Daniele Margarone, Ph.D.
The Department of Ion Acceleration and Applications of High Energy Particles focus is a generation of laser-plasma based ion sources and demonstration of proof-of-principle experiments aimed at envisioning future societal applications in multidisciplinary areas with special attention paid to biomedical ones (ELIMED ). Thus, the optimization of particle beam quality and reproducibility (spatial profile, pointing, divergence and energy stability) is a priority of the Department scientific agenda, trying to fulfil the expectations of the scientific user community which are summarized in the ELI-White Book.
Laser-driven particle acceleration is a novel field of physics that is rapidly evolving thanks to the continuing development of high power laser systems, thus allowing researchers to investigate the interaction of relativistic laser intensities (> 1019 W/cm2) with the matter. As a result of such interaction, extremely high electric and magnetic fields are generated. Such tremendous fields, which can be supported only in plasma, allow for the acceleration of particles by way of very compact approaches. In particular, spectacular progress in the acceleration of protons through energy transfer from relativistic electrons in thin solid targets has been achieved [Macchi et al ], currently leading to record proton energies around 100-MeV [Higginson et al ].
A photo of the Ion Acceleration and Applications of High Energy Particles team in charge of the ELIMAIA ion beamline user operations
LASER-PLASMA ION ACCELERATION
According to the state of the art in laser-driven ion acceleration, maximum proton energies approaching the 100-MeV level have been experimentally achieved with a relatively high yield (1010-1012 protons/pulse) [Macchi et al, Higginson et al]. However, laser-accelerated ion beams are still not mature for several applications in which additional features, such as low divergence, narrow energy spread, spatial profile uniformity, or shot-to-shot stability, are essential. Nevertheless, new laser technologies that will soon be available, e.g. at ELI Beamlines, will allow the scientific community to investigate novel plasma acceleration regimes that are very promising in terms of future use of laser-driven ion beams for multidisciplinary applications.
In general, such high energy ion beams are produced in thin solid targets by a strong electric field, which is, in turn, generated by a collective displacement of a large number of electrons. Such fields accelerate ions until charge neutrality is restored and they move together with electrons in a ballistic way. Until now, most of the experiments on laser-driven proton acceleration have been performed in a regime known as Target Normal Sheath Acceleration (TNSA). This scheme is based on the space-charge field generated at the rear surface of a micrometer-thick target. Such a quasi-electrostatic field is generated by high energy electrons, also known as “hot electrons”, which are accelerated by a high intensity laser at the target front surface, then cross the target bulk and attempt to escape in a vacuum from the rear side (see figure below). Currently, thanks to higher laser intensities achievable with newly installed laser systems, along with the use of thinner and specially engineered targets, new acceleration mechanisms are emerging. Protons can be consequently accelerated not only by longitudinal electric fields (established because of space charge separation), but also by laser radiation pressure, plasma shock waves and relativistic-induced transparency acceleration mechanisms [Macchi et al].