Development and Application of Laser Systems for Ablation
Abstract
A possible system for the remote sampling of contaminated solids by laser ablation utilises capillary transport of the ablation products in a high speed laminar gas stream. The ablation products are removed from the gas stream by an in-line membrane filter and subsequently analysed by ICP-MS or other analytical technique. Ablation and collection have been characterised using several laser systems: Q-switched, fixed-Q, mode-locked and frequency doubled Nd:YAG, Q-switched and fixed-Q ruby, long pulse XeCl excimer and chirped pulse regeneratively amplified Ti:sapphire. The ablation of 18/10 Cr/Ni stainless steel samples is analysed qualitatively over a wide range of laser pulse lengths, 150 mus, 200 ns, 10 ns, 35 ps and 130 fs, and also for different surface finishes, the effect of which was found to influence the long pulse ablation characteristics. Comparison of the ablation with a simple theory of thermal interaction with decoupled electron and lattice temperatures closely describes many of the ablation features. The ablation efficiencies at high fluences have been found to be lowest for pulses in the ns range, and the highest collection rates were recorded for similar energy 100mus pulses. Very short pulse ablation was found to be efficient but the low available energy per pulse limited the measured collection rate. The effects of high vacuum and helium atmospheres were found to have little effect on the ablation outcome. Optical fibre delivery of high energy laser pulses is also explored, as are methods of Q-switched laser pulse stretching to enable higher delivery energies. This involved modification of the Q-switched ruby laser system by the use of an intra-cavity nonlinear scattering device, and also by modification of the Q-switch electronics. By the former method, smooth temporal profiles of 500 ns FWHM have been obtained. Ablation of stainless steel has been demonstrated with fibre delivered beams, both stretched and unstretched. Ablation of BNFL supplied concrete has been studied for pulse lengths of 150 mus and 20 ns, the latter often displaying a smaller degree of thermal melting. ICP-MS analysis of 18/10 stainless steel ablated with high energy (1-3 J) 150 mus pulses is also presented to assess the level of elementary fractionation with long laser pulses. Within the large experimental error, no significant fractionation was observed at the high intensities used. Particle size analysis results of stainless steel ablation products are presented for mus, ns and ps pulses in the visible and infrared, and indicate a range of diameters from 20-1000 nm. Laser drilling of 130 mum thicknesses of ferry metal alloy (45% Ni, 55% Cu) was performed with 100 mus Nd:YAG pulses at 1064 nm to assess the size and consistency of holes produced in this way. Parallel holes of diameter 100mum with a tolerance of < 15 mum were possible with the correct choice of laser parameters and focusing optics.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1996
- Bibcode:
- 1996PhDT........71W
- Keywords:
-
- Applied physics;Optics