Experimental investigation of laser ablation of stone polycrystalline targets

We report the results of an experimental investigation of ablation of stone polycrystalline targets of complex multicomponent composition, which imitate the substance of asteroids. The targets were irradiated by nanosecond pulses of a neodymium laser at an energy density Φ_L of up to 5 × 10⁴ J cm^-2. The experiments demonstrated the existence of two ablation regimes, with the boundary between them lying at Φ_L ≈ 4000 J cm^-2. The regime change is characterised by a change in the form of the dependence of the surface mass density of removed target material on the laser energy density and by the appearance of a minimum in the dependence of specific energy of destruction on Φ_L. This is supposedly related to the passage from a one-dimensional plasma plume expansion to the three-dimensional one and the corresponding decrease in the efficiency of energy transfer from the laser beam to the target due to a lowering of laser-produced plasma density. Our experiments also showed the existence of a maximum in impulse coupling coefficient C_m as a function of laser energy density (C_m ≈ 6.3 × 10^-5 N W^-1 for Φ_L = Φ_opt ≈ 25 J cm^-2). Maxima were also recorded in the dependences of the ablation efficiency and average ablation flow velocity on Φ_L. For Φ_L > Φ_opt, the decrease in the function C_m(Φ_L) turns out to be much steeper than for metals and polymer materials. The difference is supposedly due to the lower strength and lower plasticity of the polycrystalline stone targets.