The surface fireball effect caused by the Coulomb particles in laser-generated plasmas: the plasma explosion regime

Abstract

Laser-material processing in the regime of plasma explosion in vacuum and in pressurized (reactive) gaseous atmospheres is a new emerging technology. The characteristics are considered of the explosion of a “colloidal” laser-generated plasma, i.e. a plasma containing Coulomb particles, and their effects on a titanium surface. Being very hot ( T = 3200 K) they produce fireball effects and leave a present pattern on the metal surface, which reveals their spatial order in the plasma during the pulse, and in the regime of adiabatic expansion after pulse termination (S Lugomer and G Bitelli, Vacuum (in press)). Among all the cases studied (Ta in N 2; Ta in O 2; Mo in N 2; Mo in O 2; Ti in N 2) only the system Ti in O 2 reveals the laser-plasma explosion. Coulomb particles generate a very specific type of pattern whose morphological and statistical characteristics are described in this paper. A posteriori analysis of a Ti surface after a plasma explosion revealed an elliptical spot with the flower-like (wavy) envelope characteristic of the Richtmyer-Meshkov instability. The inner side of the surface spot indicated a dominance of the jetting phenomena. It is shown that the jetting pattern is a multifractal pattern of the Cantor type with a spatial distribution of jetting, which can be expressed as the ratio series: 3 25 ; 3 23 ; 3 20 ; 1 5 ; 1 3 ; 1 2 ; 3 5 ;, 3 4 ; 3 3 . The particle spin and the average periodicity Δ of jetting over the whole spot, appear to be the universal properties connected with explosion of laser-generated “colloidal” plasma, not reported previously. Molten titanium in the central zone and in the jets on the periphery oxidizes quickly and transforms into an amorphous glassy phase after pulse termination.