Speaker
Description
The research on Inertial Confinement Fusion (ICF) is always requiring the development of new types of materials. The structure of the fusion capsule has to be precisely tailored to ensure an optimal performance for the implosion and ignition, and also a reliable reproducibility of the plasma behavior from shot to shot, in the view of a future fusion reactor. Among the various processes affecting the performance of the capsule, hydrodynamic instabilities, such as the Rayleigh-Taylor instability, and parametric instabilities, such as the stimulated Brillouin scattering, the stimulated Raman scattering and the two-plasmon decay, play a detrimental role. Hydrodynamic instabilities degrade the compression efficiency, while the parametric instabilities reduce the absorption of the laser energy in the fusion target, scattering the laser light and producing suprathermal particles which cause a typically unwanted preheat of the fuel.
Micro-structured low-density materials, such as porous materials, or foams, with a randomly arranged internal structure have been shown in the last decades to be able to reduce to some extent the aforementioned issues. They can increase the efficiency of the absorption of the laser in the plasma [1], reduce hydrodynamic and parametric instabilities [2] and also increase the pressure at the shock front [3]. All these advantages are due to the peculiar features of their internal structure, constituted by solid parts, in the shape of filaments and membranes, and by empty spaces. The size of the empty spaces can be either of the order of 1 μm or of tens of μm and the average density can range from a few mg/cm3 to hundreds of mg/cm3. Therefore, foams with an average density lower or higher than the critical density for the given laser wavelength can be selected.
In the decades, a notable control on the average material parameters during manufacturing has been achieved, but the techniques traditionally used do not give the ability to finely tune the internal structure on the basis of the experimental needs. Each foam is a unique piece and the parameters, such as the density and the size of the empty spaces are usually the same over the whole sample. Also, stacking various layers of foams with different parameters can be hardly done with the traditional techniques. However, accessing these capabilities may improve the performances of the material during the interaction with a high-power laser and permit the design of even complicated geometries for a new class of targets.
The techniques for manufacturing micro- and nano-structured materials with the use of ultrashort pulsed lasers which are currently being developed allow to realize complex structures and internal architectures with a high precision, which can be as lower as a few tens of nanometers.
Ultrafast laser direct write 3D micro-/nano-lithography (also known as two-photon polymerization or multi-photon lithography) is becoming an established tool for precission 3D printing. Using tight focusing and exploiting spatially confined light-matter interaction allow fabrication of arbitrary objects with accurately defined features out of diverse cross-linkable organic and hybrid organic- inorganic materials [4]. The state-of-the-art technique empowers additive manufacturing of structures having ~100 nm individual resolved features with overall object dimensions up to mm in scales [5].
Micro-structured materials fabricated in this way will have several advantages. Every feature of the internal structure can be engineered with precision and freedom, by realizing a computer model which will then be faithfully reproduced by the printing process. This allows to tune the sample parameters, such as the density, the thickness or the separation between the printed filaments, with precision and freedom, also ensuring a high reproducibility of the targets from shot to shot.
In this work we will present the preliminary results of a campaign conducted at the ABC laser facility in the ENEA Research Center in Frascati, in which we irradiated micro-structured materials realized at the Vilnius Laser Research Center by additive manufacturing. We will discuss the fabrication strategy of the targets, which involved two different printing techniques. The foam sample was obtained by direct laser writing and it was kept in place on the target holder in the experimental chamber by a support structure 3D printed with a UV table-top lithographic commercial printer (Asiga Pico 2 UV). We will then describe the behavior of the plasma under irradiation at about 1014 W/cm2 at the fundamental wavelength of the ABC laser. The data collected show a high reproducibility from shot to shot and peculiar features compared to foams produced with traditional methods.