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In the high power laser treatment of metal surfaces (Ti, Ta, Mo) in of O2 and N2, at the pressure P = 2–6 atm, with the CO2 laser beam of energy density Q ∼ 20–50 J/cm2, of the power density ∼ 109 W/cm2, and of the pulse duration τ ∼ 150 ns, some new effects have been observed. The laser-material interaction occurred in the highly absorptive plasma regime, meaning that the metal surface was effectively screened from the beam. Because of the neutral particles immersed, the plasma was a type of “colloidal” or “dusty” plasma. The neutral particles (probably the metal clusters or clusters of metal oxides) “dressed” with (+) ions, of the size ∼ 10 μm to ∼ 30 μm, of the volume density ∼ 20 × 106 cm−3, move radially with expanding plasma and strike the metal surface. A high particle's temperature (T ∼ 3200 K) causes them to behave as micro-scale fireballs leaving the hot spots on the surface. Metal surface reveals two-dimensional orthogonal projection of the Coulomb system: (i) with random structure which represents either amorphous (glassy) solid, or liquid, (ii) with the short range-order of hcp hexagonal lattice which represents a Coulomb liquid, (iii) with the middle-range order showing the coexistence of the hcp hexagonal and bcc cubic lattices, which represents a polycrystal (near the solid-liquid transition), (iv) with the long-range order with the bcc cubic lattice, which represents the Coulomb solid and finally, (v) with the bcc → fcc deformation induced transition, caused by the shear waves. A local geometrical order was found to appear for the critical surface charge on the particles Qp ∼ 2,1 × 704 e (of elementary charge). Debye shielding radius λD was estimated to be ∼ 70 μm (in adiabatic plasma expression), and ∼ 52 μm (in isothermal expression during the laser pulse). |
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