The role of nanostructured targets in Laser-Produced Plasmas for Nuclear Astrophysics studies

Abstract

This PhD thesis documents the experimental study of plasmas produced from the interaction of a high-power laser in ns domain with nanostructured materials compared with ordinary bulk target. The study is focused on the effect of solid targets with different physical and geometric characteristics, and has the purpose to asses the effects of nanoscale structures in laser-matter interaction and in plasma formation. The motivation for these experiments arises from the fact that there is the possibility of producing plasmas with density and temperature characteristics suitable for nuclear fusion studies, relevant in astrophysics. The optimization of the specific characteristics of nanomaterials, containing metal nanowires, could lead to a stagnant, hotter and denser plasma and to implement the above mentioned studies successfully. The nanostructured targets used in this study are metamaterials consisting in aligned metal nanowires grown by electrodeposition into a porous alumina matrix, obtained on a thick aluminium substrate. These materials were developed with different length, diameter, metal and deposition technique in order to maximize absorption in the visible and IR wavelengths. Various diagnostics were employed for the characterization of the Laser Produced Plasma (LPP). In particular, an Intensified CCD camera in visible domain has been a useful diagnostic tools to understand the expansion dynamics of laser created plumes, by providing a two-dimensional snap shots of the three-dimensional LPP propagation. Depending upon the target material, the generated plume s ion emission features (velocity, flux) as well as plasma properties (temperature, density) are varied even at constant laser intensity. The use of a CCD-camera in X-rays domain has allowed to investigate the X-ray emissivity of laser-produced plasmas. By coupling the detector with an array of pinhole, spectral selection of X-ray emission has been implemented. The Time of Flight measurements have provided a technique to determine the velocity distribution of the plasma at large distances from the target surface, complementarily to velocity estimated by visible imaging close to target surface. Moreover, morphological analysis of craters formed for the laser irradiation was performed by using an optical microscope. The cross-analysis of various diagnostics has immediately showed the differences between an ordinary Al-bulk target and nanostructured materials: bulk aluminium plasma has shorter duration, X-ray flux and ablation efficiency than all other. Finally, preliminary investigations of ion energy spectra, obtained with a Thomson Parabola Spectrometer, were carried out to better understand the nuclear fusion process in a plasma. Temperature estimated are in good agreement with the occurrence of nuclear fusions. All these experimental evidences have taken a further step towards the application of laser driven nuclear reactions.