Surface heating in metals irradiated by fast I.R. laser pulses
For this laser output to be optimized to the requirements of the workpiece the behaviour of the irradiated material must be known with reasonable accuracy. This requires modelling of the interaction processes for the materials concerned. The validity of the Fourier conduction theory within the context of high powered laser irradiation has been raised by several workers who have proved it to be invalid both in terms of its resolution of sharp energy gradients and inability to cope with non-equilibrium energy transport between electrons and lattice phonons. An alternative theory of energy transport based on the Electron Kinetic theory is therefore presented and the results compared with those obtained using the Fourier conduction theory. It is found that the results obtained using the Electron theory are in better agreement with available experimental results. The new model is then extended to include evaporation effects. Previous computer simulation of high mean power, high pulse repetition frequency (p.r.f.) lasers have predicted the characteristics of the first output pulse only. This pulse, however, is not representative of the subsequent pulses as the simulation is initiated using conditions based on thermodynamic equilibrium. Using a modified kinetic model which incorporates plasma temperature variation, optical cavity characteristics and transverse gas flow, the simulation was extended to include the second output power pulse. A substantial difference was found between the first and second pulse profiles. This extended model is essential as it identifies further the control variables which can be used to optimize beam characteristics for material processing applications. It also gives closer agreement with experimental measurements made under continuously running conditions.