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In recent years, the continuous emergence of numerous facilities capable of generating highly intense, short-duration (~fs) laser pulses with an high grade of precision and reproducibility has driven a significant increase in interest in laser-plasma applications. One of the most promising and extensively studied possibilities is the acceleration of protons and ions to energies of several MeV through the interaction of an high-intensity laser pulse (I > 10$^{18}$ W/cm$^2$) with a thin micrometric solid foil. This accelaration mecchanism, known as Target Normal Sheath Acceleration (TNSA), represents a promising and compact alternative to conventional methods based on large electromagnetic accelerators, with potential applications in material science [1], cultural heritage [2] and medical field [3].
Various strategies can be adopted to achieve an effective acceleration mechanism: critical-dense gas targets [4], liquid targets [5], solid targets [6], or more advanced solutions such as cryogenic targets [7]. The common goal among these approaches is to ensure that the accelerated ion beam exhibits good reproducibility in terms of maximum energy and number of accelerated ions, and that this can be achieved at a high-repetition rate. Furthermore, to make this technology practically viable, these aspects need to be improved. To this end, various strategies to increase the cut-off energy and the number of the accelerated particles have been developed. One particularly promising solution is represented by the so-called Double Layer Targets (DLTs). These targets consist of a solid substrate, typically a micrometric-thin metallic or polymer foil, onto which a nanostructured layer is deposited to be interposed at the laser–plasma interaction interface. The purpose of this additional layer is to tailor the density profile of the target enhancing laser absorption and promoting more efficient generation of hot electrons. The optimal condition is achieved when the density of the nanostructured layer approaches the critical density for the given laser wavelength. Under these conditions, the laser energy can be more effectively transferred to the plasma, resulting in higher sheath fields and enhanced ion acceleration [9]. With this approach, several types of targets have been developed, exploiting different kinds of coatings such as nanosphere particles [10], nanowires [11], or more disorganized and porous structures like nanofoams [12]. In particular, DLTs based on carbon nanofoams are currently subject of extensive studies at the Micro and Nanostructured Materials Lab (NanoLab) of Politecnico di Milano, as promising advanced target solution to ehnance the acceleration process.
Despite the several advantages introduced by these types of targets, the use of DLTs also introduces significant experimental challenges. The nanostructurd layer in DLTs is extremely delicate, and any damage occurring before the interaction with the laser pulse can lead to a significant loss in acceleration performances. This premature damage can occur not only at the focal spot due to the interaction with the laser pre-pulse and pedestal [13, 14], but also, in the surrounding region of the interaction point due to the propagation of shock and heat waves, the interaction with any unfocused light or the impact of debris produced during the laser–target interaction [15]. As a result, the morphological and density characteristics of the layer may change, thereby affecting both the integrity of the target and the efficiency of the TNSA mechanism. This effect becomes critical in high-repetition-rate facilities. Here, the decrease in efficiency of the laser-plasma coupling may occur during subsequent shots because of foam removal due to previous ones, affecting the stability of the accelerated beam. In this context, the present work investigates how the density, geometric thickness, and mass thickness of carbon nanofoams used in DLTs influence both their damage resistance and TNSA performance. The analysis is based on data collected in two distinct experimental campaigns.
In the first campaign, carried out at ELI Beamlines (Czech Republic) as part of a study on Proton Induced X-ray Emission (PIXE) diagnostics. Twelve DLTs were fabricated with three different areal densities and six specific combinations of foam structural parameters – i.e. thickness and mass density - allowing for a systematic comparison of how each parameter affects the interaction. The main goal of this analysis was to identify correlations between the nanofoam characteristics and the resulting proton spectra, as well as to evaluate the extent of physical damage observed on the targets after laser irradiation.
In the second campaign, performed at NanoLab of Politecnico di Milano, four types of nanofoam samples with different combinations of structural parameters were irradiated under controlled, low-intensity conditions in order to investigate the behavior and structural response of carbon nanofoams when directly irradiated by a laser pulse.
The results of these two campaigns have highlighted the importance of both the nanofoam and laser parameters for the design of robust DLTs in laser-plasma experiments. This way, the study contributes to the understanding of how nanofoams behave under extreme laser conditions and provides important guidelines for their integration in high-repetition-rate applications.
References
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