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A high power laser named FLAME with an intensity up to 1021 W/cm2, a repetition rate of 10 Hz and a contrast value (between main pulse and pre-pulse) of 1010 is installed at the LNF – INFN in Frascati. In this frame an experiment of light ions acceleration through laser interaction with thin metal targets (SL-LILIA) has been proposed and funded. The aim of the SL-LILIA experiment is to study, design and verify a scheme which foresees the production, the characterization and the transport of a proton beam toward a stage of post acceleration (high frequency compact Linac). Now the maximum operating laser intensity is limited to 1019 W/cm2 due to the lack of a parabola with a focal length shorter that the current used. In this configuration, according to the interaction theory by short pulse laser and to performed numerical simulations, we expect a proton beam with maximum energy of a few MeV and a number of protons/shot up to 1010-1012.
In the past few years, various interesting experiments have been started in order to study the interaction of ultrahigh-power laser pulses (with intensities beyond 1018 W/cm2 and duration time ranging between 40-1000 fs) with thin solid films (thickness of the order of 0.5-100 µm) of different elements both metallic (Au, Cu, Pd, Al) and dielectric (polymers). These experiments have shown that, as a result of the laser-target interaction, protons and ions with energies up to 60 MeV are emitted. These protons, mostly originated by contaminated hydrocarbon surface used as target, are accelerated due to their higher charge-mass ratio with respect to other ions. Nevertheless, it is possible even to accelerate various species of ions by etching the target utilizing different methods. The total number of accelerated particles is strongly correlated to the specific target conditions and the experimental set-up: typical values are in the range from 109 to 1013 particles for laser pulse.
Laser driven acceleration is characterized by specific interesting properties, which mark a strong difference with respect to the traditional accelerating techniques. The most relevant features may be summarized in the following points: a) the possibility to accelerate ions at tens of MeV in very compact structures (of the order of a few tens of microns) due to the very high electric fields available with respect to the size of well known accelerators; b) an excellent beam quality with a transverse emittance less than 10-8 mm·mrad; c) a very short duration of the proton bunch (of the order of a few ps); d) the possibility to synchronize the proton beam with the laser beam up to a scale of a few fs to obtain multiple, synchronized sources of different particles (electrons, protons) and radiation (monochromatic X rays).
Due to these considerations laser driven acceleration holds the promise of compact accelerating structures which may be useful in different applications such as medicine (radioisotope production for PET, hadrontherapy), nuclear physics, inertial fusion (proton induced fast ignition), advanced diagnostic (proton imaging of fast electromagnetic fields) or material properties analysis and advanced imaging applications.