SEARCHING FOR CHAOS IN FLUCTUATIONS OF A PLASMA-INDUCED DURING CW-CO2LASER-WELDING

Fluctuations of cold (T-e approximate to 0.8 eV) and dense (n(e) appro ximate to 5 x 10(16) cm(-3)) plasmas which burn above metallic surface s during welding with a cw CO2 laser are registered in monochromatic r adiation. The aim of the present work is to check whether low-dimensio nal deterministic chaos can be an explanation for the irregular, rando m-like oscillations of the surface plasma induced during laser welding . The standard procedures of nonlinear time series analysis like the e mbedding technique, noise reduction by projection on the local manifol d, calculations of the correlation dimension using the G-P algorithm, and looking for the largest Lypunov exponent are applied. After cleani ng original time series from the noise, 3D phase portraits displaying more or less regular structures of attractors were reconstructed. The values of the estimated fractal dimensions of these attractors lay in the range ]2, 3[, far various cleaning conditions. The plots of the av erage orbits' 'divergence' calculated for the estimation of the larges t Lyapunov exponent display behaviour which is typically observed in c haotic systems. The value of the largest Lyapunov exponent obtained fr om the slope of the orbits' divergence curves is for our welding condi tions, about 5 x 10(-3) s(-1) and the average 'period' of the plasma o scillation is estimated to be 10(-3) s. This means that the system is highly unpredictable, These results indicate that fluctuations of lase r induced plasmas represent deterministic chaos and a nonlinear dynami cal system consisting of several ordinary differential equations can b e used for modelling laser welding.