Energy enhancement of proton beam driven by ultra high intensity lasers and characterization of proton beams accelerated by ultra short laser pulses

Aula Divisione Acceleratori (Laboratori Nazionali di Frascati, Via Enrico Fermi 40, Frascati)

Aula Divisione Acceleratori

Laboratori Nazionali di Frascati, Via Enrico Fermi 40, Frascati

Via Enrico Fermi 40 00044 Frascati

Technological improvements since the 1980s have allowed for important developments in the field of high power lasers, thus paving the way for relativistic laser-matter interactions. With laser intensities higher than 1.1018, the scientific community could explore a new physics, full of promising applications. Historically, laser–plasma interaction research initially aimed at exploring fusion by inertial confinement, but, with the breakthrough of ultra-high power lasers, the scope of research could now be broadened to laboratory astrophysics, radiation generation (harmonics, betatron, X) and the production of high energy particles (electrons, ions). Regarding the latter, radiation and ion sources are of such excellent quality that they might in the future replace current conventional sources like synchrotrons or accelerators, which require very expensive facilities. This thesis focuses particularly on laser-driven ion acceleration, whose accelerated beams have already demonstrated strong potential, e.g. in ultrafast imaging or warm dense matter generation. Within this domain, the present work focused on strategies developed to increase the ion beam energy in the ultrahigh intensity regime (higher than 1.1019, exploiting as well moderate (400 fs) and short (25 fs) pulses facilities available as a result of the collaboration between the LULI laboratory in France and the INRS-EMT in Quebec. Innovative acceleration techniques have been explored at the LULI laboratory using moderately short laser pulses (400 fs to a few ps). This has been done first by improving our understanding of acceleration physics. Then, confinement of the laser-driven fast electrons that are at the source of the ion acceleration could be obtained by using reduced size targets. With such targets, electron confinement in the acceleration area could be achieved, inducing improvement of the laser to ions conversion efficiency, the ion beam cut off energy, and the ion beam quality. Another strategy that was exploited was to use refocalizing plasma optics to produce strongly reduced laser focal spot sizes. This induces laser intensity increase and thus improvement of the ion beam cut off energy. Finally, we used the combination of two laser pulses to have the electrons accelerated by each laser pulse interact together. When this was the case, we noted an increase of the ion beam cut off energy along with a modification of the beam typology. Complementarily, the experiments carried out using the 200 TW laser system in Quebec aimed at improving our understanding of femtosecond ion acceleration regimes, as only a few experimental studies were yet available, and to confirm the relevance of these regimes for ion acceleration. The results obtained with this laser clearly show the important role of the laser pulse contrast ratio, and the need for it to be extremely high to obtain efficient ion acceleration in this ultrashort regime. The systematic recording of accelerated ion beams has showed that a quasi-symmetric acceleration from the target front and rear sides is possible. These results have also proved that the highest proton energy is not necessarily obtained with the shortest pulse duration when the laser energy is kept constant. Thus, we demonstrated that the shortest pulses available today (i.e. 25 fs) are not the most efficient to accelerate ion beams.

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