Measurement of laser-produced plasma impulses on molten metals for thermophysical property determination

A high-power pulsed laser excitation of a material surface generates a well-separated sequence of plasma, fluid flow, and acoustic events. When the move ment of the surface due to evaporation by laser heating is kept in pace with the thermal diffusion front, the ablative mass loss from a solid surface becomes strongly correlated with the thermal diffusivity of the target matter. The other thermophysical properties which figure in this correlation are the mass density, heal of formation, and molecular weight. The functional relationship, which is given in this text for the first time, can be exploited to measure the thermophysical properties. We have now extended such an approach to measurement of the thermal diffusivity of molten specimens by developing a new instrumentation for determining the ablative mass loss due to a single laser pulse. This has been accomplished by combining a facility for controlled generation of a molten specimen and a novel transducer for real-time measurement of the impulse imparted to the molten target by a laser-produced plasma plume. The transducer design, calibration, signal recovery, and method of extracting the mass loss per laser excitation are detailed by comparing the results for metallic specimens in the solid and molten state.