Investigation on the evolution and distribution of plasma in magnetic field assisted laser-induced plasma micro-machining

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Magnetically Controlled Laser-induced Plasma Micro-Machining (MC-LIPMM), compared to traditional direct laser ablation, has the ability of achieving better machining characteristics including larger aspect ratios of machined features, a smaller heat affected zone and more diverse micro-structures. To reveal the process mechanisms affected by the magnetic field, this paper studies the evolution and distribution of plasma in MC-LIPMM by analytical and experimental methods. An analysis of the confinement effect and enhanced collision of electrons-ions in the plasma in the presence of a magnetic field is performed for revealing the influence of the magnetic field on its diffusion and geometric size. To validate our theoretical predictions, experiments were conducted to analyze the effects of key process parameters on the geometrical size, shape and fluence of the plasma. Edge contour extracting and elliptic formula fitting algorithms were applied for quantitatively evaluating the changes. The experimental results show that transverse magnetic fields generate a larger plasma plume and change its shape from spherical to ellipsoidal compared to that without a magnetic field. The eccentricity of the ellipsoidal plasma exhibits a parabolic growth while the cross-sectional area of the fitted ellipse shows a linear growth as the transverse magnetic field strength increases. Greater growth in geometric size and intensity of the plasma was obtained under a longitudinal compared to a transverse magnetic field, and its shape changed into an elongated ellipsoid. Moreover, a two-dimensional magnetic field, combining both the transverse and the longitudinal magnetic field component, was shown to produce a larger area and intensity of spherical plasma under an appropriate transverse and longitudinal magnetic field strength compared to that without a magnetic field.