Effects of laser radiation on immobile and fast-moving targets

We have studied the effects of laser pulses up to 50 J in energy and up to 1100 μs in duration, with a rise time of ~300 μs, on immobile and moving (linear velocity of up to 10⁴ cm s^-1) stainless steel targets. The results indicate that, in the vapour above an immobile target, optical breakdown develops at a spot-averaged power density of 5.5×10⁶ W cm^-2, against 7.3×10⁶ W cm^-2 in air. This undesirable effect can be eliminated even at an order of magnitude higher incident power density by rapidly rotating the target (linear velocity of ~50 cm s^-1 relative to the laser beam). At incident power densities in the range (0.6—1.2)×10⁷ W cm^-2 (incident powers from 12 to 24 kW), evaporation from a rotating target is intermittent. To interpret this effect, a numerical model is developed which takes into account the influence of target burning. Its predictions agree with the evaporation behaviour observed in our experiments. The thicknesses of the evaporated and molten layers are evaluated as functions of laser beam parameters and target velocity, with reference to the optimisation of laser surface processing conditions.