Laser plasma formation in metals vapour : kinetic approach

The break-down of metals vapour (Al, Cu) are simulated near the break-down threshold values of laser radiation. The approximations of collision-radiation model are used, including the detail kinetics of exited states with radiation and collision transitions, step-by-step ionisation process, unexitation and recombination. Energy balance is based on two-temperature approximation : for electrons and for heavy particles. It is shown that the particles concentration in exited states and plasma species composition differ rather from Saha-Boltzmann's distribution. Strong dependence of the break-down threshold intensity on initial temperature of neutral and charged particles is shown. 1. Basis of Physical and Mathematical Model It is assumed that the metal vapours passing through the Knudsen's layer are initially in the equilibrium state, i.e. initial concentration of atoms and ions in the ground and in the excited states are determined by the Saha-Boltzmann's equations. The main elementary processes taken into account by the collisional-radiational model are as follows : . Spontaneous radiative decay of excited states where azmn 1s-l] is the rate constant for radiation transition from n-state to m-state ho released quantum. . Excitation and un excitation of atoms and ions by electron collision where kzmn [cm3.s-l] is electron-particle reaction rate constant for z-charged particle excitation from state m to state n: kZnm unexcitation rate constant for the n th level. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994421 C4-102 JOURNAL DE PHYSIQUE IV . Ionization of the ground and excited states of atoms and ions by electron impact where az, [cm3.s-l] is electron-particle reaction rate constant for the n-th level ionization of z-charged particle. . Reaction reversible to the collision ionization : three particles recombination (the third particle is an electron) where g X n [cm6.s-l] is reaction rate constant. For example, we have listed in table 1 the various energy levels take into account in the model for aluminium species.[l] Table 1 : Energy levels of A1 neutral atom Al+, A12+ ions used in the model. On this basis, the distribution of ions and level-to-level kinetics of neutral atoms and ions is calculated by a system of nnon linear differential equations 121. 2. Results and Discussion The values of laser intensity used in the simulation correspond the near break-down threshold values (ht1.06 pm, G=1O6 + lo9 W/cm2 for Al, G=106 + lo1 O W/cm2 for Cu). The initial values of vapour temperature To and density EN=, are equal for both metals and correspond to the Knudsen's layer values during intensive evaporation : To 3.0.2 eV, ENx, =6.1018 cm3 , Two typical regimes are considered : . Laser intensity is below the threshold values of avalanche-like ionization (G<109 W/cm2 for Al, Gt2. lo8 W/cm2 for Cu); . Optical break-down development when a partially ionized gas transforms into a fully ionized plasma.