Sustained plasma burning using continuous and pulsed lasers operating at different infrared and optical frequencies and various pulse repetition rates

Abstract

Free-burning plasmas can be pre-ignited using electric arcs or lasers that can be either continuous or else pulsed. The plasma so formed can be sustained by the action of continuous CO2 or CO lasers, or else pseudo-continuous Nd:YAG laser light. A sustained plasma is only possible if the power absorbed in the plasma from the laser is greater than a certain threshold pulse level. Razier et al (1969, 1970) and Steverding (1972) have developed a simplified model of plasmas with laser light and Generalov et al (1970) have carried out experiments in this area. In the work of Razier and others, no allowance was made for laser defocussing effects due to ionisation in the plasma. This defocussing effect is taken into account in the present work and a more comprehensive comparison with experiment is considered. Further improvements in the present analysis are that a full numerical solution to the heat conduction equation in the plasma is obtained using finite difference methods. At the same time, the Saha equation is solved for the degree of ionisation. The principal mechanism for absorption in the plasma is the inverse bremsstrahlung process. The results of very high pulse repetition rates will be compared with the corresponding cases of continuous laser operation. Comparisons of the results of averaging the effects in the pulse laser case will also be made with the corresponding cases of continuous laser operation. In this way the phenomenon of sustained plasma burning will be studied for the cases of both continuous and pulsed lasers under a variety of conditions. Thus insight will be gained into some of the conditions that need to be fulfilled to ensure the formation and support of a keyhole in laser welding for laser light at a variety of different wavelengths.Free-burning plasmas can be pre-ignited using electric arcs or lasers that can be either continuous or else pulsed. The plasma so formed can be sustained by the action of continuous CO2 or CO lasers, or else pseudo-continuous Nd:YAG laser light. A sustained plasma is only possible if the power absorbed in the plasma from the laser is greater than a certain threshold pulse level. Razier et al (1969, 1970) and Steverding (1972) have developed a simplified model of plasmas with laser light and Generalov et al (1970) have carried out experiments in this area. In the work of Razier and others, no allowance was made for laser defocussing effects due to ionisation in the plasma. This defocussing effect is taken into account in the present work and a more comprehensive comparison with experiment is considered. Further improvements in the present analysis are that a full numerical solution to the heat conduction equation in the plasma is obtained using finite difference methods. At the same time, the Saha equation...