Target Normal Sheath Acceleration (TNSA) is arguably the most robust and well known laser-driven ion acceleration scheme, nevertheless it has one main issue, namely the low efficiency of laser conversion into energetic ions. In order to overcome this limit, one possibility is to use a double-layer target with a near-critical coating . Nevertheless, the electron critical density for the widely used Ti:Sapphire lasers corresponds to a mass density of approximately $6\ mg/cm^3$, which is intermediate between solid and gas densities and thus very challenging to obtain.
An extremely low density material, called "Carbon foam", has been developed at the Micro and Nano-structured Materials Laboratory (Nanolab) at Politecnico di Milano, exploiting the Pulsed Laser Deposition technique (PLD). By virtue of the foam fractal structure, the material can reach a high void fraction and a density down to few $mg/cm^3$  and the material can thus be suitably exploited as a near-critical coating. In addition, thanks to PLD features, the foam can be deposited directly onto any kind of substrates (metal/plastic, down to $100\ nm$) to form a double-layer target. It has been experimentally proven, in different ultra-intense laser facilities, that a foam-based target can be suitably exploited to increase maximum energy and number of laser accelerated ions .
Nevertheless, to fully exploit the potential of this kind of target, a deep investigation through a multidisciplinary approach is required. In particular, the ongoing research at Nanolab deals with both the experimental optimization of the target and with the theoretical analysis of the laser-target interaction, carried out with the help of Particle-In-Cell simulations.
In this contribution we present the progresses that we have achieved in these fields. In particular, we developed a technique for the production of substrate of the double-layer target, based on the High-Power Impulse Magnetron Sputtering (HiPIMS), which enables the fine tunability of the substrate thickness and composition. Moreover, we have thoroughly investigated the growth dynamics of the near-critical coating, namely the C foam, which is largely unexplored . We exploited the new insights on the physical process to achieve a better control of the foam properties, as the density and homogeneity. This has been possible also thanks to an improvement of the density measurement technique based on Energy Dispersive X-rays Spectroscopy (EDS) .
On the other hand, we used this knowledge to model the near-critical fractal material and to perform PIC simulations of the laser interaction with a realistic foam structure . In addition, we carried out a large simulation parametric scan in order to determine the optimal target parameters and to reveal the physical mechanism underlying the ion energy enhancement.
We finally present the experimental achievements in the enhanced laser-driven ion acceleration with foam-based targets. Thanks to the promising results, we believe that this kind of target could be exploited in the near future for some interesting application, such as ion beam analysis  or neutron generation.
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|Working group||Laser-driven ion acceleration|