• absorption
• emission
• foam
• laser
• plasma
• X-ray

The incessant need of energy has stimulated the development of new technologies for its production. Nowadays the next step to take seems to be toward nuclear fusion.
In this context plasma physics plays a central role. Indeed fusion reactions can occur only if the relative energy of the nuclei involved is high enough to overcome Coulomb repulsion and can be shown that, in order to favor fusion reactions, it is convenient to work in a ionized gas with high density and temperature.
These requirements led thermonuclear fusion research to develop two different approaches: Inertial Confinement Fusion (ICF) and Magnetic Confinement Fusion. The main difference between these two techniques arises from the strategy adopted to reach ignition condition, i.e. to obtain self-sustained fusion reactions. Both methods work at high temperatures but, while magnetic confinement, to facilitate fusion, tries to increase the time that ions spend near each other by using intense pulsed magnetic fields, ICF relies on the inertia of the fuel mass to provide confinement in order to fuse nuclei before target explosion occurs. In fact at the high plasma density typical of ICF, the magnetic fields required to obtain confinement would be so high that are impossible to achieve.
In inertial Confinement fusion the necessary heating and fuel compression can be provided by high power laser beams. Two different schemes of irradiation have been proposed: direct and indirect drive. In indirect drive, a pellet containing the fuel, typically a mixture of deuterium and tritium, is located inside an high Z case, an hohlraum. Laser energy is first absorbed by this enclosure which, once heated, emits X-rays which fill the hohlraum and drive the capsule implosion. On the other hand, in direct drive approach laser beams deliver their energy directly onto the outer layer of the pellet.
The irradiation of the fuel target represents the crucial point of the process, in fact during laser-matter interaction, due to irradiation inhomogeneities, both parametric and hydrodynamic instabilities could be generated, leading to target pre-heating and compression inefficiency. Avoiding this phenomenon is one of the challenges we have to face to move a further step toward the access of a new source of energy. One of the key points to make this problem less severe lies into target design.

In this respect, since the late 90's it was considered of interest to coat fusion pellet with light foams which would act as absorbers of the laser radiation. Made from plastic polymer, these foams present a porous structure with cavities separated by solid filaments or membranes, with dimensions of the cavities much greater than those of the solid elements. This inhomogeneous structure induces peculiar absorption mechanisms when laser light impinges on its surface. In particular radiation reaches regions of the material deeper than in the corresponding solid and an homogenization process takes place in which the solid filaments evaporate and create a plasma which fills the surrounding cavities. In this conditions absorption doesn't involve only the superficial layer of the material but actually concerns a considerable part of the volume. This distinctive property could be useful in reducing the unwanted effects of irradiation inhomogeneities and enhancing absorption in ICF scheme. Indeed it has been shown that covering the outer shell of the pellet with a layer of a porous plastic material, laser imprinting could be sensibly reduced with subsequent inhibition of undesired hydrodynamic instabilities. Moreover this materials seems to enhance the efficiency of conversion of laser energy into shock wave energy.

The aim of this thesis is to study the behavior of such porous material interacting with high energy laser beams. In particular we are going to compare the response of different materials to irradiation, focusing our attention on the differences between an homogeneous solid material and the analogous inhomogeneous porous one.

The present work of thesis has been carried out in the frame of a collaboration between the Physics Department of Pisa University and the ENEA research center in Frascati (Rome). The experimental campaign that I attended has been performed at ABC laser facility led by Dr. Riccardo De Angelis. The ABC laser is able to deliver two beams, each one up to $100$ J energy and, thanks to the different diagnostics at our disposal, from a single shot we were able to collect a set of data pertinent with a wide range of various aspects of laser-matter interaction. This allows us to have a quite complete view of the subject investigated.
Nomarski interferometer allows us to perform the plasma electron density profile reconstruction; spectral analysis of the X-Ray radiation is performed by means of a transmission diffraction grating; visible streak camera provides information about temporal evolution, hence on expansion velocity, which can be related to plasma temperature. X-ray emission was also monitored with a system of semiconductor detectors, which, with an adequate filtering, provides information on different spectral regions. In order to persecute our aim, in addition to the diagnostic system set up and data collection, a consistent part of the work was that of preparing different targets of porous polystyrene, so as to have a wide collection of different irradiation situations.