, and in particular, for highly reflective and/or refractory materials. The available laser power from CO2, N2 and He gas discharges is critically dependent on the gas pressure, gas composition, pumping power and the extraction efficiency of the resonator. The stability of the discharge during the pumping pulse is of paramount importance to ensure that laser output is maintained. Moreover, arcing can damage circuit components and seriously degrade the electrodes through spark erosion and sputtering processes. These factors are considered in the development of the laser. To create laser pulses for enhanced material processing a novel, large volume, low energy density, transverse gas discharge system is developed which uses the conventional double discharge preionisation technique. The auxiliary discharge is capacitively coupled to the discharge anode - brass planar electrodes are used. Two Roots blowers operate in parallel and yield maximum gas velocities of 100 ms-1 within the electrode region. A multi-pass, stable, half-symmetric resonator is used to extract the laser power. The criteria for obtaining stable, uniform, glow discharges over a range of gas mixtures and pressures of up to 200 torr are critically assessed. The affects of preionisation and the applied discharge overvoltage on the discharge stability are examined for the operating regime of the laser. The discharge stability and the laser output is quantified for a range of pumping circuit configurations. A high voltage, low inductance, preionising circuit is fabricated and tested as an alternative method of preionisation. The uniformity of the gas velocity between the electrodes is examined in two dimensions by using a computer controlled, Pitot-static tube and stepper motor assembly. The maximum discharge power ratings, prior to the onset of arcing, are determined for different flow-shaping geometries. Clearing ratios of less than unity are observed for low discharge energy densities. However, for higher values, the clearing ratio is above two. It is concluded that the non-uniformity of the gas flow is a major factor limiting the performance, of the planar electrode discharge geometry, to frequencies of about 1.5 kHz. Two ballasted electrodes with total resistances of 0.8 Q and 8.0 were fabricated and tested. They offered substantial improvement in the maximum discharge power loading and operating frequencies prior to the development of discharge instabilities. Typically, 11 kW mean power was deposited into the discharge, with frequencies above 5 kHz. The extraction efficiency for different resonator configurations is investigated for all electrode geometries for a high inductance pumping circuit. A 5 pass, half-symmetric, stable resonator yielded the maximum laser power from the ballasted electrode system with pulse energies of 0.13 J, and plateau powers in excess of 17 kW. This delivered the laser pulse characteristics that are suitable for achieving the non-conduction limited material interaction process. It is shown that the laser output power is substantially increased by reducing the circuit inductance and increasing the gas pressure so that the network and discharge impedances are nearly matched. Laser pulse energies of over 0.1 J are observed for a single pass resonator. The non-conduction limited interaction process has been demonstrated by cutting and drilling holes in aluminium. In conclusion, future work is proposed in order to fully extend the operating regime of the laser so that it is capable of a performance that would be attractive to industry.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: C Charwin
Keywords: Mechanical engineering, Materials science, Electrical engineering
Date of Award: 1993
Depositing User: Enlighten Team
Unique ID: glathesis:1993-75710
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 18:32
Last Modified: 19 Nov 2019 18:32
URI: http://theses.gla.ac.uk/id/eprint/75710

Actions (login required)

View Item View Item


Downloads per month over past year