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Study of matter behaviour dynamics governed by the interaction with laser pulses and external strong currents

Kaselouris Evangelos

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Summary

This thesis studies the physical phenomena that occur during the deposition of energy in matter and consists of two parts. In the first part, the nanosecond pulsed laser irradiation of thin metal films-substrate systems is investigated for thermoelastic, melting and ablation regimes, using the finite element method (FEM). The numerical simulations are compared and validated with experimental results, obtained by dynamic imaging interferometry and white-light tomographic interferometry. In the second part, the sequential stages of explosion of a Z-pinch copper wire from solid to plasma formation and its plasma expansion are investigated using multiphysics coupled numerical simulations, validated by experimental results obtained by interferometry, shadowgraphy, schlieren and diffraction imaging techniques. In more details, for the first part coupled thermal-structural, transient models based on FEM are developed to give a comprehensive spatiotemporal numerical solution of the physical phenomena occurring in laser matter interactions. Temperature dependent material properties of metal coatings deposited on glass substrates are used for the simulations. Initially, a 2D axisymmetric model is developed to study the generation and propagation of laser generated ultrasounds, for laser fluences below the melting threshold of metallic film-substrates. Afterwards, a 3D quarter-symmetric FEM model is developed and used for all regimes of interest. The developed FEM modeling provides a simultaneous analysis of the thermal and structural parameters, as defined by the solution of the heat conduction and wave propagation equations. The wave equation determines the displacements of the target imposed by the laser energy deposition, while the heat conduction equation predicts the temperature distribution induced by absorption of the laser pulse in the target. The 3D model computes the phase changes of matter by taking into account the latent heats of melting and vaporization, depending on the laser fluence. Ablation is simulated by the ‘killing’ of the elements that exceed the boiling temperature and are subsequently deactivated. The attenuation of the laser irradiance at the target surface due to plasma absorption is also taken into account. With regard to loading conditions, a heat source term is used, representing the laser energy absorbed by the sample and described by a Gaussian distribution in time and space. The Lagrangian mesh is locally adaptive, depending on the simulation needs. Moreover, the elastic and plastic behaviour of thin films is being investigated by taking into account stress-strains temperature-dependent curves of different materials until fracture into the 3D model. Furthermore, the finite element model was further extended to simulate the presence of surface and solid volume defects (gaps) and their subsequent influence on the generation and propagation of ultrasonic waves (surface acoustic waves). For the second part that concerns the matter behaviour dynamics, governed by the interaction with external strong currents, a 3D coupled mechanical/thermal FEM model, simulating the initial stages of explosion of a Z-pinch metallic copper wire, was initially developed. The model conveyed a simultaneous analysis of the thermal and structural parameters, as defined by the solution of the heat conduction and mechanical motion equations. The mechanical equation determines the displacements of the wire imposed by an alternative nanosecond pulsed current while the heat equation predicts the temperature distribution. The source term of the heat generation rate is the Joule heating term and is used as loading condition, while the pulsed imposed current is provided from experiments.In order to investigate the crucial physical phenomena that take place in the initial stages of wire explosion, a 3D electromagnetic-thermal-structural hydrodynamic FEM model is developed. Large volumetric deformation is considered by taking into account the material’s hydrodynamic behavior via equations of state (Gruneisen and multiphase tabular) and the elastoplastic behavior is also considered by a flow stress constitutive model (Johnson-Cook). A Lagrangian transient analysis has been carried out. The loading condition of the simulation is a nanosecond pulsed alternative current, which is provided from experiments. Τhe sequential stages of explosion of a Z-pinch copper wire from solid to plasma formation and its plasma expansion are investigated. The expansion of the exploded material is being investigated with a magnetohydrodynamic code that uses as initial condition the density distribution and radius instabilities from the above aforementioned 3D FEM model.

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