| Astronomy is primarily a science based on observations through telescopes or satellites having different wavelength ranges (from radio to hard X-rays). By analysing the observed spectra or images, which are generated by various kinds of physics pro-cesses, appropriate models have to be proposed to explain astrophysical phenomena, origins and evolutions of astrophysical objects. On one side, laboratory astrophysics can contribute to the measurement of various kind of parameters that are needed to model, for example, stars formation and evolution, to interpret data, such as accurate wavelengths, oscillator strengths, specific collision cross sections and so on. On the other side, theoretical models can be directly benchmarked with experimental results in laboratory. During the past two decades, as the development of modern high power laser facilities and magnetic pinch equipments, high energy density physics (HEDP) developed very quickly. Combined with HEDP, laboratory astrophysics can access the properties of matter under extreme states, such as metal opacity under high temperature and high density in stellar interiors, equation of states and so on, which will expand our understandings of formations, structures and interior dynamics of stars.Opacity is an essential issues to the star evolutions. In laboratory, Nova laser and Saturn Z-pinch machines were used to measure iron opacities under different temper-ature and density conditions. By comparison with experimental results, opacity code OPAL has been greatly improved. Using the new iron theoretical opacity, the long standing problems of period-luminosity relations of Cepheid variables were solved. However, there are still lots of problems are unresolved which are related to the opac-ity, such as the position of solar convection boundaries(namely CZ problems), the energy radiation from super novae explosion and so on. Silicon.is one of the main constituent for opacity sources of cool star atmospheres, and also of solar convection and core. Thus, a good knowledge of Si opacity, in conditions of objects mentioned above, is fundamental. In this work, Si K-shell opacity has been measured for the first time in laboratory. In our experiments, eight main laser beams with a total energy of 2400 J were injected into a gold cylindrical Hohlraum. The Planckian radiation generated by Hohlraum were used to radiatively heat the 250μm thick SiO2 foil to high ionized plasma. The foil was located at the center of the Hohlraum. The backlighter was generated by ir-radiating a solid gold target with another 150 ps laser beam. By alternating the delay between the backlighter and the main laser beams, we have measured time-resolved spectra within wavelength range from 6.6 to 7.1 A. For this wavelength range, we have calculated atomic data of Si ranging from He-like to F-like ions, including energy levels, wavelengths and radiative decay rates. Furthermore, the natural broadening, Doppler broadening and autoionization resonance broadening have been determined. Besides, we have assessed the accuracy of these atomic data. Under local thermo-dynamical equilibrium, theoretical spectra have been calculated using a detailed level accounting model. The effect of oxygen have been taken into account, and we find it can't be neglected in our experiments. The contribution to the opacity from highly excited states of Si ions are included, and we neglected the states with principal quan-tum number n> 8. The plasma temperatures that we determined ranged from 65 to 30 eV as time increasing, while the densities decreased from 80 to 18μg/cm2 by comparison between theoretical and experimental results. When delay time gets longer, two-component model have to be used to fit the absorption spectra, the reason is that parts of plasmas expanding out of gold Hohlraum. Good agreements have been achieved between the measured and simulated spectra.The second part of my thesis is related to Warm Dense Matter physics (WDM) mainly related to equation of state which is a relation between relations between den-sity, temperature, pressure and internal energy. My work was related to iron properties, which is the main component of Earth core. Precise knowledge of iron or iron alloys EOS in the inner-outer core boundary (330GPa, about 5000K) are poor, in particu-lar there are still controversies about iron fusion temperature at this pressure. This greatly limits current modeling of earth constitution and dynamics. Directly laser driven method were used to compress iron along an isentropic path. The laser pro- file was shaped to provide an increasing laser intensity, and the iron target was directly ablated by the laser beam to achieve isentropic compression (IC). Rear surface veloc-ities of the iron targets were recorded by VISAR (Velocity Interferometer System for Any Reflector) diagnostic. The maximum pressure achieved in our experiment was 2.5 Mbar. The iron elastic-plastic precursor andαto (?) phase transformation were clearly observed. Experimental results were compared with simulations by an hydrodynamic code (SHYLAC)taking into accounted phase transformation and kinetic effects. Good agreements were achieved both for the phase transitions and spallation effects. This new technique is a new way to study iron in the inner earth core conditions by using IC experiments.In EOS measurements, error bars are one of the main issues. In order reduce those, new parameters beside velocities need to be measured. To this end, mass den-sity is constitutes one of the best candidate. To get a direct density measurement, we performed a detailed study on hard X-ray (18-60 KeV) sources generated by short-pulse laser. Experiments were carried out on different targets (W, Mo, Dy) using var-ious laser parameters. High quality radiographs with good resolutions were obtained with W wire target. We have shown that frequency doubling the laser did not affect the conversion efficiency for comparable intensities while the signal to noise level was significantly reduced in the case of Mo Ka radiation. Finally experimental geometry showed significant influences on noise level and conversion efficiency in the case of Mo Ka radiation.
|
PDF Full Text Request