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Exploring the New Science and engineering unveiled by Ultraintense ultrashort Radiation interaction with mattEr

Periodic Reporting for period 4 - ENSURE (Exploring the New Science and engineering unveiled by Ultraintense ultrashort Radiation interaction with mattEr)

Reporting period: 2020-03-01 to 2020-08-31

The aim of the ENSURE project was the theoretical, numerical and experimental investigation of novel ion acceleration mechanisms in the interaction of ultrashort (10^-12 to 10^-14 s), superintense (10^19-10^23 W/cm2) laser pulses with solid targets whose properties (composition, thickness, density profile) are controlled down to the nanoscale.

The topic of ion acceleration by intense laser pulses has attracted an impressive amount of experimental and theoretical work and can be considered as one of the most active and innovative areas of laser and plasma physics. The appealing features of laser-driven ion beams (e.g. the extremely short acceleration length, high collimation, very low emittance, picosecond duration of the ion bunch), not found for conventional ion sources, are promising for several scientific, technological and societal applications which are based on the unique properties of localized energy deposition by ion beams in dense matter.

The ENSURE project had a strong focus towards the use of these beams for novel key applications in materials (material irradiation and advanced characterization) & nuclear (ion-driven secondary particle/exotic nuclei sources) science and engineering. These goals have been pursued integrating in an unprecedented way advanced expertise and methods from materials science and engineering, laser-plasma physics and computational science into a single team.
"The research activities carried out within ENSURE are organized in four work packages (WP), as detailed below:

WP1: The main scope of the task was the development of materials with desired and often unconventional properties to be used as Double-Layer Targets (DLTs) in laser-driven acceleration experiments, with the ultimate goal of enhancing the acceleration process.
DLT are made of two components: a ultra-low density material (density is so low that the material weights few time the air) called “nanofoam” on top of a thin (less than one tenth of a hair width) foil called substrate.
As a first step, we worked on the production of nanofoams exploiting the equipment already present at the HI via the nanosecond Pulsed Laser Deposition (ns-PLD) technique. In particular we studied -both theoretically and experimentally- how the foams actually grow when using the ns-PLD production technique, achieving an accurate control on important properties such as density and uniformity.
Then, we expanded our capability of producing DLT by setting up two new laboratories, one mainly focused on the production of substrates using the High Power Impulse Magnetron Sputtering (HiPIMS) technique, the other on producing nanofoam with femtosecond PLD, a technique that is complementary to ns-PLD. Combining any of the two PLD technique with the HiPIMS, we are now able to produce by ourselves a DLT with specific features that cannot be found in commercially available targets.
IN parallel, we devolepd advanced techniques to characterize the properties of the DLT materials.

WP2: Investigation of applications of these laser-driven ion beams for materials/nuclear science and engineering.
Ion beams are widely used in many applications, ranging from nuclear medicine to material science. Today, ion beams are provided via conventional accelerators that are tyically bulky and expensive. Laser-driven ion sources are attractive because have potential for cost reduction and portability. Among the potential applications, we considered the Proton Induced X-Ray Emission (PIXE), a non-destructive technique used to retrieve the properties of a sample (e.g. elemental composition of an artwork). We designed a system suitable for a “proof-of-principle” laser-driven PIXE experiment carried out at the CLPU laser facility in Spain. We demonstrated the feasibility of laser-driven PIXE to retrieve quantitative information on the sample.
In addition, we studied the possibility of using laser-accelerated ions to generate neutrons. This part of the research lead to the ERC-PoC INTER project, in the context of which we have worked to find solutions for a compact laser-driven neutron source based on moderate power laser systems coupled with DLTs.



WP3: Design and realization of experimental campaigns aimed to the study of the acceleration regimes identified in the project, to be conducted on world class facilities.

We also worked on the design, realization and interpretation of laser-driven ion acceleration experiments in world-class laser facilities. Several experimental campaigns, with different purposes and goals, have been carried out in collaboration with GIST (South Korea) in 2016, HZDR (Germany) in 2017 and CLPU (Spain) in 2018 and 2019. All these activities have confirmed the potential of the approach based on double-layer targets to enhance the ion acceleration process. In particular, DLTs have proven to be capable of increasing the maximum energy and the total number of the accelerated ions by a factor up to 3 and 4, respectively.

WP4: Systematic study of superintense laser-ion acceleration regimes and laser-ion interaction with matter.

It is difficult to achieve a satisfactory theoretical understanding of a complex physical phenomenon as the interaction of superintense laser pulses with DLTs. We addressed this issue with two complementary approach. On the one side we used numerical simulations (typically run on supercomputers) to study the interaction between laser pulses and nanostructured low-density plasmas, such as those that arise when DLT are used. At the beginning simulations have been carried under some simplifying assumptions (e.g. without considering the complicated morphology of nanofoams); then, more realistic ones have been carried out, supporting the interpretation of experiments where DLTs were adopted. On the other side we worked on the analytical modeling of different aspects of the laser acceleration process, such as the generation of high-energy electrons and their role in accelerating the ions.
Combining numerical simulations with an analytical description we formulated a ""recipe"" to find the DLT parameters - i.e. the nanofoam thickness and average density - that maximize the ion energy. Comparisons with experimental results show very nice agreement with our model."
In the course of its five years, the ENSURE project has focused on studying non-conventional mechanisms for the acceleration of ions driven by high-power laser systems, and their applications. Thanks to ENSURE we were able to push further the research frontiers in the field of laser-driven ion acceleration in different directions. Overall, we successfully demonstrated that a multidisciplinary approach combining and integrating theoretical and experimental skills is the key towards the full development of laser-driven ion sources and their applications. We deem the demonstration of laser-driven materials characterization via PIXE to be a major breakthrough, it being among the very few practical applications of laser-driven sources with a potential widespread diffusion.
Compact, cost effective laser-driven accelerators could have a significant societal impact also in the fields of nuclear engineering and radioisotope production for diagnostic and therapeutic nuclear medicine.
Together with its aims and adopted methodology, these results and impact enlighten the ground-breaking, non-conventional, interdisciplinary, high-risk/high-gain nature of the ENSURE project.
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